Java Programming/Print version

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Contents


Overview

Preface

The beautiful thing about learning is nobody can take it away from you.
B.B. King (5 October 1997)

Learning a computer programming language is like a toddler's first steps. You stumble, and fall, but when you start walking, programming becomes second nature. And once you start programming, you never cease evolving or picking up new tricks. Learn one programming language, and you will "know" them all — the logic of the world will begin to unravel around you.

Are you new to programming?

This stunning image of the sunset on planet Mars wouldn't have been possible without Java.

If you have chosen Java as your first programming language, be assured that Java is also the first choice for computer science programs in many universities. Its simple and intuitive syntax, or grammar, helps beginners feel at ease with complex programming constructs quickly.

However, Java is not a basic programming language. In fact, NASA used Java as the driving force (quite literally) behind its Mars Rover missions. Robots, air traffic control systems and the self-checkout barcode scanners in your favorite supermarkets can all be programmed in Java.

Programming with Java™

By now, you might truly be able to grasp the power of the Java programming language. With Java, there are many possibilities. Yet not every programmer gets to program applications that take unmanned vehicles onto other planets. Software that we encounter in our daily life is somewhat humble in that respect. Software in Java, however, covers a vast area of the computing ecosphere. Here are just a few examples of the ubiquitous nature of Java applications in real-life:

  • OpenOffice.org, a desktop office management suite that rivals the Microsoft Office suite has been written in Java.
  • The popular building game Minecraft is written in Java.
  • Online browser-based games like Runescape, a 3D massively multi-player online role playing game (MMORPG), run on graphics routines, 3D rendering and networking capabilities powered by the Java programming language.
  • Two of the world's renowned digital video recorders, TiVo and BSkyB's Sky+ use built-in live television recording software to record, rewind and play your favorite television shows. These applications make extensive use of the Java programming language.

The above mentioned applications illustrate the reach and ubiquity of Java applications. Here's another fact: almost 80% of mobile phone vendors adopt Java as their primary platform for the development of applications. The most widely used mobile-based operating system, Android, uses Java as one of its key application platforms — developers are encouraged to develop applications for Android in the Java programming language.

What can Java not do?

Well, to be honest, there is nothing that Java can't do. Java is a "complete" language; the only limits are programmer imagination and ability. This book aims to get you acquainted with the basics of the language so you can create the software masterpiece of your dreams.



About This Book

The Java Programming Wikibook is a shared effort in amassing a comprehensive guide of the complete Java platform — from programming advice and tutorials for the desktop computer to programming on mobile phones. The information presented in this book has been conceptualised with the combined efforts of various contributors, and anonymous editors.

The primary purpose of this book is to teach the Java programming language to an audience of beginners, but its progressive layout of tutorials increasing in complexity, it can be just as helpful for intermediate and experienced programmers. Thus, this book is meant to be used as:

  • a collection of tutorials building upon one another in a progressive manner;
  • a guidebook for efficient programming with the Java programming language; and,
  • a comprehensive manual resource for the advanced programmer.

This book is intended to be used in conjunction with various other online resources, such as:

Who should read this book?

Everything you would need to know to write computer programs would be explained in this book. By the time you finish reading, you will find yourself proficient enough to tackle just about anything in Java and programs written using it. This book serves as the first few stepping stones of many you would need to cross the unfriendly waters of computer programming. We have put a lot of emphasis in structuring this book in a way that lets you start programming from scratch, with Java as your preferred language of choice. This book is designed for you if any one of the following is true.

  • You are relatively new to programming and have heard how easy it is to learn Java.
  • You had some BASIC or Pascal in school, and have a grasp of basic programming and logic.
  • You already know and have been introduced to programming in earlier versions of Java.
  • You are an experienced developer and know how to program in other languages like C++, Visual Basic, Python, Ruby, etc.
  • You've heard that Java is great for web applications and web services programming.

Although this book is generally meant to be for readers who are beginning to learn programming; it can be highly beneficial for intermediate and advanced programmers who may have missed out on some vital information. After completing this book you should be able to solve many complicated problems using the Java skills presented in the following chapters. Once you finish, you are also encouraged to undertake ambitious programming projects of your own.

This book assumes that the reader has no prior knowledge of programming in Java, or for that matter, any object-oriented programming language. Practical examples and exercises following each topic and module make it easy to understand the software development methodology. If you are a complete beginner, we suggest that you move slowly through this book and complete each exercise at your own pace.

How to use this book

This book is a reference book of the Java language and its related technologies. Its goal is to give a complete picture of Java and its technologies. While the book can be read from the beginning to end, it is also designed to have individual sections that can be read independently. To help find information quickly, navigation boxes are given in the online version for access to individual topics.

This book is divided to sections. Pages are grouped together into section topics. To make this book expandable in the future via the addition of new sections, the sections navigation-wide are independent from each other. Each section can be considered as a mini book by itself. Pages that belong to the same topic can be navigated by the links on the right hand side.

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History

On 23 May 1995, John Gage, the director of the Science Office of the Sun Microsystems along with Marc Andreesen, co-founder and executive vice president at Netscape announced to an audience of SunWorldTM that Java technology wasn't a myth and that it was a reality and that it was going to be incorporated into Netscape Navigator.[1]

At the time the total number of people working on Java was less than 30.[1] This team would shape the future in the next decade and no one had any idea as to what was in store. From being the mind of an unmanned vehicle on Mars to the operating environment on most of the consumer electronics, e.g. cable set-top boxes, VCRs, toasters and also for personal digital assistants (PDAs).[2] Java has come a long way from its inception. Let's see how it all began.

Earlier programming languages

Before Java emerged as a programming language, C++ was the dominant player in the trade. The primary goals that the creators of Java was to create a language that could tackle most of the things that C++ offered while getting rid of some of the more tedious tasks that came with the earlier languages.

Computer hardware went through a performance and price revolution from 1972 to 1991. Better, faster hardware was available at lesser price and the demand for big and complex software exponentially increased. To accommodate the demand, new development technologies were invented.

The C language developed in 1972 by Dennis Ritchie had taken a decade to become the most popular language amongst programmers. But, with time programmers found that programming in C became tedious with its structural syntax.[3] Although, people attempted solving this problem, it would be later that a new development philosophy was introduced, one named Object-Oriented Programming. With OOP, a person may write a certain code which may be reused later without rewriting the code over and over again. In 1979, Bjarne Stroustrup developed C++, an enhancement to the C language with included OOP fundamentals and features.

The Green team

James Gosling, architect and designer of the compiler for the Java technology

Behind closed doors, a project was initiated in December of 1990, whose aim was to create a programming tool that could render obsolete the C and C++ programming languages. Engineer Patrick Naughton had become extremely frustrated with the state of Sun's C++ and C APIs (Application Programming Interfaces) and tools. While he was considering to move towards NeXT, he was offered a chance to work on new technology and the Stealth Project was started, a secret nobody but he knew.

This Stealth Project was later named the Green Project when James Gosling and Mike Sheridan joined Patrick.[1] Over the period of time that the Green Project teethed, the prospects of the project started becoming clearer to the engineers working on it. No longer was its aim to create a new language far superior to the present ones, but it aimed to target the language to devices other than the computer.

Staffed at 13 people, they began work in a small office on Sand Hill Road in Menlo Park, California. This team would be called Green Team henceforth in time. The project they underwent was chartered by Sun Microsystems to anticipate and plan for the "next-wave" in computing. For the team, this meant at least one significant trend, that of the convergence of digitally controlled consumer devices and computers.[1]

Reshaping thought

The team started thinking of replacing C++ with a better version, a faster version, a responsive version. But the one thing they hadn't thought of, as of yet, was that the language they were aiming for had to be developed for an embedded system with limited resources. An embedded system is a computer system scaled to a minimalistic interface demanding only a few functions from its design. For such a system, C++ or any successor would seem too large as all the languages at the time demanded a larger footprint than what was desired. And, other than this, the language lacked some other important features as well. The team thus had to think in a different way to go about solving all these problems.

Co-founder of Sun Microsystems, Bill Joy, envisioned a language combining the power of Mesa and C in a paper he wrote for the engineers at Sun named Further. Gathering ideas, Gosling began work on enhancing C++ and named it "C++ ++ --", a pun on the evolutionary structure of the language's name. The ++ and -- meant, putting in and taking out stuff. He soon abandoned the name and called it Oak[1] after the tree that stood outside his office.

Table 1: Who's who of the Java technology[1]
Has worked for GT (Green Team), FP (FirstPerson) and JP (Java Products Group)
Name GT FP JP Details
Lisa Friendly Yes check.svgYes Yes check.svgYes FirstPerson employee and member of the Java Products Group
John Gage Science Office (Director), Sun Microsystems
James Gosling Yes check.svgYes Yes check.svgYes Yes check.svgYes Lead engineer and key architect of the Java technology
Bill Joy Co-founder and VP, Sun Microsystems; Principal designer of the UC Berkeley, version of the UNIX® OS
Jonni Kanerva Yes check.svgYes Java Products Group employee, author of The Java FAQ1
Tim Lindholm Yes check.svgYes Yes check.svgYes FirstPerson employee and member Java Products Group
Scott McNealy Chairman, President, and CEO of Sun Microsystems
Patrick Naughton Yes check.svgYes Yes check.svgYes Green Team member, FirstPerson co-founder
George Paolini Corporate Marketing (Director), Sun's Java Software Division
Kim Polese Yes check.svgYes FirstPerson product marketing
Lisa Poulson Original director of public relations for Java technology (Burson-Marsteller)
Wayne Rosing Yes check.svgYes FirstPerson President
Eric Schmidt Former Sun Microsystems Chief Technology Officer
Mike Sheridan Yes check.svgYes Green Team member

The demise of an idea, birth of another

By now, the work on Oak had been significant but come the year 1993, people saw the demise of set-top boxes, interactive TV and the PDAs. A failure that completely ushered the inventors' thoughts to be reinvented. Only a miracle could make the project a success now. And such a miracle awaited anticipation.

National Center for Supercomputing Applications (NCSA) had just unveiled its new commercial web browser for the internet the previous year. The focus of the team, now diverted towards where they thought the "next-wave" of computing would be — the internet. The team then divulged into the realms of creating the same embeddable technology to be used in the web browser space calling it an appleta small application. Keeping all of this in mind, the team created a list of features tackling the C++ problems. In their opinion, the project should ...

  • .. be simple and gather tested fundamentals and features from the earlier languages in it,
  • .. have standard sets of APIs with basic and advanced features bundled with the language,
  • .. get rid of concepts requiring direct manipulation of hardware (in this case, memory) to make the language safe,
  • .. be platform independent and may written for every platform once (giving birth to the WORA idiom),
  • .. be able to manipulate network programming out-of-the-box,
  • .. be embeddable in web browsers, and ...
  • .. have the ability for a single program to multi-task and do multiple things at the same time.

The team now needed a proper identity and they decided on naming the new technology they created Java ushering a new generation of products for the internet boom. A by-product of the project was a cartoon named "Duke" created by Joe Parlang which became its identity then.

Finally at the SunWorldTM conference, Andreesen unveiled the new technology to the masses. Riding along with the explosion of interest and publicity in the Internet, Java quickly received widespread recognition and expectations grew for it to become the dominant software for browser and consumer applications.[2]

Initially Java was owned by Sun Microsystems, but later it was released to open source; the term Java was a trademark of Sun Microsystems. Sun released the source code for its HotSpot Virtual Machine and compiler in November 2006, and most of the source code of the class library in May 2007. Some parts were missing because they were owned by third parties, not by Sun Microsystems. The released parts were published under the terms of the GNU General Public License, a free software license.

Versions

Unlike C and C++, Java's growth is pretty recent. Here, we'd quickly go through the development paths that Java took with age.

Development of Java over the years. From version 1.0 to version 1.7, Java has displayed a steady growth.

Initial Release (versions 1.0 and 1.1)

Introduced in 1996 for the Solaris, Windows, Mac OS Classic and Linux, Java was initially released as the Java Development Kit 1.0 (JDK 1.0). This included the Java runtime (the virtual machine and the class libraries), and the development tools (e.g., the Java compiler). Later, Sun also provided a runtime-only package, called the Java Runtime Environment (JRE). The first name stuck, however, so usually people refer to a particular version of Java by its JDK version (e.g., JDK 1.0).

Java 2 (version 1.2)

Introduced in 1998 as a quick fix to the former versions, version 1.2 was the start of a new beginning for Java. The JDKs of version 1.2 and later versions are often called Java 2 as well. For example, the official name of JDK 1.4 is The Java(TM) 2 Platform, Standard Edition version 1.4.

Major changes include:
  • Rewrite the event handling (add Event Listeners)
  • Change Thread synchronizations
  • Introduction of the JIT-Just in time compilers

Kestrel (Java 1.3)

Released in 8 May 2000. The most notable changes were:

  • HotSpot JVM included (the HotSpot JVM was first released in April, 1999 for the J2SE 1.2 JVM)
  • RMI was modified to support optional compatibility with CORBA
  • JavaSound
  • Java Naming and Directory Interface (JNDI) included in core libraries (previously available as an extension)
  • Java Platform Debugger Architecture (JPDA)
  • Synthetic proxy classes

Merlin (Java 1.4)

Released in 6 February 2002, Java 1.4 has improved programmer productivity by expanding language features and available APIs:

  • Assertion
  • Regular Expression
  • XML processing
  • Cryptography and Secure Socket Layer (SSL)
  • Non-blocking I/O (NIO)
  • Logging

Tiger (version 1.5.0; Java SE 5)

Released in September 2004

Major changes include:
  • Generics - Provides compile-time type safety for collections :and eliminates the drudgery of casting.
  • Autoboxing/unboxing - Eliminates the drudgery of manual conversion between primitive types (such as int) and wrapper types (such as Integer).
  • Enhanced for - Shorten the for loop with Collections use.
  • Static imports - Lets you import all the static part of a class.
  • Annotation/Metadata - Enabling tools to generate code and deployment descriptors from annotations in the source code. This leads to a "declarative" programming style where the programmer says what should be done and tools emit the code to do it. Annotations can be inspected through source parsing or by using the additional reflection APIs added in Java 5.
  • JVM Improvements - Most of the run time library is now mapped into memory as a memory image, as opposed to being loaded from a series of class files. Large portion of the runtime libraries will now be shared among multiple JVM instances.

Mustang (version 1.6.0; Java SE 6)

Released on 11 December 2006.[4]

What's New in Java SE 6:

  • Web Services - First-class support for writing XML web service client applications.
  • Scripting - You can now mix in JavaScript technology source code, useful for prototyping. Also useful when you have teams with a variety of skill sets. More advanced developers can plug in their own scripting engines and mix their favorite scripting language in with Java code as they see fit.
  • Database - No more need to find and configure your own JDBC database when developing a database application. Developers will also get the updated JDBC 4.0, a well-used API with many important improvements, such as special support for XML as an SQL datatype and better integration of Binary Large OBjects (BLOBs) and Character Large OBjects (CLOBs) into the APIs.
  • More Desktop APIs - GUI developers get a large number of new tricks to play like the ever popular yet newly incorporated SwingWorker utility to help you with threading in GUI apps, JTable sorting and filtering, and a new facility for quick splash screens to quiet impatient users.
  • Monitoring and Management - The really big deal here is that you don't need to do anything special to the startup to be able to attach on demand with any of the monitoring and management tools in the Java SE platform.
  • Compiler Access - Really aimed at people who create tools for Java development and for frameworks like JavaServer Pages (JSP) or Personal Home Page construction kit (PHP) engines that need to generate a bunch of classes on demand, the compiler API opens up programmatic access to javac for in-process compilation of dynamically generated Java code. The compiler API is not directly intended for the everyday developer, but for those of you deafened by your screaming inner geek, roll up your sleeves and give it a try. And the rest of us will happily benefit from the tools and the improved Java frameworks that use this.
  • Pluggable Annotations allows programmer to write annotation processor so that it can analyse your code semantically before javac compiles. For example, you could write an annotation processor that verifies whether your program obeys naming conventions.
  • Desktop Deployment - At long last, Java SE 6 unifies the Java Plug-in technology and Java WebStart engines, which just makes sense. Installation of the Java WebStart application got a much needed makeover.
  • Security - Java SE 6 has simplified the job of its security administrators by providing various new ways to access platform-native security services, such as native Public Key Infrastructure (PKI) and cryptographic services on Microsoft Windows for secure authentication and communication, Java Generic Security Services (Java GSS) and Kerberos services for authentication, and access to LDAP servers for authenticating users.
  • The -lities: Quality, Compatibility, Stability - Bug fixes ...

Dolphin (version 1.7.0; Java SE 7)

Released on 28 July 2011.

Feature additions for Java 7 include:[5]

  • JVM support for dynamic languages, following the prototyping work currently done on the Multi Language Virtual Machine
  • Compressed 64-bit pointers[6] Available in Java 6 with -XX:+UseCompressedOops
  • Small language changes (grouped under a project named Coin):[7]
  • Strings in switch[8]
  • Automatic resource management in try-statement[9]
  • Improved type inference for generic instance creation[10]
  • Simplified varargs method declaration[11]
  • Binary integer literals[12]
  • Allowing underscores in numeric literals[13]
  • Catching multiple exception types and rethrowing exceptions with improved type checking[14]
  • Concurrency utilities under JSR 166[15]
  • New file I/O library to enhance platform independence and add support for metadata and symbolic links. The new packages are java.nio.file and java.nio.file.attribute[16][17]
  • Library-level support for Elliptic curve cryptography algorithms
  • An XRender pipeline for Java 2D, which improves handling of features specific to modern GPUs
  • New platform APIs for the graphics features originally planned for release in Java version 6u10
  • Enhanced library-level support for new network protocols, including SCTP and Sockets Direct Protocol
  • Upstream updates to XML and Unicode

Lambda (Java's implementation of lambda functions), Jigsaw (Java's implementation of modules), and part of Coin were dropped from Java 7. Java 8 will be released with the remaining features in summer 2013.[18]

References

  1. a b c d e f "Java Technology: The Early Years". Sun Microsystems. http://java.sun.com/features/1998/05/birthday.html. Retrieved 9 May 2008. 
  2. a b "History of Java". Lindsey, Clark S.. http://www.particle.kth.se/~lindsey/JavaCourse/Book/Part1/Java/Chapter01/history.html. Retrieved 7 May 2008. 
  3. Structural syntax is a linear way of writing code. A program is interpreted usually at the first line of the program's code until it reaches the end. One can not hook a later part of the program to an earlier one. The flow follows a linear top-to-bottom approach.
  4. "Java Platform Standard Edition 6". Sun Microsystems. http://www.sun.com/aboutsun/media/presskits/2006-1211/. Retrieved 9 May 2008. 
  5. Miller, Alex. "Java 7". http://tech.puredanger.com/java7. Retrieved 2008-05-30. 
  6. "Compressed oops in the Hotspot JVM". OpenJDK. https://wikis.oracle.com/display/HotSpotInternals/CompressedOops. Retrieved 2012-08-01. 
  7. "Java Programming Language Enhancements". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/enhancements.html#javase7. Retrieved 2013-01-15. 
  8. "Strings in switch Statements". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/strings-switch.html. Retrieved 2013-01-15. 
  9. "The try-with-resources Statement". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/try-with-resources.html. Retrieved 2013-01-15. 
  10. "Type Inference for Generic Instance Creation". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/type-inference-generic-instance-creation.html. Retrieved 2013-01-15. 
  11. "Improved Compiler Warnings When Using Non-Reifiable Formal Parameters with Varargs Methods". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/non-reifiable-varargs.html. Retrieved 2013-01-15. 
  12. "Binary Literals". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/binary-literals.html. Retrieved 2013-01-15. 
  13. "Underscores in Numeric Literals". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/underscores-literals.html. Retrieved 2013-01-15. 
  14. "Catching Multiple Exception Types and Rethrowing Exceptions with Improved Type Checking". Download.oracle.com. http://download.oracle.com/javase/7/docs/technotes/guides/language/catch-multiple.html. Retrieved 2013-01-15. 
  15. "Concurrency JSR-166". http://gee.cs.oswego.edu/dl/concurrency-interest/index.html. Retrieved 2010-04-16. 
  16. "File I/O (Featuring NIO.2) (The Java™ Tutorials > Essential Classes > Basic I/O)". Java.sun.com. 2008-03-14. http://java.sun.com/docs/books/tutorial/essential/io/fileio.html. Retrieved 2013-01-15. 
  17. "Legacy File I/O Code (The Java™ Tutorials > Essential Classes > Basic I/O)". Java.sun.com. 2012-02-28. http://java.sun.com/docs/books/tutorial/essential/io/legacy.html. Retrieved 2013-01-15. 
  18. "JavaOne 2011 Keynote". Oracle. http://blogs.oracle.com/javaone/resource/java_keynote/slide_16_full_size.gif. 



Java Overview

The new features and upgrades included into Java changed the face of programming environment and gave a new definition to Object Oriented Programming (OOP in short). But unlike its predecessors, Java needed to be bundled with standard functionality and be independent of the host platform.

The primary goals in the creation of the Java language:

  • It is simple.
  • It is object-oriented.
  • It is independent of the host platform.
  • It contains language facilities and libraries for networking.
  • It is designed to execute code from remote sources securely.

The Java language introduces some new features that did not exist in other languages like C and C++.

Object orientation

Object oriented programming can be represented with UML diagrams.

Object orientation ("OO"), refers to a method of programming and language technique. The main idea of OO is to design software around the "things" (i.e. objects) it manipulates, rather than the actions it performs.

As the hardware of the computer advanced, it brought about the need to create better software techniques to be able to create ever increasing complex applications. The intent is to make large software projects easier to manage, thus improving quality and reducing the number of failed projects. Object oriented solution is the latest software technique.

Assembly languages 
Software techniques started with the assembly languages, that was close to machine instruction and was easy to convert into executable code. Each hardware had its own assembly language. Assembly language contains low level instructions like move data from memory to hardware registers, do arithmetic operations, and move data back to memory. Programmers had to know the detailed architecture of the computer in order to write programs.
Procedural languages 
After the assembly languages, high level languages were developed. Here the language compiler is used to convert the high level program to machine instructions, freeing up the programmers the burden of knowing the computer hardware architecture. To promote the re-use of code, and to minimize the use of GOTO instruction, "procedural" techniques were introduced. This simplified the creation and maintenance of software control flow, but they left out the organization of data. It became a nightmare to debug and maintain programs having many global variables. Global variables contain data that can be modified anywhere in the application.
Object oriented languages 
In OO languages, data is taken seriously with information hiding. Procedures were replaced by Objects. Objects contain data as well as control flow. Our thinking has to shift from procedures to interaction between objects.

Platform dependence

In C or C++ programming, you start to write a source code:
Gnome-mime-text-x-c.svg

... you compile it to a machine code file:
Gnome-mime-text-x-c.svg Gnome-mime-application-x-executable.svg

... and then you execute it:
Gnome-mime-text-x-c.svg Gnome-mime-application-x-executable.svg Window Dialog.svg

In this situation, the machine code file and its execution are specific to the platform (Windows, Linux, Mac OS, ...) it was compiled for, that is to say to the targeted platform:

Gnome-mime-text-x-c.svg

Gnome-mime-application-x-executable.svgWindow Dialog.svg


... because the compiled file is a machine code file designed to work on a specific platform and hardware. It would have produced a different results/output for another platform. So if you want your program to run on several platforms, you have to compile your program several times:

Gnome-mime-text-x-c.svg

Gnome-mime-application-x-executable.svgGnome-mime-application-x-executable.svgGnome-mime-application-x-executable.svg

Windows icon.svgApple Logo.svgTux-shaded.svg

It poses greater vulnerability risks. Note here that when a certain code is compiled into an executable format, the executable cannot be changed dynamically. It would need to be recompiled from the changed code for the changes to be reflected in the finished executable. Modularity (dividing code into modules) is not present in Java's predecessors. If instead of a single executable, the output application was in the form of modules, one could easily change a single module and review changes in the application. In C/C++ on the other hand, a slight change in code required the whole application to be recompiled.

The idea of Java is to compile the source code into an intermediate language that will be interpreted.

Text-x-java-source.svg
Application-x-executable.svg
Java.png
The source code The intermediate file The interpretor

The intermediate language is the byte code. The interpretor is the Java Virtual Machine (JVM). The byte code file is universal and the JVM is platform specific:

Text-x-java-source.svg

Application-x-executable.svg

Java.pngJava.pngJava.png

Windows icon.svgApple Logo.svgTux-shaded.svg

So a JVM should be coded for each platform. And that's the case. So you just have to generate a unique byte code file (a .class file).

The first implementations of the language used an interpreted virtual machine to achieve portability, and many implementations still do. These implementations produce programs that run more slowly than the fully-compiled programs created by the typical C++ compiler, so the language suffered a reputation for producing slow programs. Since Java 1.2, Java VM produces programs that run much faster, using multiple techniques.

The first of these is to simply compile directly into native code like a more traditional compiler, skipping bytecode entirely. This achieves great performance, but at the expense of portability. This is not really used any more.

Another technique, the just-in-time (JIT) compiler, compiles the Java bytecode into native code at the time the program is run, and keep the compiled code to be used again and again. More sophisticated VMs even use dynamic recompilation, in which the VM can analyze the behavior of the running program and selectively recompile and optimize critical parts of the program. Both of these techniques allow the program to take advantage of the speed of native code without losing portability.

Portability is a technically difficult goal to achieve, and Java's success at that goal is a matter of some controversy. Although it is indeed possible to write programs for the Java platform that behave consistently across many host platforms, the large number of available platforms with small errors or inconsistencies led some to parody Sun's "Write once, run anywhere" slogan as "Write once, debug everywhere".

Standardization

C++ was built atop the C language and as a result divergent ways of doing the same thing manifested around the language. For instance, creating an object could be done in three different ways in C++. Furthermore, C++ did not come with a standard library bundled with its compilers. Instead, it relied on resources created by other programmers; code which rarely fit together.

In Java, standardized libraries are provided to allow access to features of the host machines (such as graphics and networking) in unified ways. The Java language also includes support for multi-threaded programs—a necessity for many networking applications.

Platform independent Java is, however, very successful with server side applications, such as web services, servlets, or Enterprise JavaBeans.

Swing does not rely on the underlying native user interface.

Java also made progress on the client side, first it had Abstract Window Toolkit (AWT), then Swing, and the most recent client side library is the Standard Widget Toolkit (SWT). It is interesting to see how they tried to handle the two opposing consuming forces. Those are :

Efficient, fast code; port to most popular hardware (write once, test anywhere) 
Use the underlying native subroutine to create a GUI component. This approach was taken by AWT, and SWT.
Portability to any hardware where JVM ported (write once, run anywhere) 
To achieve this to the latter, the Java toolkit should not rely on the underlying native user interface. Swing tooks this approach.

It is interesting to see how the approach was switched back and forth. AWT → Swing → SWT.

Secure execution

The segmentation fault, one of the most recurrent issues in C programming.

With the high-level of control built into the language to manipulate hardware, a C/C++ programmer could access almost any resource, either hardware or software on the system. This was intended to be one of the languages' strong points, but this very flexibility led to confusion and complex programming practices.

Error handling

The old way of error handling was to let each function return an error code then let the caller check what was returned. The problem with this method was that if the return code was full of error-checking codes, this got in the way of the original one that was doing the actual work, which in turn did not make it very readable.

In the new way of error handling, functions/methods do not return error codes. Instead, when there is an error, an exception is thrown. The exceptions can be handled by the catch keyword at the end of a try block. This way, the code that is calling the function does not need to be mangled with error checking codes, thus making the code more readable. This new way of error handling is called Exception handling.

Exception handling was also added to C++. However, there are two differences between Java and C++ Exception handling:

  • In Java, the exception that is thrown is a Java object like any other object in Java. It only has to implement Throwable interface.
  • In Java, the compiler checks whether an exception may be caught or not. The compiler gives an error if there is no catch block for a thrown exception.

The optional exception handling in the Java predecessors leads the developers not to care about the error handling. As a consequence, unexpected errors often occur. Java forces the developers to handle exceptions. The programmer must handle exception or declare that the user must handle it. Someone must handle it.

Networking capabilities

However powerful, the predecessors of Java lacked a standard feature to network with other computers, and usually relied on the platforms' intricate networking capabilities. With almost all network protocols being standardized, the creators of Java technology wanted this to be a flagship feature of the language while keeping true to the spirit of earlier advances made towards standardizing Remote Procedure Call. Another feature that the Java team focused on was its integration in the World Wide Web and the Internet.

The Java platform was one of the first systems to provide wide support for the execution of code from remote sources. The Java language was designed with network computing in mind.

An applet could run within a user's browser, executing code downloaded from a remote HTTP server. The remote code runs in a highly restricted "sandbox", which protects the user from misbehaving or malicious code; publishers could apply for a certificate that they could use to digitally sign applets as "safe", giving them permission to break out of the sandbox and access the local file system and network, presumably under user control.

Dynamic class loading

In conventional languages like C and C++, all code had to be compiled and linked to one executable program, before execution. In Java, classes are compiled as needed. If a class is not needed during an execution phase, that class is not even compiled into byte code.

This feature comes in handy especially in network programming when we do not know, beforehand, what code will be executed. A running program could load classes from the file system or from a remote server.

Also this feature makes it theoretically possible for a Java program to alter its own code during execution, in order to do some self-learning behavior. It would be more realistic to imagine, however, that a Java program would generate Java code before execution, and then, that code would be executed. With some feedback mechanism, the generated code could improve over time.

Automatic memory garbage collection

In conventional languages like C and C++, the programmer has to make sure that all memory that was allocated is freed. Memory leaks became a regular nuisance in instances where the programmers had to manually allocate the system's memory resources.

Memory resources or buffers have specific modes of operation for optimal performance. Once a buffer is filled with data, it needs to be cleaned up after there is no further use for its content. If a programmer forgets to clean it in his/her code, the memory is easily overloaded. Programming in C/C++ languages became tedious and unsafe because of these very quirks, and programs built in these languages were prone to memory leakages and sudden system crashes — sometimes even harming the hardware itself. Freeing memory is particularly important in servers, since it has to run without stopping for days. If a piece of memory is not freed after use and the server just keeps allocating memory, that memory leak can take down the server.

In Java, freeing up memory is taken out of the programmers hands; the Java Virtual Machine keeps track of all used memory. When memory is not used any more it is automatically freed up. A separate task is running in the background by the JVM, freeing up unreferenced, unused memory. That task is called the Garbage Collector.

The Garbage Collector is always running. This automatic memory garbage collection feature makes it easy to write robust server side programs in Java. The only thing the programmer has to watch for is the speed of object creation. If the application is creating objects faster than the Garbage Collector can free them, it can cause memory problems. Depending on how the JVM is configured, the application either can run out of memory by throwing the NotEnoughMemoryException, or can halt to give time for the Garbage Collector to do its job.

Applet

The Java creators created the concept of the applet. A Java program can be run in a client browser program. Java was released in 1995; the time when the Internet was becoming more available and familiar to the general public. The promise of Java was in the client browser-side in that code would be downloaded and executed as a Java applet in the client browser program.

See also Java Programming/Applets.

Forbidden bad practices

Over the years, some features in C/C++ programming become abused by the programmers. Although the language allows it, it was known as bad practices. So the creators of Java have disabled them:

  • Operator overloading
  • Multiple inheritance
  • Friend classes (access an other object private members)
  • Restrictions of explicit type casting (related to memory management)

Evaluation

In most people's opinions, Java technology delivers reasonably well on all these goals. The language is not, however, without drawbacks. Java tends to be more high-level than similar languages (such as C++), which means that the Java language lacks features such as hardware-specific data types, low-level pointers to arbitrary memory addresses, or programming methods like operator overloading. Although these features are frequently abused or misused by programmers, they are also powerful tools. However, Java technology includes Java Native Interface (JNI), a way to call native code from Java language code. With JNI, it is still possible to use some of these features.

Some programmers also complain about its lack of multiple inheritance, a powerful feature of several object-oriented languages, among others C++. The Java language separates inheritance of type and implementation, allowing inheritance of multiple type definitions through interfaces, but only single inheritance of type implementation via class hierarchies. This allows most of the benefits of multiple inheritance while avoiding many of its dangers. In addition, through the use of concrete classes, abstract classes, as well as interfaces, a Java language programmer has the option of choosing full, partial, or zero implementation for the object type he defines, thus ensuring maximum flexibility in application design.

There are some who believe that for certain projects, object orientation makes work harder instead of easier. This particular complaint is not unique to the Java language but applies to other object-oriented languages as well.



The Java Platform

The Java platform is the name given to the computing platform from Oracle that helps users to run and develop Java applications. The platform does not just enable a user to run and develop a Java application, but also features a wide variety of tools that can help developers work efficiently with the Java programming language.

The platform consists of two essential softwares:

  • the Java Runtime Environment (JRE), which is needed to run Java applications and applets; and,
  • the Java Development Kit (JDK), which is needed to develop those Java applications and applets. If you have installed the JDK, you should know that it comes equipped with a JRE as well. So, for all the purposes of this book, you would only require the JDK.

In this section, we would explore in further detail what these two software components of the Java platform do.

Java Runtime Environment (JRE)

Any piece of code written in the Java programming language can be run on any operating system, platform or architecture — in fact, it can be run on any device that supports the Java platform. Before Java, this amount of ubiquity was very hard to achieve. If a software was written for a Unix-based system, it was impossible to run the same application on a Windows system — in this case, the application was native only to Unix-based systems.

A major milestone in the development of the Java programming language was to develop a special runtime environment that would execute any Java application independent of the computer's operating system, platform or architecture.

The Java Runtime Environment (JRE) sits on top of the machine's operating system, platform and architecture. If and when a Java application is run, the JRE acts as a liaison between the underlying platform and that application. It interprets the Java application to run in accordance with the underlying platform, such that upon running the application, it looks and behaves like a native application. The part of the JRE that accomplishes this complex liaison agreement is called the Java Virtual Machine (JVM).


Figure 1: Java applications can be written once and run anywhere. This feature of the Java platform
is commonly abbreviated to WORA in formal Java texts.
Explaining Java Runtime Environment.svg


Executing native Java code (or byte-code)

Native Java applications are preserved in a special format called the byte-code. Byte-code remains the same, no matter what hardware architecture, operating system, or software platform it is running under. On a file-system, Java byte-code resides in files that have the .class (also known as a class file) or the .jar (also known as a Java archive) extension. To run byte-code, the JRE comes with a special tool (appropriately named java).

Suppose your byte-code is called SomeApplication.class. If you want to execute this Java byte-code, you would need to use the following command in Command Prompt (on Windows) or Terminal (on Linux or Mac OS):

Standard input or output Execution
$ java SomeApplication

If you want to execute a Java byte-code with a .jar extension (say, SomeApplication.jar), you would need to use the following command in Command Prompt (on Windows) or Terminal (on Linux or Mac OS):

Standard input or output Execution with a jar
$ java -jar SomeApplication.jar
Note Not all Java class files or Java archives are executable. Therefore, the java tool would only be able to execute files that are executable. Non-executable class files and Java archives are simply called class libraries.

Do you have a JRE?

Most computers come with a pre-installed copy of the JRE. If your computer doesn't have a JRE, then the above commands would not work. You can always check what version of the JRE is installed on the computer by writing the following command in Command Prompt (on Windows) or Terminal (on Linux or Mac OS):

Standard input or output Java version
$ java -version

Java Virtual Machine (JVM)

Quite possibly, the most important part of the JRE is the Java Virtual Machine (JVM). The JVM acts like a virtual processor, enabling Java applications to be run on the local system. It's main purpose is to interpret (read translate) the received byte-code and make it appear as native code. The older Java architecture used this process of interpretation to execute Java byte-code. Even though the process of interpretation brought the WORA principle to diverse machines, it had a drawback — it consumed a lot of time and clocked the system processor intensively to load an application.


Figure 2: A JVM interpreter translates the byte-code line-by-line to make it appear as if a native application is being executed.
How JVM Interpreter works.svg


Just-in-Time Compilation

Since version 1.2, the JRE features a more robust JVM. Instead of interpreting byte-code, it down-right converts the code straight into equivalent native code for the local system. This process of conversion is called just-in-time compilation or JIT-compilation. This process only occurs when the byte-code is executed for the first time. Unless the byte-code itself is changed, the JVM uses the compiled version of the byte-code on every successive execution. Doing so saves a lot of time and processor effort, allowing applications to execute much faster at the cost of a small delay on first execution.


Figure 3: A just-in-time compiler only compiles the byte-code to equivalent native code at first execution. Upon every successive
execution, the JVM merely uses the already compiled native code to optimize performance.
How JIT Compilation works.svg


Native optimization

The JVM is an intelligent virtual processor. It has the ability to identify areas within the Java code itself that can be optimized for faster and better performance. Based on every successive run of your Java applications, the JVM would optimize it to run even better.

Note There are portions of Java code that do not require it to be JIT-compiled at runtime, e.g., the Reflection API; therefore, code that uses such functions are not necessarily fully compiled to native code.

Was JVM the first virtual machine?

Java was not the first virtual-machine-based platform, though it is by far the most successful and well-known. Previous uses for virtual machine technology primarily involved emulators to aid development for not-yet-developed hardware or operating systems, but the JVM was designed to be implemented entirely in software, while making it easy to efficiently port an implementation to hardware of all kinds.

Java Development Kit (JDK)

The JRE takes care of running the Java code on multiple platforms, however as developers, we are interested in writing pure code in Java which can then be converted into Java byte-code for mass deployment. As developers, we do not need to write Java byte-code, rather we write the code in the Java programming language (which is quite similar to writing C or C++ code).

Upon downloading the JDK, a developer ensures that their system has the appropriate JRE and additional tools to help with the development of applications in the Java programming language. Java code can be found in files with the extension .java. These files are called Java source files. In order to convert the Java code in these source files to Java byte-code, you need to use the Java compiler tool installed with your JDK.

The Java compiler

The Java compiler tool (named javac in the JDK) is the most important utility found with the JDK. In order to compile a Java source file (say, SomeApplication.java) to its respective Java byte-code, you would need to use the following command in Command Prompt (on Windows) or Terminal (on Linux or Mac OS):

Standard input or output Compilation
javac SomeApplication.java

This command would convert the SomeApplication.java source file into its equivalent Java byte-code. The resultant byte-code would exist in a newly created file named SomeApplication.class. This process of converting Java source files into their equivalent byte-codes is known as compilation.


Figure 4: The basic Java compilation process
Java Compilation Basics.svg


A list of other JDK tools

There are a huge array of tools available with the JDK that will all be explained in due time as you progress with the book. These tools are briefly listed below in order of their usage:

Applet development

  • appletviewer — Java applets require a particular environment to execute. Typically, this environment is provided by a browser with a Java plug-in, and a web server serving the applet. However, during development and testing of an applet it might be more convenient to start an applet without the need to fiddle with a browser and a web server. In such a case, Oracle's appletviewer from the JDK can be used to run an applet.

Annotation processing

For more about annotation processing, read this

In Java 1.5 (alias Java 5.0) Oracle added a mechanism called annotations. Annotations allow the addition of meta-data to Java source code, and even provide mechanisms to carry that meta-data forth into a compiled .class files.

  • apt — An annotation processing tool which digs through source code, finds annotation statements in the source code and executes actions if it finds known annotations. The most common task is to generate some particular source code. The actions apt performs when finding annotations in the source code are not hard-coded into apt. Instead, one has to code particular annotation handlers (in Java). These handlers are called annotation processors. It can also be described in a simple way without the Oracle terminology: apt can be seen as a source code preprocessor framework, and annotation processors are typically code generators.

Integration of non-Java and Java code

  • javah — A Java class can call native, or non-Java, code that has been prepared to be called from Java. The details and procedures are specified in the JNI (Java Native Interface). Commonly, native code is written in C (or C++). The JDK tool javah helps to write the necessary C code, by generating C header files and C stub code.

Class library conflicts

  • extcheck — It can be used prior to the installation of a Java extension into the JDK or JRE environment. It checks if a particular Jar file conflicts with an already installed extension. This tool appeared first with Java 1.5.

Software security and cryptography tools

The JDK comes with a large number of tools related to the security features of Java. Usage of these tools first requires study of the particular security mechanisms. The tools are:

  • keytool — To manage keys and certificates
  • jarsigner — To generate and verify digital signatures of JARs (Java ARchives)
  • policytool — To edit policy files
  • kinit — To obtain Kerberos v5 tickets
  • klist — To manage Kerberos credential cache and key table
  • ktab — To manage entries in a key table

The Java archiver

  • jar — (short for Java archiver) is a tool for creating Java archives or jar files — a file with .jar as the extension. A Java archive is a collection of compiled Java classes and other resources which those classes may require (such as text files, configuration files, images) at runtime. Internally, a jar file is really a .zip file.

The Java debugger

  • jdb — (short for Java debugger) is a command-line console that provides a debugging environment for Java programs. Although you can use this command line console, IDE's normally provide easier to use debugging environments.

Documenting code with Java

As programs grow large and complex, programmers need ways to track changes and to understand the code better at each step of its evolution. For decades, programmers have been employing the use of special programming constructs called comments — regions that help declare user definitions for a code snippet within the source code. But comments are prone to be verbose and incomprehensible, let alone be difficult to read in applications having hundreds of lines of code.

  • javadoc — Java provides the user with a way to easily publish documentation about the code using a special commenting system and the javadoc tool. The javadoc tool generates documentation about the Application Programming Interface (API) of a set of user-created Java classes. javadoc reads source file comments from the .java source files and generates HTML documents that are easier to read and understand without looking at the code itself.
  • javap — Where Javadoc provide a detailed view into the API and documentation of a Java class, the javap tool prints information regarding members (constructors, methods and variables) in a class. In other words, it lists the class' API and/or the compiled instructions of the class. javap is a formatting disassembler for Java bytecode.

The native2ascii tool

native2ascii is an important, though underappreciated, tool for writing properties files — files containing configuration data — or resource bundles — files containing language translations of text.

Such files can contain only ASCII and Latin-1 characters, but international programmers need a full range of character sets. Text using these characters can appear in properties files and resource bundles only if the non-ASCII and non-Latin-^1 characters are converted into Unicode escape sequences (\uXXXX notation).

The task of writing such escape sequences is handled by native2ascii. You can write the international text in an editor using the appropriate character encoding, then use native2ascii to generate the necessary ASCII text with embedded Unicode escape sequences. Despite the name, native2ascii can also convert from ASCII to native, so it is useful for converting an existing properties file or resource bundle back to some other encoding.

native2ascii makes most sense when integrated into a build system to automate the conversion.

Remote Method Invocation (RMI) tools

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Java IDL and RMI-IIOP Tools

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Deployment & Web Start Tools

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Browser Plug-In Tools

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Monitoring and Management Tools / Troubleshooting Tools

With Java 1.5 a set of monitoring and management tools have been added to the JDK, in addition to a set of troubleshooting tools.

The monitoring and management tools are intended for monitoring and managing the virtual machine and the execution environment. They allow, for example, monitoring memory usage during the execution of a Java program.

The troubleshooting tools provide rather esoteric insight into aspects of the virtual machine. (Interestingly, the Java debugger is not categorized as a troubleshooting tool.)

All the monitoring and management and troubleshooting tools are currently marked as "experimental" (which does not affect jdb). So they might disappear in future JDKs.

Java class libraries (JCL)

In most modern operating systems, a large body of reusable code is provided to simplify the programmer's job. This code is typically provided as a set of dynamically loadable libraries that applications can call at runtime. Because the Java platform is not dependent on any specific operating system, applications cannot rely on any of the existing libraries. Instead, the Java platform provides a comprehensive set of standard class libraries, containing much of the same reusable functions commonly found in modern operating systems.

The Java class libraries serve three purposes within the Java platform. Like other standard code libraries, they provide the programmer with a well-known set of functions to perform common tasks, such as maintaining lists of items or performing complex string parsing. In addition, the class libraries provide an abstract interface to tasks that would normally depend heavily on the hardware and operating system. Tasks such as network access and file access are often heavily dependent on the native capabilities of the platform. The Java java.net and java.io libraries implement the required native code internally, then provide a standard interface for the Java applications to perform those tasks. Finally, some underlying platforms may not support all of the features a Java application expects. In these cases, the class libraries can either emulate those features using whatever is available, or provide a consistent way to check for the presence of a specific feature.

Similar concepts

The success of the Java platform and the concepts of the write once, run anywhere principle has led to the development of similar frameworks and platforms. Most notable of these is the Microsoft's .NET framework and its open-source equivalent Mono.

The .NET framework

The .NET framework borrows many of the concepts and innovations of Java — their alternative for the JVM is called the Common Language Runtime (CLR), while their alternative for the byte-code is the Common Intermediate Language (CIL). In fact, the .NET platform had an implementation of a Java-like language called Visual J# (formerly known as J++).

J# is normally not supported with the JVM because instead of compiling it in Java byte-code, the .NET platform compiles the code into CIL, thus making J# different from the Java programming language. Furthermore, because J# implements the .NET Base Class Libraries (BCL) instead of the Java Class Libraries, J# is nothing more than a non-standard extension of the Java programming language. Due to the lack of interest from developers, Microsoft had to withdraw their support for J#, and focused on a similar programming language: C#.

Third-party compilers targeting the JVM

The word Java, by itself, usually refers to the Java programming language which was designed for use with the Java platform. Programming languages are typically outside of the scope of the phrase "platform". However, Oracle does not encourage the use of any other languages with the platform, and lists the Java programming language as a core part of the Java 2 platform. The language and runtime are therefore commonly considered a single unit.

There are cases where you might want to program using a different language (say, Python) and yet be able to generate Java byte-code (instead of the Python compiled code) to be run with the JVM. Many third-party programming language vendors provide compilers that can compile code written in their language to Java byte-code. For instance, Python developers can use Jython compilers to compile Python code to the Java byte-code format (as illustrated below).


Figure 5: Third-party JVM-targeted compilation for non-Java source compilation to Java byte-code. Illustrated example
shows Python source being compiled to both Python compiled code and Java byte-code.
Third-party JVM-targeted Compilation.svg


Of late, JVM-targeted third-party programming and scripting languages have seen tremendous growth. Some of these languages are also used to extend the functionalities of the Java language itself. A few examples include the following:


Getting started

Getting started

Understanding systems

We conceptualize the world around us in terms of systems. A system is a web of interconnected objects working together in tandem. In the systems theory, a system is set out as a single entity within a world surrounded by an environment. A system interacts with its surrounding environment using messages of two distinct types:

  • inputs: messages received from the surrounding environment; and,
  • outputs: messages given back to the surrounding environment.


Figure 1: A simple system interacting with its environment using input and output messages.
Understanding Systems.svg


Life is a complicated mess of interconnected objects sending signals and messages. See the illustration below in figure 2 demonstrating a complex system for an economic ecosphere for a single company. Imagine what this system diagram would be like if you were to add a few more companies and their sub-systems. Computer software systems in general are a complex web of further interconnected sub-systems – where each sub-systems may or may not be divided into further sub-systems. Each sub-system communicates with others using feedback messages – that is, inputs and outputs.


Figure 2: Example of a complex system with multiple sub-systems and interactions
Complex System.svg


The process of abstraction

Programming is essentially thinking of solutions to problems in real life as a system. With any programming language, you need to know how to address real-life problems into something that could be accurately represented within a computer system. In order to begin programming with the Java programming language (or in fact, with any programming language), a programmer must first understand the basics of abstraction.

Abstraction is the process of representing real-life problems and object into your programs.

Suppose a novelist, a painter and a programmer were asked to abstract (i.e., represent) a real-life object in their work. Suppose, the real-life object that needs to be abstracted is an animal. Abstraction for a novelist would include writing the description of the animal whilst the painter would draw a picture of the animal – but what about a computer programmer?

The Java programming language uses a programming paradigm called object-oriented programming (OOP), which shows you exactly what a programmer needs to be doing. According to OOP, every object or problem in real-life can be translated into a virtual object within your computer system.

Thinking in objects

In OOP, every abstraction of a real-life object is simply called an object within your code. An object is essentially the most basic representation of a real-life object as part of a computer system. With Java being an object-oriented language, everything within Java is represented as an object. To demonstrate this effect, if you were to define an abstraction of an animal in your code, you would write the following lines of code (as you would for any other abstraction):


Computer code
class Animal { }


The code above creates a space within your code where you can start defining an object; this space is called a class (or type) definition. All objects need to be defined using a class definition in order for them to be used in your program. Notice the curly brackets – anything you write within these brackets would serve as a definition or specification for your object. In the case of the example above, we created a class definition called Animal for objects that could serve as an abstract representation of any animal in real-life. The way that a Java environment evaluates this code to be a class definition is by looking at the prefix word we used to begin our class definition (i.e., class). Such predefined words in the Java language are known as keywords and make up the grammar for the language (known as programming syntax).

Note:
Class definitions have different names in different languages. They are sometimes called type definitions, object specifications or templates as well

Understanding class definitions and types

Aristotle was perhaps the first person to think of abstract types or typologies of objects. He started calling them classes – e.g., classes of birds, classes of mammals. Class definitions therefore serve the purpose well in defining the common characteristics or types of objects you would be creating. Upon declaring a class definition, you can create objects based on that definition. In order to do so however, you need to write a special syntax that goes like this:

Computer code
Animal dog = new Animal();


The code above effectively creates an object called dog based on the class definition for Animal. In non-programmer parlance, the code above would translate into something akin to saying, "Create a new object dog of type Animal." A single class definition enables you to create multiple objects as the code below indicates:

Computer code
Animal dog = new Animal();
Animal cat = new Animal();
Animal camel = new Animal();


Basically, you just have to write the code for your class or type definition once, and then use it to create countless numbers of objects based on that specification. Although you might not grasp the importance of doing so, this little exercise saves you a lot of time (a luxury that was not readily available to programmers in the pre-Java days).

Expanding your class definitions

Although each object you create from a class definition is essentially the same, there has to be a way of differentiating those objects in your code. Object fields (or simply fields) are what makes your objects unique from other objects. Let's take our present abstraction for instance. An animal could be a dog, cat, camel or a duck but since this abstraction is of a very generic kind, you need to define fields that are common to all of these animals and yet makes the animals stand apart. For instance, you can have two fields: name (a common name given to any one of these animals) and legs (the number of limbs any one of these animals would require to walk). As you start defining your objects, they start to look like this:


Computer code
class Animal {

  String name;
  int legs;
}


In the code above you defined two object fields:

  • a field called name of type String; and,
  • a field called legs of type int.

These special pre-defined types are called data types. The String data type is used for fields that can hold textual values like names, while the int (integer) data type is used for fields that can hold numeric values

Note:
Fields are called different things in different languages. They may be called state identifiers, properties or member variables in other programming language syntax. Java uses the words fields and properties in different contexts, as would be understood from upcoming sections.

Figure 3: In order to denote the Animal object as a system within the Java Environment,
you present it as such. Note how fields are presented.
Object in Java Environment.svg


In order to demonstrate how fields work, we will go ahead and create objects from this amended version of our class definition as such:

Computer code
Animal animal1 = new Animal();
Animal animal2 = new Animal();

animal1.name = "dog";
animal1.legs = 4;

animal2.name = "duck";
animal2.legs = 2;


You can access the fields of your created objects by using the . (dot) or membership operator. In the example above, we created two objects: animal1 and animal2 of type Animal. And since, we had established that each Animal has two fields namely name and legs, we accessed and modified these fields for each of our objects using the membership operator to set the two apart. By declaring different values for different objects, we can manipulate their current state. So, for instance:

  • the animal1 object is a "dog" with 4 legs to walk with; while,
  • the animal2 object is a "duck" with 2 legs to walk with.

What sets the two objects apart is their current state. Both the objects have different states and thus stand out as two different objects even though they were created from the same template or class definition.

Adding behavior to objects

At this point, your objects do nothing more than declare a bunch of fields. Being a system, your objects should have the ability to interact with its environment and other systems as well. To add this capability for interaction, you need to add interactive behavior to your object class definitions as well. Such behavior is added to class definitions using a programming construct called method.

In the case of the Animal, you require your virtual representation of an animal to be able to move through its environment. Let's say, as an analogy, you want your Animal object to be able to walk in its environment. Thus, you need to add a method named walk to our object. To do so, we need to write the following code:


Computer code
class Animal {

  String name;
  int legs;

  void walk() { }
}


As you write this code, one thing becomes immediately apparent. Just like the class description, a method has curly brackets as well. Generally, curly brackets are used to define an area (or scope) within your object. So the first set of curly brackets defined a scope for your class definition called the class-level scope. This new set of curly brackets alongside a method defines a scope for the further definition of your method called the method-level scope.

In this instance, the name of our method is walk. Notice however that the name of our method also features a set of round brackets as well. More than just being visual identifiers for methods, these round brackets are used to provide our methods with additional input information called arguments.

A method therefore enables an object to:

  1. Accept input: Receive some argument(s);
  2. Process information: work on the received argument(s) within its curly brackets; and,
  3. Generate ouput: occasionally, return something back.

In essence, methods are what makes an object behave more like a system.

Notice the keyword void before the name of the method – this tells us that the method walk returns nothing. You can set a method to return any data type – it can be a String or an int as well.

Note:
Methods are known by different names in different programming language. They might be called functions, procedures, routines or behaviors.

Figure 4: The Animal object can now be denoted as having an interaction behavior within the Java Environment
as illustrated here. Note the difference between the presentation of fields and methods.
Object Behavior in Java Environment.svg


The process of encapsulation

By now, we thoroughly understand that any object can interact with its environment and in turn be influenced by it. In our example, the Animal object exposed certain fields – name and legs, and a method – walk() to be used by the environment to manipulate the object. This form of exposure is implicit. Using the Java programming language, a programmer has the power to define the level of access other objects and the environment have on a certain object.

Using access modifiers

Alongside declaring and defining objects, their fields and methods, a programmer also has the ability to define the levels of access on those elements. This is done using keywords known as access modifiers.

Let's modify our example to demonstrate this effect:

Computer code
class Animal {

  public String name;
  public int legs;

  public void walk() { }
}


By declaring all fields and methods public, we have ensured that they can be used outside the scope of the Animal class. This means that any other object (other than Animal) has access to these member elements. However, to restrict access to certain member elements of a class, we can always use the private access modifier (as demonstrated below).

Computer code
class Animal {

  private String name;
  private int legs;

  public void walk() { }
}


In this example, the fields name and legs can only be accessed within the scope of the Animal class. No object outside the scope of this class can access these two fields. However, since the walk() method still has public access, it can be manipulated by actors and objects outside the scope of this class. Access modifiers are not just limited to fields or methods, they can be used for class definitions as well (as is demonstrated below).

Computer code
public class Animal {

  private String name;
  private int legs;

  public void walk() { }
}


The following list of keywords show the valid access modifiers that can be used with a Java program:

keyword description
public Opens access to a certain field or method to be used outside the scope of the class.
private Restricts access to a certain field or method to only be used within the scope of the class.
protected Access to certain field or methods is reserved for classes that inherit the current class.
More on this would be discussed in the section on inheritance.



Installation

In order to make use of the content in this book, you would need to follow along each and every tutorial rather than simply reading through the book. But to do so, you would need access to a computer with the Java platform installed on it — the Java platform is the basic prerequisite for running and developing Java code, thus it is divided into two essential software:

  • the Java Runtime Environment (JRE), which is needed to run Java applications and applets; and,
  • the Java Development Kit (JDK), which is needed to develop those Java applications and applets.

However as a developer, you would only require the JDK which comes equipped with a JRE as well. Given below are installation instruction for the JDK for various operating systems:

Installation instructions for Windows

Availability check for JRE

The Java Runtime Environment (JRE) is necessary to execute Java programs. To check which version of Java Runtime Environment (JRE) you have, follow the steps below.

1. For Window Vista or Windows 7, click StartControl PanelSystem and MaintenanceSystem.
For Windows XP, click StartControl PanelSystem.
For Windows 2000, click StartSettingsControl PanelSystem.
Alternatively, you can also press Win+R to open the Run dialog. With the dialog open, type cmd at the prompt:
Figure 1.1: Run dialog
2. In the command window with black background graced with white text, type the following command:
Standard input or output JRE availability check
java -version

If you get an error, such as:

Standard input or output Other output error
Bad command or file name

..then the JDK may not be installed or it may not be in your path.

To learn more about the Command Prompt syntax, take a look at this MS-DOS tutorial.

You may have other versions of Java installed; this command will only show the first in your PATH. You will be made familiar with the PATH environment variable later in this text. For now, if you have no idea what this is all about. Read through towards the end and we will provide you with a step-by-step guide on how to set your own environment variables.

You can use your system's file search utilities to see if there is a javac.exe executable installed. If it is, and it is a recent enough version (Java 1.4.2 or Java 1.5, for example), you should put the bin directory that contains javac in your system path. The Java runtime, java, is often in the same bin directory.

If the installed version is older (i.e. it is Java 1.3.1 or Java 1.4.2 and you wish to use the more recent Java 5 release), you should proceed below with downloading and installing a JDK.

It is possible that you have the Java runtime (JRE), but not the JDK. In that case the javac program won't be found, but the java -version will print the JRE version number.

Availability check for JDK

Some Windows based systems come built-in with the JRE, however for the purposes of writing Java code by following the tutorials in this book, you would require the JDK nevertheless. The Java Development Kit (JDK) is necessary to build Java programs. First, check to see if a JDK is already installed on your system. To do so, first open a command window and execute the command below.

Standard input or output Availability check
javac -version

If the JDK is installed and on your executable path, you should see some output which tells you the command line options. The output will vary depending on which version is installed and which vendor provided the Java installation.

Advanced availability check options on Windows platform

On a machine using the Windows operating system, one can invoke the Registry Editor utility by typing REGEDIT in the Run dialog. In the window that opens subsequently, if you traverse through the hierarchy HKEY_LOCAL_MACHINE > SOFTWARE > JavaSoft > Java Development Kit on the left-hand.

The resultant would be similar to figure 1.2, with the only exception being the version entries for the Java Development Kit. At the time of writing this manuscript, the latest version for the Java Development Kit available from the Internet was 1.7 as seen in the Registry entry. If you see a resultant window that resembles the one presented above, it would prove that you have Java installed on your system, otherwise it is not.

Figure 1.2: Registry Editor
Note Caution should be exercised when traversing through the Registry Editor. Any changes to the keys and other entries may change the way your Windows operating system normally works. Even minor changes may result into catastrophic failures of the normal working of your machine. Better that you don't modify or tend to modify anything whilst you are in the Registry Editor.

Download instructions

To acquire the latest JDK (version 7), you can manually download the Java software from the Oracle website.

For the convenience of our readers, the following table presents direct links to the latest JDK for the Windows operating system.

Operating system Setup Installer License
Windows x86 Download Oracle Binary Code License Agreement
Windows x64 Download Oracle Binary Code License Agreement

You must follow the instructions for the setup installer wizard step-by-step with the default settings to ensure that Java is properly installed on your system. Once the setup is completed, it is highly recommended to restart your Windows operating system.

If you kept the default settings for the setup installer wizard, your JDK should now be installed at C:\Program Files\Java\jdk1.7.0_01. You would require the location to your bin folder at a later time — this is located at C:\Program Files\Java\jdk1.7.0_01\bin It may be a hidden file, but no matter. Just don't use Program Files (x86)\ by mistake unless that's were installed Java.

Updating environment variables

In order for you to start using the JDK compiler utility with the Command Prompt, you would need to set the environment variables that points to the bin folder of your recently installed JDK. To set permanently your environment variables, follow the steps below.

1. Type the following command in the command/terminal window:
Standard input or output System properties
rundll32 shell32.dll,Control_RunDLL sysdm.cpltief
2. Navigate to the Advanced tab on the top, and select Environment Variables...
3. Under System variables, select the variable named Path and click Edit...
4. In the Edit System Variable dialog, go to the Variable value field. This field is a list of directory paths separated by semi-colons (;).
5. To add a new path, append the location of your JDK bin folder separated by a semi-colon (;).
6. Click OK on every opened dialog to save changes and get past to where you started.

Start writing code

Once you have successfully installed the JDK on your system, you are ready to program code in the Java programming language. However, to write code, you would need a decent text editor. Windows comes with a default text editor by default — Notepad. In order to use notepad to write code in Java, you need to follow the steps below:

1. Click StartAll ProgramsAccessoriesNotepad to invoke the application.
Alternatively, you can also press Win+R to open the Run dialog. With the dialog open, type the following command at the prompt:
Standard input or output Notepad launching
notepad
2. Once the Notepad application has fired up, you can use the editor to write code for the Java programming language.
Installation instructions for GNU/Linux

Availability check for JRE

The Java Runtime Environment (JRE) is necessary to execute Java programs. To check which version of JRE you have, follow the steps below.

1. Open the Terminal window.
2. Type the following command:
Standard input or output JRE availability check
java -version

If you get something like this:

Standard input or output Output on a particular Kubuntu 12.10 installation
java version "1.7.0_09"
OpenJDK Runtime Environment (IcedTea7 2.3.3) (7u9-2.3.3-0ubuntu1~12.10.1)
OpenJDK Client VM (build 23.2-b09, mixed mode, sharing)

... then a JRE is installed. If you get an error, such as:

Standard input or output Output error
java: command not found

... then the JDK may not be installed or it may not be in your path.

You may have other versions of Java installed; this command will only show the first in your PATH. You will be made familiar with the PATH environment variable later in this text. For now, if you have no idea what this is all about, read through towards the end and we will provide you with a step-by-step guide on how to set your own environment variables.

You can use your system's file search utilities to see if there is a javac executable installed. If it is, and it is a recent enough version, you should put the bin directory that contains javac in your system path. The Java runtime, java, is often in the same bin directory.

If the installed version is older (i.e. it is Java 5 and you wish to use the more recent Java 7 release), you should proceed below with downloading and installing a JDK.

It is possible that you have the Java runtime (JRE), but not the JDK. In that case the javac program won't be found, but the java -version will print the JRE version number.

Availability check for JDK

The Java Development Kit (JDK) is necessary to build Java programs. For our purposes, you must use a JDK. First, check to see if a JDK is already installed on your system. To do so, first open a terminal window and execute the command below.

Standard input or output Availability check
javac -version

If the JDK is installed and on your executable path, you should see some output which tells you the command line options. The output will vary depending on which version is installed and which vendor provided the Java installation.

Installation using Terminal

Downloading and installing the Java platform on Linux machines (in particular Ubuntu Linux) is very easy and straight-forward. To use the terminal to download and install the Java platform, follow the instructions below.

1. Open the Terminal window.
2. At the prompt, write the following (your package manager may be other than APT, so change the command accordingly):
Standard input or output Retrieving the java packages
$ sudo apt-get install openjdk-7-jdk openjdk-7-doc
3. All Java softwares should be installed and instantly available now.

Download instructions

Alternatively, you can manually download the Java software from the Oracle website.

For the convenience of our readers, the following table presents direct links to the latest JDK for the Linux operating system.

Operating system RPM Tarball License
Linux x86 Download Download Oracle Binary Code License Agreement
Linux x64 Download Download Oracle Binary Code License Agreement

Start writing code

The most widely available text editor on GNOME desktops is Gedit, while on the KDE desktops, one can find Kate. Both these editors support syntax highlighting and code completion and therefore are sufficient for our purposes.

However, if you require a robust and standalone text-editor like the Notepad++ editor on Windows, you would require the use of the minimalistic editor loaded with features – SciTE. Follow the instructions below if you wish to install SciTE:

1. Open the Terminal window.
2. At the prompt, write the following:
Standard input or output Retrieving the java packages
$ sudo apt-get install scite
3. You should now be able to use SciTE for your programming needs. You may also want to try Geany. Installation instructions are similar to those for SciTE.
Installation instructions for Mac OS

On Mac OS, both the JRE and the JDK are already installed. However, the version installed was the latest version when the computer was purchased, so you may want to update it.

Updating Java for Mac OS

  1. Go to the Java download page.
  2. Mechanically accept Oracle's license agreement.
  3. Click on the link for Mac OS X.
  4. Run the installer package.

Availability check for JDK

The Java Development Kit (JDK) is necessary to build Java programs. For our purposes, you must use a JDK. First, check to see if a JDK is already installed on your system. To do so, first open a terminal window and execute the command below.

Standard input or output Availability check
java -version

If the JDK is installed and on your executable path, you should see some output which tells you the command line options. The output will vary depending on which version is installed and which vendor provided the Java installation.

Installation instructions for Solaris
Clipboard

To do:
Add a section describing the installation of Java onto Solaris machines. Incorporate some of the information provided in the commented section of this page.

No Install Option for Programming Online

If you already have the JRE installed, you can use the Java Wiki Integrated Development Environment (JavaWIDE) to code directly in your browser, no account or special software required.

Click here to visit the JavaWIDE Sandbox to get started.

For more information, click here to visit the JavaWIDE site.



Compilation

In Java, programs are not compiled into executable files; they are compiled into Bytecode (as discussed earlier), which the JVM then executes at runtime. Java source code is compiled into bytecode when we use the javac compiler. The bytecode gets saved on the disk with the file extension .class. When the program is to be run, the bytecode is converted, using the Just-In-Time (JIT) compiler. The result is machine code which is then fed to the memory and is executed.

So Java has two step compilation:

  1. To create byte-code
  2. To create machine level code

The Java classes/Byte Codes are compiled to machine code and loaded into memory by the JVM when needed the first time. This is different than other languages like C/C++ where the whole program had to be compiled to machine code and linked to create an executable file, before the program could start.

Quick compilation procedure

To execute your first Java program, follow the instructions below:

1. Proceed only if you have successfully installed and configured your system for Java as discussed here.
2. Open your preferred text editor — this is the editor you set while installing the Java platform.
For example, Notepad or Notepad++ on Windows; Gedit, Kate or SciTE on Linux; or, XCode on Mac OS, etc.
3. Write the following lines of code in a new text document:
Computer code Code listing 2.5: HelloWorld.java
public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello World!");
  }
}
4. Save the file as HelloWorld.java — the name of your file should be the same as the name of your class definition and followed by the .java extension. This name is case-sensitive, which means you need to capitalize the precise letters that were capitalized in the name for the class definition.
5. Next, open your preferred command-line application.
For example, Command Prompt on Windows; and, Terminal on Linux and Mac OS.
6. In your command-line application, navigate to the directory where you just created your file. If you do not know how to do this, consider reading through our crash courses for command-line applications for Windows or Linux.
7. Compile the Java source file using the following command which you can copy and paste in if you want:
Computer code Compilation
javac HelloWorld.java
Warning If you obtain an error message like error: cannot read: HelloWorld.java 1 error, your file is not in the current folder or it is badly spelled. Did you navigate to the program's location in the command prompt using the CD (Change Directory) function?

If you obtain another message ending by 1 error or ... errors, your file may be badly written. Are you sure all words are spelled correctly and with the exact case as shown? Are there semicolons and brackets in the appropriate spot? Are you missing a quote? Usually, modern IDEs would try coloring the entire source as a quote in this case.

If your computer emits beeps, then you may have illegal characters in your HelloWorld.java.

If no HelloWorld.class file has been created in the same folder, then you've got an error. Are you launching the javac program correctly?

8. Once the compiler returns back to the prompt, run the application using the following command:
Computer code Execution
java HelloWorld
Warning If you obtain an error message like Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld, the HelloWorld.class file is not in the current folder or it is badly spelled.

If you obtain an error message like Exception in thread "main" java.lang.NoSuchMethodError: main, your source file may have been badly written.

9. The above command should result in your command-line application displaying the following result:
Computer code Output
Hello World!
Ask for help if the program did not execute properly in the Discussion page for this chapter.

Automatic Compilation of Dependent Classes

In Java, if you have used any reference to any other java object, then the class for that object will be automatically compiled, if that was not compiled already. These automatic compilations are nested, and this continues until all classes are compiled that are needed to run the program. So it is usually enough to compile only the high level class, since all the dependent classes will be automatically compiled.

Computer code Main class compilation
javac ... MainClass.java

However, you can't rely on this feature if your program is using reflection to create objects, or you are compiling for servlets or for a "jar", package. In these cases you should list these classes for explicit compilation.

Computer code Main class compilation
javac ... MainClass.java ServletOne.java ...

Packages, Subdirectories, and Resources

Each Java top level class belongs to a package (covered in the chapter about Packages). This may be declared in a package statement at the beginning of the file; if that is missing, the class belongs to the unnamed package.

For compilation, the file must be in the right directory structure. A file containing a class in the unnamed package must be in the current/root directory; if the class belongs to a package, it must be in a directory with the same name as the package.

The convention is that package names and directory names corresponding to the package consist of only lower case letters.

Top level package

A class with this package declaration

Example Code section 2.1: Package declaration
package example;

has to be in a directory named example

Subpackages

A class with this package declaration

Example Code section 2.2: Package declaration with sub-packages
package org.wikibooks.en;

has to be in a directory named en which has to be a sub-directory of wikibooks which in turn has to be a sub-directory of org resulting in org/wikibooks/en.

Java programs often contain non-code files such as images and properties files. These are referred to generally as resources and stored in directories local to the classes in which they're used. For example, if the class com.example.ExampleApp uses the icon.png file, this file could be stored as /com/example/resources/icon.png. These resources present a problem when a program is complied, because javac does not copy them to wherever the .class files are being complied to (see above); it is up to the programmer to move the resource files and directories.

Filename Case

The Java source file name must be the same as the public class name, the file contains. There can be only one public class defined per file. The Java class name is case sensitive, as is the source file name.

The naming convention for the class name is for it to start with a capital letter.

Compiler Options

Debugging and Symbolic Information

Ant

For comprehensive information about all aspects of Ant, please see the Ant Wikibook.

The best way to build your application is to use a build tool. The build tool would check all the needed dependencies and compile only the needed class for the build. The Ant tool is one of the best and the most popular build tools currently available. Ant is a build management tool designed to replace MAKE as the tool for automated builds of large Java applications. Like Java, and unlike MAKE, Ant is designed to be platform independent.

Using Ant you would build your application from the command line by typing:

Computer code Ant building
ant build.xml

The build.xml file contains all the information needed to build the application.

Building a Java application requires certain tasks to be performed defined in a build.xml file. Those tasks may include not only compiling the code, but also copying code, packaging the program to Jar, creating EJBs, to running automated tests, doing ftp for the code to remote site, and so on. For some tasks a condition can be assigned, for example a compile only changed code, or do the task if that was not already done so. Tasks dependency can also be specified, that will make sure that the order of executions of the tasks are in the right order. For example, compile the code before package them to jar, the package-to-jar task depend on the compilation task.

Note In rare cases, your code may appear to compile correctly but the program behaves as if you were using an old copy of the source code (or otherwise reports errors during runtime.) When this occurs, you may need to clean your compilation folder by either deleting the class files or using the Clean command from an IDE.

The build.xml file is generally kept in the root directory of the java project. Ant parses this file and executes the tasks therein. Below we give an example build.xml file.

Ant tool is written in Java and is open source, so it can be extended if there is a task you'd like to be done during the build that is not in the pre-defined tasks list. It is very easy to hook your ant task code to the other tasks: your code only needs to be in the classpath, and the Ant tool will load it at runtime. For more information about writing your own Ant tasks, please see the project website at http://ant.apache.org/.

Example build.xml file.

The next most popular way to build applications is using an Integrated Development Environment (IDE).

The JIT compiler

The Just-In-Time (JIT) compiler is the compiler that converts the byte-code to machine code. It compiles byte-code once and the compiled machine code are re-used again and again, to speed up execution. Early Java compilers compiled the byte-code to machine code each time it was used, but more modern compilers cache this machine code for reuse on the machine. Even then, java's JIT compiling was still faster than an "interpreter-language", where code is compiled from high level language, instead of from byte-code each time it was used.

The standard JIT compiler runs on demand. When a method is called repeatedly, the JIT compiler analyzes the bytecode and produces highly efficient machine code, which runs very fast. The JIT compiler is smart enough to recognize when the code has already been compiled, so as the application runs, compilation happens only as needed. As Java applications run, they tend to become faster and faster, because the JIT can perform runtime profiling and optimization to the code to meet the execution environment. Methods or code blocks which do not run often receive less optimization; those which run often (so called hotspots) receive more profiling and optimization.



Execution

There are various ways in which Java code can be executed. A complex Java application usually uses third party APIs or services. In this section we list the most popular ways a piece of Java code may be packed together and/or executed.

JSE code execution

Java language first edition came out in the client-server era. Thick clients were developed with rich GUI interfaces. Java first edition, JSE (Java Standard Edition) had/has the following in its belt:

  • GUI capabilities (AWT, Swing)
  • Network computing capabilities (RMI)
  • Multi-tasking capabilities (Threads)

With JSE the following Java code executions are possible:

Figure 1: Stand alone execution
Stand alone Java application 
(Figure 1) Stand alone application refers to a Java program where both the user interface and business modules are running on the same computer. The application may or may not use a database to persist data. The user interface could be either AWT or Swing.
The application would start with a main() method of a Class. The application stops when the main() method exits, or if an exception is thrown from the application to the JVM. Classes are loaded to memory and compiled as needed, either from the file system or from a *.jar file, by the JVM.
Invocation of Java programs distributed in this manner requires usage of the command line. Once the user has all the class files, he needs to launch the application by the following command line (where Main is the name of the class containing the main() method.)
Computer code Execution of class
java Main
Java 'jar' class libraries 
Utility classes, framework classes, and/or third party classes are usually packaged and distributed in Java ' *.jar' files. These 'jar' files need to be put in the CLASSPATH of the java program from which these classes are going to be used.
If a jar file is executable, it can be run from the command line:
Computer code Execution of archive
java -jar Application.jar
Figure 2: Applet Execution
Java Applet code 
(Figure 2) Java Applets are Java code referenced from HTML pages, by the <APPLET> tag. The Java code is downloaded from a server and runs in the client browser JVM. Java has built-in support to render applets in the browser window.
Sophisticated GUI clients were found hard to develop, mostly because of download time, incompatibilities between browser JVM implementations, and communication requirements back to the server. Applets are rarely used today, and are most commonly used as small, separate graphic-like animation applets. The popularity of Java declined when Microsoft withdrew its Java support from Internet Explorer default configuration, however, the plugin is still available as a free download from java.com.
More information can be found about applets at the Applet Chapter, in this book. Also, Wikipedia has an article about Java Applets.
Client Server applications 
The client server applications consist of a front-end, and a back-end part, both running on a separate computer. The idea is that the business logic would be on the back-end part of the program, which would be reused by all the clients. Here the challenge is to achieve a separation between front-end user interface code, and the back-end business logic code.
The communication between the front-end and the back-end can be achieved by two ways.
  • One way is to define a data communication protocol between the two tiers. The back-end part would listen for an incoming request. Based on the protocol it interprets the request and sends back the result in data form.
  • The other way is to use Java Remote Invocation (RMI). With the use of RMI, a remote object can be created and used by the client. In this case Java objects are transmitted across the network.
More information can be found about client-server programming, with sample code, at the Client Server Chapter in this book.
Web Applications 
For applications needed by lots of client installations, the client-server model did not work. Maintaining and upgrading the hundreds or thousands of clients caused a problem. It was not practical. The solution to this problem was to create a unified, standard client, for all applications, and that is the Browser.
Having a standard client, it makes sense to create a unified, standard back-end service as well, and that is the Application Server.
Web Application is an application that is running in the Application Server, and it can be accessed and used by the Browser client.
There are three main area of interest in Web Applications, those are:
Wikipedia also has an article about Web application.

J2EE code execution

As the focus was shifting from reaching GUI clients to thin client applications, with Java version 2, Sun introduced J2EE (Java 2 Extended Edition). J2EE added :

With J2EE the following Java component executions are possible:

Figure 3: Servlet Execution
Java Servlet code 
(Figure 3) Java got its popularity with server side programming, more specifically with J2EE servlets. Servlets are running in a simple J2EE framework to handle client HTTP requests. They are meant to replace CGI programming for web pages rendering dynamic content.
The servlet is running in a so called servlet-container/web container. The servlet's responsibility is to:
  • Handle the request by doing the business logic computation,
  • Connecting to a database if needed,
  • Create HTML to present to the user through the browser
The HTML output represents both the presention logic and the results of the business computations. This represents a huge problem, and there is no real application relying only on servlets to handle the presention part of the responsibility. There are two main solutions to this:
  • Use a template tool (Store the presentation part in an HTML file, marking the areas that need to be replaced after business logic computations).
  • Use JSP (See next section)
Wikipedia also has an article about Servlets.
Figure 4: Jsp Execution
Java Server Pages (JSP) code 
(Figure 4) JSP is an HTML file with embedded Java code inside. The first time the JSP is accessed, the JSP is converted to a Java Servlet. This servlet outputs HTML which has inside the result of the business logic computation. There are special JSP tags that helps to add data dynamically to the HTML. Also JSP technology allows to create custom tags.
Using the JSP technology correctly, business logic computations should not be in the embedded Java part of the JSP. JSP should be used to render the presentation of the static and dynamic data. Depending on the complexity of the data, 100% separation is not easy to achieve. Using custom tags, however may help to get closer to 100%. This is advocated also in MVC architecture (see below).
Figure 5: EJB Execution
EJB code 
(Figure 5) In the 1990s, with the client server computing, a trend started, that is to move away from Mainframe computing. That resulted in many small separate applications in a Company/Enterprise. Many times the same data was used in different applications. A new philosophy, "Enterprise Computing", was created to address these issues. The idea was to create components that can be reused throughout the Enterprise. The Enterprise Java Beans (EJBs) were supposed to address this.
An EJB is an application component that runs in an EJB container. The client accesses the EJB modules through the container, never directly. The container manages the life cycle of the EJB modules, and handles all the issues that arise from network/enterpise computing. Some of those are security/access control, object pooling, transaction management, ... .
EJBs have the same problems as any reusable code: they need to be generic enough to be able to be reused and the changes or maintenance of EJBs can affect existing clients. Many times EJBs are used unnecessarily when they are not really needed. An EJB should be designed as a separate application in the enterprise, fulfilling one function.


Figure 6: MVC Execution
Combine J2EE components to create an MVC architecture 
This leads us to the three layers/tiers as shown in (Figure 6).
In modern web applications, with lots of static data and nice graphics, how the data is presented to the user became very important and usually needs the help of a graphic artist.
To help programmers and graphic artists to work together, the separation between data, code, and how it is presented became crucial.
  • The view (User Interface Logic) contains the logic that is necessary to construct the presentation. This could be handled by JSP technology.
  • The servlet acts as the controller and contains the logic that is necessary to process user events and to select an appropriate response.
  • The business logic (model) actually accomplishes the goal of the interaction. This might be a query or an update to a database. This could be handled by EJB technology.
For more information about MVC, please see MVC.

Jini

After J2EE Sun had a vision about the next step of network computing. That is Jini. The main idea is that in a network environment, there would be many independent services and consumers. Jini would allow these services/consumers to interact dynamically with each other in a robust way. The basic features of Jini are:

  • No user intervention is needed when services are brought on or offline. (In contrast to EJBs where the client program has to know the server and port number where the EJB is deployed, in Jini the client is supposed to find, to discover, the service in the network.)
  • Self healing by adapting when services (consumers of services) come and go. (Services periodically need to renew a lease to indicate that they are still available.)
  • Consumers of JINI services do not need prior knowledge of the service's implementation. The implementation is downloaded dynamically and run on the consumer JVM, without configuration and user intervention. (For example, the end user may be presented with a slightly different user interface depending upon which service is being used at the time. The implementation of the user interface code would be provided by the service being used.)

A minimal Jini network environment consists of:

  • One or more services
  • A lookup-service keeping a list of registered services
  • One or more consumers

Jini is not widely used at the current writing (2006). There are two possible reasons for it. One is Jini a bit complicated to understand and to set it up. The other reason is that Microsoft pulled out from Java, which caused the industry to turn to the use of proprietary solutions.



Understanding a Java Program

This article presents a small Java program which can be run from the console. It computes the distance between two points on a plane. You do not need to understand the structure and meaning of the program just yet; we will get to that soon. Also, because the program is intended as a simple introduction, it has some room for improvement, and later in the module we will show some of these improvements. But let's not get too far ahead of ourselves!

The Distance Class: Intent, Source, and Use

This class is named Distance, so using your favorite editor or Java IDE, first create a file named Distance.java, then copy the source below and paste it into the file and save the file.

Computer code Code listing 2.1: Distance.java
  1.  public class Distance {
    
  2.    private java.awt.Point point0, point1;
    
  3.  
    
  4.    public Distance(int x0, int y0, int x1, int y1) {
    
  5.      point0 = new java.awt.Point(x0, y0);
    
  6.      point1 = new java.awt.Point(x1, y1);
    
  7.    }
    
  8.  
    
  9.    public void printDistance() {
    
  10.      System.out.println("Distance between " + point0 + " and " + point1
    
  11.                      + " is " + point0.distance(point1));
    
  12.    }
    
  13.  
    
  14.    public static void main(String[] args) {
    
  15.      Distance dist = new Distance(
    
  16.               intValue(args[0]), intValue(args[1]),
    
  17.               intValue(args[2]), intValue(args[3]));
    
  18.      dist.printDistance();
    
  19.    }
    
  20.  
    
  21.    private static int intValue(String data) {
    
  22.      return Integer.parseInt(data);
    
  23.    }
    
  24.  }
    

At this point, you may wish to review the source to see how much you might be able to understand. While perhaps not being the most literate of programming languages, someone with understanding of other procedural languages such as C, or other OO languages such as C++ or C#, will be able to understand most if not all of the sample program.

Once you save the file, compile the program:

Computer code Output for the compilation
$ javac Distance.java

(If the javac command fails, review the Java installation instructions.)

To run the program, you supply it with the x and y coordinates of two points on a plane separated by a space. For this version of Distance, only integer points are supported. The command sequence is java Distance <x0> <y0> <x1> <y1> to compute the distance between the points (x0, y0) and (x1, y1).

Warning If you have a NumberFormatException exception, some arguments are not a number. If you will have a java.lang.ArrayIndexOutOfBoundsException exception, there is not enough arguments.

Here are two examples:

Computer code Output for the distance between the points (0, 3) and (4, 0)
$ java Distance 0 3 4 0
Distance between java.awt.Point[x=0,y=3] and java.awt.Point[x=4,y=0] is 5.0
Computer code Output for the distance between the points (-4, 5) and (11, 19)
$ java Distance -4 5 11 19
Distance between java.awt.Point[x=-4,y=5] and java.awt.Point[x=11,y=19] is 20.518284528683193

We'll explain this strange looking output, and also show how to improve it, later.

Detailed Program Structure and Overview

As promised, we will now provide a detailed description of this Java program. We will discuss the syntax and structure of the program and the meaning of that structure.

Introduction to Java Syntax

public class Distance {
   private java.awt.Point point0, point1;

   public Distance(int x0, int y0, int x1, int y1) {
     point0 = new java.awt.Point(x0, y0);
     point1 = new java.awt.Point(x1, y1);
   }

   public void printDistance() {
     System.out.println("Distance between " + point0 + " and " + point1
                     + " is " + point0.distance(point1));
   }

   public static void main(String[] args) {
     Distance dist = new Distance(
              intValue(args[0]), intValue(args[1]),
              intValue(args[2]), intValue(args[3]));
     dist.printDistance();
   }

   private static int intValue(String data) {
     return Integer.parseInt(data);
   }
}

Figure 2.1: The different tokens of the Java syntax in the Distance class.

For a further treatment of the syntax elements of Java, see also Syntax.

The syntax of a Java class is the characters and symbols and their structure used to code the class using Unicode characters. Java programs consist of a sequence of tokens. There are different kinds of tokens. For example, there are word tokens such as class and public which represent keywords (in purple above) — special words with reserved meaning in Java. Other words such as Distance, point0, x1, and printDistance are not keywords but identifiers (in grey). Identifiers have many different uses in Java but primarily they are used as names. Java also has tokens to represent numbers, such as 1 and 3; these are known as literals (in orange). String literals (in blue), such as "Distance between ", consist of zero or more characters embedded in double quotes, and operators (in red) such as + and = are used to express basic computation such as addition or String concatenation or assignment. There are also left and right braces ({ and }) which enclose blocks. The body of a class is one such block. Some tokens are punctuation, such as periods . and commas , and semicolons ;. You use whitespace such as spaces, tabs, and newlines, to separate tokens. For example, whitespace is required between keywords and identifiers: publicstatic is a single identifier with twelve characters, not two Java keywords.

Declarations and Definitions

public class Distance {

  private java.awt.Point point0, point1;


  public Distance(int x0, int y0, int x1, int y1) {
    point0 = new java.awt.Point(x0, y0);
    point1 = new java.awt.Point(x1, y1);
  }


  public void printDistance() {
    System.out.println("Distance between " + point0 + " and " + point1
      + " is " + point0.distance(point1));
  }


  public static void main(String[] args) {
    Distance dist = new Distance(
      intValue(args[0]), intValue(args[1]),
      intValue(args[2]), intValue(args[3]));
    dist.printDistance();
  }


  private static int intValue(String data) {
    return Integer.parseInt(data);
  }


}

Figure 2.2: The different declarations and definitions of the Java syntax in the Distance class.

Sequences of tokens are used to construct the next building blocks of Java classes as shown above: declarations and definitions. A class declaration provides the name and visibility of a class. For our example, public class Distance is the class declaration. It consists (in this case) of two keywords, public and class followed by the identifier Distance.

This means that we are defining a class named Distance. Other classes, or in our case, the command line, can refer to the class by this name. The public keyword is an access modifier which declares that this class and its members may be accessed from other classes. The class keyword, obviously, identifies this declaration as a class. Java also allows declarations of interfaces and (as of Java 5) annotations.

The class declaration is then followed by a block (surrounded by curly braces) which provides the class's definition (in blue in figure 2.2). The definition is the implementation of the class — the declaration and definitions of the class's members. This class contains exactly six members, which we will explain in turn.

  1. Two field declarations, named point0 and point1 (in green)
  2. A constructor declaration (in orange)
  3. Three method declarations (in red)

Example: Instance Fields

The declaration

Example Code section 2.1: Declaration.
  1. private java.awt.Point point0, point1;
    

...declares two instance fields. Instance fields represent named values that are allocated whenever an instance of the class is constructed. When a Java program creates a Distance instance, that instance will contain space for point0 and point1. When another Distance object is created, it will contain space for its own point0 and point1 values. The value of point0 in the first Distance object can vary independently of the value of point0 in the second Distance object.

This declaration consists of:

  1. The private access modifier,
    which means these instance fields are not visible to other classes.
  2. The type of the instance fields. In this case, the type is java.awt.Point.
    This is the class Point in the java.awt package.
  3. The names of the instance fields in a comma separated list.


These two fields could also have been declared with two separate but more verbose declarations,

Example Code section 2.2: Verbose declarations.
  1.  private java.awt.Point point0;
    
  2.  private java.awt.Point point1;
    

Since the types of these fields is a reference type (i.e. a field that refers to or can hold a reference to an object value), Java will implicitly initialize the values of point0 and point1 to null when a Distance instance is created. The null value means that a reference value does not refer to an object. The special Java literal, null is used to represent the null value in a program. While you can explicitly assign null values in a declaration, as in

Example Code section 2.3: Declarations and assignments.
  1.  private java.awt.Point point0 = null;
    
  2.  private java.awt.Point point1 = null;
    

It is not necessary and most programmers omit such default assignments.

Example: Constructor

A constructor is a special method in a class which is used to construct an instance of the class. The constructor can perform initialization for the object, beyond that which the Java VM does automatically. For example, Java will automatically initialize the fields point0 and point1 to null.

Example Code section 2.4: The constructor for the class
  1. public Distance(int x0, int y0, int x1, int y1) {
    
  2.    point0 = new java.awt.Point(x0, y0);
    
  3.    point1 = new java.awt.Point(x1, y1);
    
  4. }
    

The constructor above consists of five parts:

  1. The optional access modifier(s).
    In this case, the constructor is declared public
  2. The constructor name, which must match the class name exactly: Distance in this case.
  3. The constructor parameters.
    The parameter list is required. Even if a constructor does not have any parameters, you must specify the empty list (). The parameter list declares the type and name of each of the method's parameters.
  4. An optional throws clause which declares the exceptions that the constructor may throw. This constructor does not declare any exceptions.
  5. The constructor body, which is a Java block (enclosed in {}). This constructor's body contains two statements.

This constructor accepts four parameters, named x0, y0, x1 and y1. Each parameter requires a parameter type declaration, which in this example is int for all four parameters. The parameters in the parameter list are separated by commas.

The two assignments in this constructor use Java's new operator to allocate two java.awt.Point objects. The first allocates an object representing the first point, (x0, y0), and assigns it to the point0 instance variable (replacing the null value that the instance variable was initialized to). The second statement allocates a second java.awt.Point instance with (x1, y1) and assigns it to the point1 instance variable.

This is the constructor for the Distance class. Distance implicitly extends from java.lang.Object. Java inserts a call to the super constructor as the first executable statement of the constructor if there is not one explicitly coded. The above constructor body is equivalent to the following body with the explicit super constructor call:

Example Code section 2.5: Super constructor.
  1. {
    
  2.    super();
    
  3.    point0 = new java.awt.Point(x0, y0);
    
  4.    point1 = new java.awt.Point(x1, y1);
    
  5. }
    

While it is true that this class could be implemented in other ways, such as simply storing the coordinates of the two points and computing the distance as \sqrt{(x_1-x_0)^2 + (y_1-y_0)^2}, this class instead uses the existing java.awt.Point class. This choice matches the abstract definition of this class: to print the distance between two points on the plane. We take advantage of existing behavior already implemented in the Java platform rather than implementing it again. We will see later how to make the program more flexible without adding much complexity, because we choose to use object abstractions here. However, the key point is that this class uses information hiding. That is, how the class stores its state or how it computes the distance is hidden. We can change this implementation without altering how clients use and invoke the class.

Example: Methods

Methods are the third and most important type of class member. This class contains three methods in which the behavior of the Distance class is defined: printDistance(), main(), and intValue()

The printDistance() method

The printDistance() method prints the distance between the two points to the standard output (normally the console).

Example Code section 2.6: printDistance() method.
  1. public void printDistance() {
    
  2.   System.out.println("Distance between " + point0
    
  3.     + " and " + point1
    
  4.     + " is " + point0.distance(point1));
    
  5. }
    

This instance method executes within the context of an implicit Distance object. The instance field references, point0 and point1, refer to instance fields of that implicit object. You can also use the special variable this to explicitly reference the current object. Within an instance method, Java binds the name this to the object on which the method is executing, and the type of this is that of the current class. The body of the printDistance method could also be coded as

Example Code section 2.7: Explicit instance of the current class.
  1.   System.out.println("Distance between " + this.point0
    
  2.     + " and " + this.point1
    
  3.     + " is " + this.point0.distance(this.point1));
    

to make the instance field references more explicit.

This method both computes the distance and prints it in one statement. The distance is computed with point0.distance(point1); distance() is an instance method of the java.awt.Point class (of which point0 and point1 are instances). The method operates on point0 (binding this to the object that point0 refers to during the execution of the method) and accepting another Point as a parameter. Actually, it is slightly more complicated than that, but we'll explain later. The result of the distance() method is a double precision floating point number.

This method uses the syntax

Example Code section 2.8: String concatenation.
  1.  "Distance between " + this.point0
    
  2.  + " and " + this.point1
    
  3.  + " is " + this.point0.distance(this.point1)
    

to construct a String to pass to the System.out.println(). This expression is a series of String concatenation methods which concatenates Strings or the String representation of primitive types (such as doubles) or objects, and returns a long string. For example, the result of this expression for the points (0,3) and (4,0) is the String

Computer code Output
"Distance between java.awt.Point[x=0,y=3] and java.awt.Point[x=4,y=0] is 5.0"

which the method then prints to System.out.

In order to print, we invoke the println(). This is an instance method from java.io.PrintStream, which is the type of the static field out in the class java.lang.System. The Java VM binds System.out to the standard output stream when it starts a program.

The main() method

The main() method is the main entry point which Java invokes when you start a Java program from the command line. The command

Computer code Output
java Distance 0 3 4 0

instructs Java to locate the Distance class, put the four command line arguments into an array of String values, then pass those arguments to the public static main(String[]) method of the class. We will introduce arrays shortly. Any Java class that you want to invoke from the command line or desktop shortcut must have a main method with this signature or the following signature: public static main(String...).

Example Code section 2.9: main() method.
  1. public static void main(String[] args) {
    
  2.    Distance dist = new Distance(
    
  3.          intValue(args[0]), intValue(args[1]),
    
  4.          intValue(args[2]), intValue(args[3]));
    
  5.    dist.printDistance();
    
  6. }
    

The main() method invokes the final method, intValue(), four times. The intValue() takes a single string parameter and returns the integer value represented in the string. For example, intValue("3") will return the integer 3.

People who do test-first programming or perform regression testing write a main() method in every Java class, and a main() function in every Python module, to run automated tests. When a person executes the file directly, the main() method executes and runs the automated tests for that file. When a person executes some other Java file that in turn imports many other Java classes, only one main() method is executed -- the main() method of the directly-executed file.

The intValue() method

The intValue() method delegates its job to the Integer.parseInt() method. The main method could have called Integer.parseInt() directly; the intValue() method simply makes the main() method slightly more readable.

Example Code section 2.10: intValue() method.
  1. private static int intValue(String data) {
    
  2.    return Integer.parseInt(data);
    
  3. }
    

This method is private since, like the fields point0 and point1, it is part of the internal implementation of the class and is not part of the external programming interface of the Distance class.

Static vs. Instance Methods

Both the main() and intValue() methods are static methods. The static keyword tells the compiler to create a single memory space associated with the class. Each individual object instantiated has its own private state variables and methods but use the same static methods and members common to the single class object created by the compiler when the first class object is instantiated or created. This means that the method executes in a static or non-object context — there is no implicit separate instance available when the static methods run from various objects, and the special variable this is not available. As such, static methods cannot access instance methods or instance fields (such as printDistance()) or point0) directly. The main() method can only invoke the instance method printDistance() method via an instance reference such as dist.

Data Types

Most declarations have a data type. Java has several categories of data types: reference types, primitive types, array types, and a special type, void.

Primitive Types

The primitive types are used to represent boolean, character, and numeric values. This program uses only one primitive type explicitly, int, which represents 32 bit signed integer values. The program also implicitly uses double, which is the return type of the distance() method of java.awt.Point. double values are 64 bit IEEE floating point values. The main() method uses integer values 0, 1, 2, and 3 to access elements of the command line arguments. The Distance() constructor's four parameters also have the type int. Also, the intValue() method has a return type of int. This means a call to that method, such as intValue(args[0]), is an expression of type int. This helps explain why the main method cannot call:

Warning Code section 2.11: Wrong type.
  1. new Distance(args[0], args[1], args[2], args[3]) // This is an error
    

Since the type of the args array element is String, and our constructor's parameters must be int, such a call would result in an error because Java cannot automatically convert values of type String into int values.

Java's primitive types are boolean, byte, char, short, int, long, float and double. Each of which are also Java language keywords.

Reference Types

In addition to primitive types, Java supports reference type. A reference type is a Java data type which is defined by a Java class or interface. Reference types derive this name because such values refer to an object or contain a reference to an object. The idea is similar to pointers in other languages like C.

Java represents sequences of character data, or String, with the reference type java.lang.String which is most commonly referred to as String. String literals, such as "Distance between " are constants whose type is String.

This program uses three separate reference types:

  1. java.lang.String (or simply String)
  2. Distance
  3. java.awt.Point
For more information see chapter: Java Programming/Classes, Objects and Types.

Array Types

Java supports arrays, which are aggregate types which have a fixed element type (which can be any Java type) and an integral size. This program uses only one array, String[] args. This indicates that args has an array type and that the element type is String. The Java VM constructs and initializes the array that is passed to the main method. See arrays for more details on how to create arrays and access their size.

The elements of arrays are accessed with integer indices. The first element of an array is always element 0. This program accesses the first four elements of the args array explicitly with the indices 0, 1, 2, and 3. This program does not perform any input validation, such as verifying that the user passed at least four arguments to the program. We will fix that later.

void

void is not a type in Java; it represents the absence of a type. Methods which do not return values are declared as void methods.

This class defines two void methods:

Example Code section 2.12: Void methods
  1. public static void main(String[] args) { ... }
    
  2. public void printDistance()  { ... }
    

Whitespace

Whitespace in Java is used to separate the tokens in a Java source file. Whitespace is required in some places, such as between access modifiers, type names and Identifiers, and is used to improve readability elsewhere.

Wherever whitespace is required in Java, one or more whitespace characters may be used. Wherever whitespace is optional in Java, zero or more whitespace characters may be used.

Java whitespace consists of the

  • space character ' ' (0x20),
  • the tab character (hex 0x09),
  • the form feed character (hex 0x0c),
  • the line separators characters newline (hex 0x0a) or carriage return (hex 0x0d) characters.

Line separators are special whitespace characters in that they also terminate line comments, whereas normal whitespace does not.

Other Unicode space characters, including vertical tab, are not allowed as whitespace in Java.

Required Whitespace

Look at the static method intValue:

Example Code section 2.13: Method declaration
  1. private static int intValue(String data) {
    
  2.   return Integer.parseInt(data);
    
  3. }
    

Whitespace is required between private and static, between static and int, between int and intValue, and between String and data.

If the code is written like this:

Example Code section 2.14: Collapsed code
  1. privatestaticint intValue(String data) {
    
  2.   return Integer.parseInt(data);
    
  3. }
    

...it means something completely different: it declares a method which has the return type privatestaticint It is unlikely that this type exists and the method is no longer static, so the above would result in a semantic error.

Indentation

Java ignores all whitespace in front of a statement. As this, these two code snippets are identical for the compiler:

Example Code section 2.15: Indented code
  1. public static void main(String[] args) {
    
  2.   Distance dist = new Distance(
    
  3.            intValue(args[0]), intValue(args[1]),
    
  4.            intValue(args[2]), intValue(args[3]));
    
  5.   dist.printDistance();
    
  6. }
    
  7.  
    
  8. private static int intValue(String data) {
    
  9.   return Integer.parseInt(data);
    
  10. }
    
Example Code section 2.16: Not indented code
  1. public static void main(String[] args) {
    
  2. Distance dist = new Distance(
    
  3. intValue(args[0]), intValue(args[1]),
    
  4. intValue(args[2]), intValue(args[3]));
    
  5. dist.printDistance();
    
  6. }
    
  7.  
    
  8. private static int intValue(String data) {
    
  9. return Integer.parseInt(data);
    
  10. }
    

However, the first one's style (with whitespace) is preferred, as the readability is higher. The method body is easier to distinguish from the head, even at a higher reading speed.



Java IDEs

What is a Java IDE?

A Java IDE (Integrated Development Environment) is a software application which enables users to more easily write and debug Java programs. Many IDEs provide features like syntax highlighting and code completion, which help the user to code more easily.

Eclipse

Eclipse on Ubuntu

Eclipse is a Free and Open Source IDE, plus a developer tool framework that can be extended for a particular development need. IBM was behind its development, and it replaced IBM VisualAge tool. The idea was to create a standard look and feel that can be extended via plugins. The extensibility distinguishes Eclipse from other IDEs. Eclipse was also meant to compete with Microsoft Visual Studio tools. Microsoft tools give a standard way of developing code in the Microsoft world. Eclipse gives a similar standard way of developing code in the Java world, with a big success so far. With the online error checking only, coding can be sped up by at least 50% (coding does not include programming).

The goals for Eclipse are twofold:

  1. Give a standard IDE for developing code
  2. Give a starting point, and the same look and feel for all other more sophisticated tools built on Eclipse

IBM's WSAD, and later IBM Rational Software Development Platform, are built on Eclipse.

Standard Eclipse features:

  • Standard window management (perspectives, views, browsers, explorers, ...)
  • Error checking as you type (immediate error indications, ...)
  • Help window as toy type (type ., or <ctrl> space, ...)
  • Automatic build (changes in source code are automatically compiled, ...)
  • Built-in debugger (full featured GUI debugger)
  • Source code generation (getters and setters, ...)
  • Searches (for implementation, for references, ...)
  • Code refactoring (global reference update, ...)
  • Plugin-based architecture (ability to build tools that integrate seamlessly with the environment, and some other tools)
  • ...
More info: Eclipse and Plugincentral.


NetBeans

NetBeans on GNU/Linux

The NetBeans IDE is a Free and Open Source IDE for software developers. The IDE runs on many platforms including Windows, GNU/Linux, Solaris and Mac OS X. It is easy to install and use straight out of the box. You can easily create Java applications for mobile devices using Mobility Pack in NetBeans. With Netbeans 6.0, the IDE has become one of the most preferred development tools, whether it be designing a Swing UI, building a mobile application, an enterprise application or using it as a platform for creating your own IDE.

More info: netbeans.org


JCreator

JCreator is a simple and lightweight JAVA IDE from XINOX Software. It runs only on Windows platforms. It is very easy to install and starts quickly, as it is a native application. This is a good choice for beginners.

More info: http://www.apcomputerscience.com/ide/jcreator/index.htm or JCreator

Processing

Processing is an enhanced IDE. It adds some extra commands and a simplified programming model. This makes it much easier for beginners to start programming in Java. It was designed to help graphic artists learn a bit of programming without struggling too much. Processing runs on Windows, GNU/Linux and Mac OS X platforms.

More info: Processing.

BlueJ

BlueJ on Mac OS X

BlueJ is an IDE that includes templates and will compile and run the applications for you. BlueJ is often used by classes because it is not necessary to set classpaths. BlueJ has its own sets of libraries and you can add your own under preferences. That sets the classpath for all compilations that come out of it to include those you have added and the BlueJ libraries.

BlueJ offers an interesting GUI for creation of packages and programs. Classes are represented as boxes with arrows running between them to represent inheritance/implementation or if on is constructed in another. BlueJ adds all those classes (the project) into the classpath at compile time.

More info: BlueJ Homesite

Kawa

Kawa is basically a Java editor developed by Tek-Tools. It does not include wizards and GUI tools, best suited to experienced Java programmers in small and midsized development teams. It looks that there is no new development for Kawa. The latest version is 4.0, you can download it.

More info: Kawa from tek-tools. See also a javaworld article

JBuilder

JBuilder is an IDE with proprietary source code, sold by Embarcadero Technologies. One of the advantages is the integration with Together, a modeling tool.

More info: Embarcadero.

DrJava

DrJava is an IDE developed by the JavaPLT group at Rice University. It is designed for students.

For more information see DrJava.

Other IDEs



Language fundamentals

Language Fundamentals

The previous chapter "Getting started" was a primer course in the basics of understanding how Java programming works. Throughout the chapter, we tackled a variety of concepts that included:

  • Objects and class definitions;
  • Abstract and data types;
  • Properties;
  • Methods;
  • Class-level and method-level scopes;
  • Keywords; and,
  • Access modifiers, etc.

From this point on, we will be looking into the above mentioned concepts and many more in finer detail with a deeper and richer understanding of how each one of them works. This chapter on Language fundamentals introduces the fundamental elements of the Java programming language in detail. The discussions in this chapter will use the concepts we have already gathered from our previous discussions and build upon them in a progressive manner.

The Java programming syntax

In linguistics, the word syntax (which comes from Ancient Greek σύνταξις where σύν [syn] means "together", and τάξις [táxis] means "an ordering") refers to "the process of arranging things". It defines the principles and rules for constructing phrases and sentences in natural languages.

When learning a new language, the first step one must take is to learn its programming syntax. Programming syntax is to programming languages what grammar is to spoken languages. Therefore, in order to create effective code in the Java programming, we need to learn its syntax — its principles and rules for constructing valid code statements and expressions.

Java uses a syntax similar to the C programming language and therefore if one learns the Java programming syntax, they automatically would be able to read and write programs in similar languages — C, C++ and C#

When learning a new language, the first step one must take is to learn its syntax rules and keywords. Combining the two, one creates statements, Programming Blocks, Classes, Interfaces, et al.

Use packages to avoid name collisions. To hide as much information as possible use the access modifiers properly.

Create methods that do one and if possible only one thing/task. If possible have separate method that changes the object state.

In an object oriented language, programs are run with objects; however, for ease of use and for historic reasons, Java has primitive types. Primitive Data Types only store values and have no methods. Primitive Types may be thought of as Raw Data and are usually embedded attributes inside objects or used as local variables in methods. Because primitive types are not subclasses of the object superclass, each type has a Wrapper Class which is a subclass of Object, and can thus be stored in a collection or returned as an object.

Java is a strong type checking language. There are two concepts regarding types and objects. One is the object type and the other the template/class the object was created from. When an object is created, the template/class is assigned to that object which can not be changed. Types of an object however can be changed by type casting. Types of an object is associated with the object reference that referencing the object and determines what operation can be performed on the object through that object reference. Assigning the value of one object reference to a different type of object reference is called type casting.

The most often used data structure in any language is a character string. For this reason java defines a special object that is String.

To aggregate same type java objects to an array, java has a special array object for that. Both java objects and primitive types can be aggregated to arrays.



Statements

Now, that we have the Java platform on our systems and have run the first program successfully, we are geared towards understanding how programs are actually made. As we have already discussed. A program is a set of instructions, which are simple tasks provided to a computer. These instructions are called statements in Java. Statements can be anything from a single line of code to a complex mathematical equation. Consider the following line:

Example Code section 3.1: A simple assignment statement.
  1. int age = 24;
    

This line is a simple instruction that tells the system to initialize a variable and set its value as 24. Within that simple definition, we talked about initialization of a variable and setting its value. If the above statement was the only one in the program, it would look similar to this:

Computer code Code listing 3.1: A statement in a simple class.
  1. public class MyProgram {
    
  2.     public static void main(String[] args) {
    
  3.         int age = 24;
    
  4.     }
    
  5. }
    

In the same style as C++, Java places its statements within a class declaration and, in the class declaration, the statements are usually placed in the method declaration, as above.

Variable declaration statement

The simplest statement is the variable declaration:

Example Code section 3.2: A simple declaration statement.
  1. int age;
    

It defines a variable that can be used by the further statement. The first token is the data type of the variable. The second token is the name of the variable. Then each declaration statement is ended by a semi-colon (;).

Assignment statements

Up until now, we've assumed the creation of variables as a single statement. In essence, we assign a value to those variables, and that's just what it is called. When you assign a value to a variable in a statement, that statement is called an assignment statement. Did you notice one more thing? The semicolon (;). It's at the end of each statement. A clear indicator that a line of code is a statement is its termination with an ending semicolon. If one was to write multiple statements, it is usually done on each separate line ending with a semicolon. Consider the example below:

Example Code section 3.3: Multiple assignment statements.
  1. int a = 10;
    
  2. int b = 20;
    
  3. int c = 30;
    

You do not necessarily have to use a new line to write each statement. Just like English, you can begin writing the next statement where you ended the first one as depicted below:

Example Code section 3.4: Multiple assignment statements on the same line.
  1. int a = 10; int b = 20; int c = 30;
    

However, the only problem with writing such code is, it's very difficult to read it back. It doesn't look that intimidating at first, but once you've got a significant amount of code, it's usually better to organize it in a way that makes sense. It would look more complex and incomprehensible written as it is in Listing 3.4.

Now that we have looked into the anatomy of a simple assignment statement, we can look back at what we've achieved. We know that...

  • A statement is a unit of code in programming.
  • If we are assigning a variable a value, the statement is called an assignment statement.
  • An assignment statement include three parts: a data type, variable name (also called an identifier) and the value of a variable. We will look more into the nature of identifiers and values in the section titled Variables later.

Now, before we move on to the next topic, you need to try and understand what the code below does.

Example Code section 3.5: Multiple assignment statements with expressions.
  1. int firstNumber = 10;
    
  2. int secondNumber = 20;
    
  3. int result = firstNumber + secondNumber;
    

The first two statements are pretty much similar to those in Section 3.3 but with different variable names. The third however is a bit interesting. We've already talked of variables as being similar to gift boxes. Think of your computer's memory as a shelf where you put all those boxes. Whenever you need a box (or variable), you call its identifier (that's the name of the variable). So calling the variable identifier firstNumber gives you the number 10, calling secondNumber would give you 20 hence when you add the two up, the answer should be 30. That's what the value of the last variable result would be. The part of the third statement where you add the numbers, i.e., firstNumber + secondNumber is called an expression and the expression is what decides what the value is to be. If it's just a plain value, nothing fancy, then it's called a literal.

With the information you have just attained, you can actually write a decent Java program that can sum up values. To learn more, continue reading.

Assertion

An assertion checks if a condition is true:

Example Code section 3.6: A return statement.
  1.     public int getAge() {
    
  2.         assert age >= 0;
    
  3.         return age;
    
  4.     }
    

Each assert statement is ended by a semi-colon (;).

Program Control Flow

Statements are evaluated in the order as they occur. The execution of flow begins at the top most statement and proceed downwards till the last statement is encountered. A statement can be substituted by a statement block. There are special statements that can redirect the execution flow based on a condition, those statements are called branching statements, described in detail in a later section.

Statement Blocks

A bunch of statements can be placed in braces to be executed as a single block. Such a block of statement can be named or be provided a condition for execution. Below is how you'd place a series of statements in a block.

Example Code section 3.7: A statement block.
  1. {
    
  2.     int a = 10;
    
  3.     int b = 20;
    
  4.     int result = a + b;
    
  5. }
    

Branching Statements

Program flow can be affected using function/method calls, loops and iterations. Of various types of branching constructs, we can easily pick out two generic branching methods.

  • Unconditional Branching
  • Conditional Branching

Unconditional Branching Statements

If you look closely at a method, you'll see that a method is a named statement block that is executed by calling that particular name. An unconditional branch is created either by invoking the method or by calling break, continue, return or throw, all of which are described below.

When a name of a method is encountered in a flow, it stops execution in the current method and branches to the newly called method. After returning a value from the called method, execution picks up at the original method on the line below the method call.

Computer code Code listing 3.8: UnconditionalBranching.java
  1. public class UnconditionalBranching {
    
  2.     public static void main(String[] args) {
    
  3.         System.out.println("Inside main method! Invoking aMethod!");
    
  4.         aMethod();
    
  5.         System.out.println("Back in main method!");
    
  6.     }
    
  7.  
    
  8.     public static void aMethod() {
    
  9.         System.out.println("Inside aMethod!");
    
  10.     }
    
  11. }
    
Standard input or output Output provided with the screen of information running the above code.
Inside main method! Invoking aMethod!
Inside aMethod!
Back in main method!

The program flow begins in the main method. Just as aMethod is invoked, the flow travels to the called method. At this very point, the flow branches to the other method. Once the method is completed, the flow is returned to the point it left off and resumes at the next statement after the call to the method.

Return statement

A return statement exit from a block so it is often the last statement of a method:

Example Code section 3.9: A return statement.
  1.     public int getAge() {
    
  2.         int age = 24;
    
  3.         return age;
    
  4.     }
    

A return statement can return the content of a variable or nothing. Beware not to write statements after a return statement which would not be executed! Each return statement is ended by a semi-colon (;).

Conditional Branching Statements

Conditional branching is attained with the help of the if...else and switch statements. A conditional branch occurs only if a certain condition expression evaluates to true.

Conditional Statements

Also referred to as if statements, these allow a program to perform a test and then take action based on the result of that test.

The form of the if statement:

if (condition) {
  do statements here if condition is true
} else {
  do statements here if condition is false
}

The condition is a boolean expression which can be either true or false. The actions performed will depend on the value of the condition.

Example:

Example Code section 3.10: An if statement.
  1. if (i > 0) {
    
  2.    System.out.println("value stored in i is greater than zero");
    
  3. } else {
    
  4.    System.out.println("value stored is not greater than zero");
    
  5. }
    

If statements can also be made more complex using the else if combination:

if (condition 1) {
   do statements here if condition 1 is true
} else if (condition 2) {
   do statements here if condition 1 is false and condition 2 is true
} else {
  do statements here if neither condition 1 nor condition 2 is true
}

Example:

Example Code section 3.11: An if/else if/else statement.
  1. if (i > 0) {
    
  2.    System.out.println("value stored in i is greater than zero");
    
  3. } else if (i < 0) {
    
  4.    System.out.println("value stored in i is less than zero");
    
  5. } else {
    
  6.    System.out.println("value stored is equal to 0");
    
  7. }
    

If there is only one statement to be executed after the condition, as in the above example, it is possible to omit the curly braces, however Oracle's Java Code Conventions explicitly state that the braces should always be used.

There is no looping involved in an if statement so once the condition has been evaluated the program will continue with the next instruction after the statement.

If...else statements

The if ... else statement is used to conditionally execute one of two blocks of statements, depending on the result of a boolean condition.

Example:

Example Code section 3.12: An if/else statement.
  1. if (list == null) {
    
  2.   // This block of statements executes if the condition is true.
    
  3. } else {
    
  4.   // This block of statements executes if the condition is false.
    
  5. }
    

Oracle's Java Code Conventions recommend that the braces should always be used.

An if statement has two forms:

if (boolean-condition)
   statement1

and

if (boolean-condition)
   statement1
else
   statement2

Use the second form if you have different statements to execute if the boolean-condition is true or if it is false. Use the first if you only wish to execute statement1 if the condition is true and you do not wish to execute alternate statements if the condition is false.

The code section 3.13 calls two int methods, f() and y(), stores the results, then uses an if statement to test if x is less than y and if it is, the statement1 body will swap the values. The end result is x always contains the larger result and y always contains the smaller result.

Example Code section 3.13: Value swap.
  1. int x = f();
    
  2. int y = y();
    
  3. if (x < y) {
    
  4.   int z = x;
    
  5.   x = y;
    
  6.   y = z;
    
  7. }
    

if...else statements also allow for the use of another statement, else if. This statement is used to provide another if statement to the conditional that can only be executed if the others are not true. For example:

Example Code section 3.14: Multiple branching.
  1. if (x == 2)
    
  2.   x = 4;
    
  3. else if (x == 3)
    
  4.   x = 6;
    
  5. else
    
  6.   x = -1;
    

The else if statement is useful in this case because if one of the conditionals is true, the other must be false. Keep in mind that if one is true, the other will not execute. For example, if the statement at line 2 contained in the first conditional were changed to x = 3;, the second conditional, the else if, would still not execute. However, when dealing with primitive types in conditional statements, it is more desirable to use switch statements rather than multiple else if statements.

Switch statements

The switch conditional statement is basically a shorthand version of writing many if...else statements. The syntax for switch statements is as follows:

switch(<variable>) {
  case <result>: <statements>; break;
  case <result>: <statements>; break;
  default: <statements>; break;
}

This means that if the variable included equals one of the case results, the statements following that case, until the word break will run. The default case executes if none of the others are true. Note: the only types that can be analysed through switch statements are char, byte, short, or int primitive types. This means that String variables can not by analyzed through switch statements.

Example Code section 3.15: A switch.
  1. int n = 2, x;
    
  2. switch (n) {
    
  3.   case 1: x = 2;
    
  4.     break;
    
  5.   case 2: x = 4;
    
  6.     break;
    
  7.   case 3: x = 6;
    
  8.     break;
    
  9.   case 4: x = 8;
    
  10.     break;
    
  11. }
    
  12. return x;
    

In this example, since the integer variable n is equal to 2, case 2 will execute, make x equal to 4. Thus, 4 is returned by the method.

Iteration Statements

Iteration Statements are statements that are used to iterate a block of statements. Such statements are often referred to as loops. Java offers four kinds of iterative statements.

  • The while loop
  • The do...while loop
  • The for loop
  • The foreach loop

The while loop

Main page: Java Programming/Keywords/while

The while loop iterates a block of code while the condition it specifies is true.

The syntax for the loop is:

while (condition) {
   statement;
 }

Here the condition is an expression. An expression as discussed earlier is any statement that returns a value. While condition statements evaluate to a boolean value, that is, either true or false. As long as the condition is true, the loop will iterate the block of code over and over and again. Once the condition evaluates to false, the loop exits to the next statement outside the loop.

The do...while loop

The do-while loop is functionally similar to the while loop, except the condition is evaluated AFTER the statement executes

do {
   statement;
 } while (condition);

The for loop

Main page: Java Programming/Keywords/for

The for loop is a specialized while loop whose syntax is designed for easy iteration through a sequence of numbers. Example:

Example Code section 3.16: A for loop.
  1. for (int i = 0; i < 100; i++) {
    
  2.   System.out.println(i + "\t" + i * i);
    
  3. }
    
Standard input or output Output for code listing 3.16 if you compile and run the statement above.
 0      0
 1      1
 2      4
 3      9
 ...
 99     9801

The program prints the numbers 0 to 99 and their squares.

The same statement in a while loop:

Example Code section 3.17: An alternative version.
  1. int i = 0;
    
  2. while (i < 100) {
    
  3.    System.out.println(i + "\t" + i * i);
    
  4.    i++;
    
  5. }
    

The foreach loop

The foreach statement allows you to iterate through all the items in a collection, examining each item in turn while still preserving its type. The syntax for the foreach statement is:

for (type item : collection) statement;

For an example, we'll take an array of Strings denoting days in a week and traverse through the collection, examining one item at a time.

Example Code section 3.18: A foreach loop.
  1. String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};
    
  2.  
    
  3. for (String day : days) {
    
  4.   System.out.println(day);
    
  5. }
    
Standard input or output Output for code listing 3.18
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday

Notice that the loop automatically exits after the last item in the collection has been examined in the statement block.

Although the enhanced for loop can make code much clearer, it can't be used in some common situations.

  • Only access. Elements can not be assigned to, eg, not to increment each element in a collection.
  • Only single structure. It's not possible to traverse two structures at once, eg, to compare two arrays.
  • Only single element. Use only for single element access, eg, not to compare successive elements.
  • Only forward. It's possible to iterate only forward by single steps.
  • At least Java 5. Don't use it if you need compatibility with versions before Java 5.

The continue and break statements

At times, you would like to re-iterate a loop without executing the remaining statement within the loop. The continue statement causes the loop to re-iterate and start over from the top most statement inside the loop.

Where there is an ability to re-iterate the loop, there is an ability to exit the loop when required. At any given moment, if you'd like to exit a loop and end all further work within the loop, the break ought to be used.

The continue and break statements can be used with a label like follows:

Example Code section 3.19: Using a label.
  1. String s = "A test string for the switch!\nLine two of test string...";
    
  2. outer: for (int i = 0; i < s.length(); i++) {
    
  3.   switch (s.charAt(i)) {
    
  4.     case '\n': break outer;
    
  5.     case ' ': break;
    
  6.     default: System.out.print(s.charAt(i));
    
  7.   }
    
  8. }
    
Standard input or output Output for code listing 3.19
 Ateststringfortheswitch!

Throw statement

A throw statement exit from a method and so on and so on or it is caught by a try/catch block. It does not return a variable but an exception:

Example Code section 3.20: A return statement.
  1.     public int getAge() {
    
  2.         throw new NullPointerException();
    
  3.     }
    

Beware not to write statements after a throw statement which would not be executed too! Each throw statement is ended by a semi-colon (;).

try/catch

A try/catch must at least contain the try block and the catch block:

Example Code section 3.21: try/catch block.
  1. try {
    
  2.   // Some code
    
  3. } catch (Exception e) {
    
  4.   // Optional exception handling
    
  5. } finally {
    
  6.   // This code is executed no matter what
    
  7. }
    
Test your knowledge

Question 3.1: How many statements are there in this class?

Computer code Code listing 3.2: AProgram.java
  1. public class AProgram {
    
  2.  
    
  3.     private int age = 24;
    
  4.  
    
  5.     public static void main(String[] args) {
    
  6.         int daysInAYear = 365;int ageInDay = 100000;
    
  7.         int localAge = ageInDay / daysInAYear;
    
  8.     }
    
  9.  
    
  10.     public int getAge() {
    
  11.         return age;
    
  12.     }
    
  13. }
    
Answer

5
One statement at line 3, two statements at line 6, one statement at line 7 and one statement at line 11.



Conditional blocks

Conditional blocks allow a program to take a different path depending on some condition(s). These allow a program to perform a test and then take action based on the result of that test. In the code sections, the actually executed code lines will be highlighted.

If

The if block executes only if the boolean expression associated with it is true. The structure of an if block is as follows:

if (boolean expression1) {
statement1
statement2
...
statementn

}

Here is a double example to illustrate what happens if the condition is true and if the condition is false:

Example Code section 3.22: Two if blocks.
  1. int age = 6;
    
  2. System.out.println("Hello!");
    
  3.  
    
  4. if (age < 13) {
    
  5.   System.out.println("I'm a child.");
    
  6. }
    
  7.  
    
  8. if (age > 17) {
    
  9.   System.out.println("I'm an adult.");
    
  10. }
    
  11.  
    
  12. System.out.println("Bye!");
    
Computer code Output for Code section 3.22
Hello!
I'm a child
Bye!
Note If only one statement is to be executed after an if block, it does not have to be enclosed in curly braces. For example, if (i == 0) i = 1; is a perfectly valid portion of Java code. This works for most control structures, such as else and while. However Oracle's Java Code Conventions explicitly state that the braces should always be used.

If/else

The if block may optionally be followed by an else block which will execute if that boolean expression is false. The structure of an if block is as follows:

if (boolean expression1) {
statement1
statement2
...
statementn

} else {

statement1bis
statement2bis
...
statementnbis

}


If/else-if/else

An else-if block may be used when multiple conditions need to be checked. else-if statements come after the if block, but before the else block. The structure of an if block is as follows:

if (boolean expression1) {
statement1.1
statement1.2
...
statementn

} else if (boolean expression2) {

statement2.1
statement2.2
...
statement2.n

} else {

statement3.1
statement3.2
...
statement3.n

}

Here is an example to illustrate:

Computer code Code listing 3.3: MyConditionalProgram.java
  1. public class MyConditionalProgram {
    
  2.     public static void main (String[] args) {
    
  3.       int a = 5;
    
  4.       if (a > 0) {
    
  5.           // a is greater than 0, so this statement will execute
    
  6.           System.out.println("a is positive");
    
  7.       } else if (a >= 0) {
    
  8.           // a case has already executed, so this statement will NOT execute
    
  9.           System.out.println("a is positive or zero");
    
  10.       } else {
    
  11.           // a case has already executed, so this statement will NOT execute
    
  12.           System.out.println("a is negative");
    
  13.       }
    
  14.     }
    
  15. }
    
Computer code Output for code listing 3.3
a is positive

Keep in mind that only a single block will execute, and it will be the first true condition.

All the conditions are evaluated when if is reached, no matter what the result of the condition is, after the execution of the if block:

Example Code section 3.23: A new value for the variable a.
  1. int a = 5;
    
  2. if (a > 0) {
    
  3.     // a is greater than 0, so this statement will execute
    
  4.     System.out.println("a is positive");
    
  5.     a = -5;
    
  6. } else if (a < 0) {
    
  7.     // a WAS greater than 0, so this statement will not execute
    
  8.     System.out.println("a is negative");
    
  9. } else {
    
  10.     // a does not equal 0, so this statement will not execute
    
  11.     System.out.println("a is zero");
    
  12. }
    
Computer code Output for code section 3.23
a is positive

Conditional expressions

Conditional expressions use the compound ?: operator. Syntax:

boolean expression1 ? expression1 : expression2

This evaluates boolean expression1, and if it is true then the conditional expression has the value of expression1; otherwise the conditional expression has the value of expression2.

Example:

Example Code section 3.24: Conditional expressions.
  1. String answer = (p < 0.05)? "reject" : "keep";
    

This is equivalent to the following code fragment:

Example Code section 3.25: Equivalent code.
  1. String answer;
    
  2. if (p < 0.05) {
    
  3.     answer = "reject";
    
  4. } else {
    
  5.     answer = "keep";
    
  6. }
    

Switch

The switch conditional statement is basically a shorthand version of writing many if...else statements. The switch block evaluates an integer (or enum, starting in J2SE 5.0; or String, starting in J2SE 7.0), and, based on the value provided, jumps to a specific case within the switch block and executes code until the break command is encountered or the end of the block. If the switch value does not match any of the case values, execution will jump to the optional default case.

The structure of a switch statement is as follows:

switch (integer1 or enum1 or String value1) {
case case value1:
statement1.1
...
statement1.n
break;
case case value2:
statement2.1
...
statement2.n
break;
default:
statementn.1
...
statementn.n

}

Here is an example to illustrate:

Example Code section 3.26: A switch block.
  1. int i = 3;
    
  2. switch(i) {
    
  3.     case 1:
    
  4.         // i doesn't equal 1, so this code won't execute
    
  5.         System.out.println("i equals 1");
    
  6.         break;
    
  7.     case 2:
    
  8.         // i doesn't equal 2, so this code won't execute
    
  9.         System.out.println("i equals 2");
    
  10.         break;
    
  11.     default:
    
  12.         // i has not been handled so far, so this code will execute
    
  13.         System.out.println("i equals something other than 1 or 2");
    
  14. }
    
Computer code Output for code section 3.26
i equals something other than 1 or 2

If a case does not end with the break statement, then the next case will be checked, otherwise the execution will jump to the end of the switch statement.

Look at this example to see how it's done:

Example Code section 3.27: A switch block containing a case without break.
  1. int i = -1;
    
  2. switch(i) {
    
  3.     case -1:
    
  4.     case 1:
    
  5.         // i is -1, so it will fall through to this case and execute this code
    
  6.         System.out.println("i is 1 or -1");
    
  7.         break;
    
  8.     case 0:
    
  9.         // The break command is used before this case, so if i is 1 or -1, this will not execute
    
  10.         System.out.println("i is 0");
    
  11. }
    
Computer code Output for code section 3.27
i is 1 or -1

Starting in J2SE 5.0, the switch statement can also be used with an enum value instead of an integer.

Though enums have not been covered yet, here is an example so you can see how it's done (note that the enum constants in the cases do not need to be qualified with the type:

Example Code section 3.28: A switch block with an enum type.
  1. Day day = Day.MONDAY; // Day is a fictional enum type containing the days of the week
    
  2. switch(day) {
    
  3.     case MONDAY:
    
  4.         // Since day == Day.MONDAY, this statement will execute
    
  5.         System.out.println("Mondays are the worst!");
    
  6.         break;
    
  7.     case TUESDAY:
    
  8.     case WEDNESDAY:
    
  9.     case THURSDAY:
    
  10.         System.out.println("Weekdays are so-so.");
    
  11.         break;
    
  12.     case FRIDAY:
    
  13.     case SATURDAY:
    
  14.     case SUNDAY:
    
  15.         System.out.println("Weekends are the best!");
    
  16.         break;
    
  17. }
    
Computer code Output for code section 3.28
Mondays are the worst!

Starting in J2SE 7.0, the switch statement can also be used with an String value instead of an integer.

Example Code section 3.29: A switch block with a String type.
  1. String day = "Monday";
    
  2. switch(day) {
    
  3.     case "Monday":
    
  4.         // Since day == "Monday", this statement will execute
    
  5.         System.out.println("Mondays are the worst!");
    
  6.         break;
    
  7.     case "Tuesday":
    
  8.     case "Wednesday":
    
  9.     case "Thursday":
    
  10.         System.out.println("Weekdays are so-so.");
    
  11.         break;
    
  12.     case "Friday":
    
  13.     case "Saturday":
    
  14.     case "Sunday":
    
  15.         System.out.println("Weekends are the best!");
    
  16.         break;
    
  17.     default:
    
  18.         throw new IllegalArgumentException("Invalid day of the week: " + day);
    
  19. }
    
Computer code Output for code section 3.29
Mondays are the worst!



Loop blocks

Loops are a handy tool that enables programmers to do repetitive tasks with minimal effort. Say we want a program that can count from 1 to 10, we could write the following program.

Computer code Code listing 3.4: Count.java
  1. class Count {
    
  2.     public static void main(String[] args) {
    
  3.         System.out.println('1 ');
    
  4.         System.out.println('2 ');
    
  5.         System.out.println('3 ');
    
  6.         System.out.println('4 ');
    
  7.         System.out.println('5 ');
    
  8.         System.out.println('6 ');
    
  9.         System.out.println('7 ');
    
  10.         System.out.println('8 ');
    
  11.         System.out.println('9 ');
    
  12.         System.out.println('10 ');
    
  13.     }
    
  14. }
    
Standard input or output Output for code listing 3.4
1
2
3
4
5
6
7
8
9
10


The task will be completed just fine, the numbers 1 to 10 will be printed in the output, but there are a few problems with this solution:

  • Flexibility: what if we wanted to change the start number or end number? We would have to go through and change them, adding extra lines of code where they're needed.
  • Scalability: 10 repeats are trivial, but what if we wanted 100 or even 1000 repeats? The number of lines of code needed would be overwhelming for a large number of iterations.
  • Maintenance: where there is a large amount of code, one is more likely to make a mistake.
  • Feature: the number of tasks is fixed and doesn't change at each execution.

Using loops we can solve all these problems. Once you get you head around them they will be invaluable to solving many problems in programming.

Open up your editing program and create a new file saved as Loop.java. Now type or copy the following code:

Computer code Code listing 3.5: Loop.java
  1. class Loop {
    
  2.     public static void main(String[] args) {
    
  3.         int i;
    
  4.         for (i = 1; i <= 10; i++) {
    
  5.             System.out.println(i + ' ');
    
  6.         }
    
  7.     }
    
  8. }
    
Standard input or output Output for code listing 3.5
1
2
3
4
5
6
7
8
9
10

If we run the program, the same result is produced, but looking at the code, we immediately see the advantages of loops. Instead of executing 10 different lines of code, line 5 executes ten times. 10 lines of code have been reduced to just 4. Furthermore, we may change the number 10 to any number we like. Try it yourself, replace the 10 with your own number.

While

while loops are the simplest form of loop. The while loop repeats a block of code while the specified condition is true. Here is the structure of a while loop:

while (boolean expression1) {
statement1
statement2
...
statementn

}

The loop's condition is checked before each iteration of the loop. If the condition is false at the start of the loop, the loop will not be executed at all. The code section 3.28 sets in squareHigherThan200 the smallest integer whose square exceeds 200.

Example Code section 3.28: The smallest integer whose square exceeds 200.
  1. int squareHigherThan200 = 0;
    
  2.  
    
  3. while (squareHigherThan200 * squareHigherThan200 < 200) {
    
  4.   squareHigherThan200 = squareHigherThan200 + 1;
    
  5. }
    
Note If a loop's condition will never become false, such as if the true constant is used for the condition, said loop is known as an infinite loop. Such a loop will repeat indefinitely unless it is broken out of. Infinite loops can be used to perform tasks that need to be repeated over and over again without a definite stopping point, such as updating a graphics display.

Do... while

The do-while loop is functionally similar to the while loop, except the condition is evaluated AFTER the statement executes It is useful when we try to find a data that does the job by randomly browsing an amount of data.

do {
statement1
statement2
...
statementn

} while (boolean expression1);

For

The for loop is a specialized while loop whose syntax is designed for easy iteration through a sequence of numbers. It consists of the keyword for followed by three extra statements enclosed in parentheses. The first statement is the variable declaration statement, which allows you to declare one or more integer variables. The second is the condition, which is checked the same way as the while loop. Last is the iteration statement, which is used to increment or decrement variables, though any statement is allowed.

This is the structure of a for loop:

for (variable declarations; condition; iteration statement) {
statement1
statement2
...
statementn

}

To clarify how a for loop is used, here is an example:

Example Code section 3.29: A for loop.
  1. for (int i = 1; i <= 10; i++) {
    
  2.     System.out.println(i);
    
  3. }
    
Standard input or output Output for code listing 3.29
1
2
3
4
5
6
7
8
9
10

The for loop is like a template version of the while loop. The alternative code using a while loop would be as follows:

Example Code section 3.30: An iteration using a while loop.
  1. int i = 1;
    
  2. while (i <= 10) {
    
  3.   System.out.println(i);
    
  4.   i++;
    
  5. }
    

The code section 3.31 shows how to iterate with the for loop using multiple variables and the code section 3.32 shows how any of the parameters of a for loop can be skipped. Skip them all, and you have an infinitely repeating loop.

Example Code section 3.31: The for loop using multiple variables.
  1. for (int i = 1, j = 10; i <= 10; i++, j--) {
    
  2.   System.out.print(i + " ");
    
  3.   System.out.println(j);
    
  4. }
    
Example Code section 3.32: The for loop without parameter.
  1. for (;;) {
    
  2.   // Some code
    
  3. }
    

For-each

Arrays haven't been covered yet, but you'll want to know how to use the enhanced for loop, called the for-each loop. The for-each loop automatically iterates through a list or array and assigns the value of each index to a variable.

To understand the structure of a for-each loop, look at the following example:

Example Code section 3.33: A for-each loop.
  1. String[] sentence = {"I", "am", "a", "Java", "program."};
    
  2. for (String word : sentence) {
    
  3.     System.out.print(word + " ");
    
  4. }
    
Standard input or output Output for code section 3.33
I am a Java program.

The example iterates through an array of words and prints them out like a sentence. What the loop does is iterate through sentence and assign the value of each index to word, then execute the code block.

Here is the general contract of the for-each loop:

for (variable declaration : array or list) {
statement1
statement2
...
statementn

}

Make sure that the type of the array or list is assignable to the declared variable, or you will get a compilation error. Notice that the loop automatically exits after the last item in the collection has been examined in the statement block.

Although the enhanced for loop can make code much clearer, it can't be used in some common situations.

  • Only access. Elements can not be assigned to, eg, not to increment each element in a collection.
  • Only single structure. It's not possible to traverse two structures at once, eg, to compare two arrays.
  • Only single element. Use only for single element access, eg, not to compare successive elements.
  • Only forward. It's possible to iterate only forward by single steps.
  • At least Java 5. Don't use it if you need compatibility with versions before Java 5.

Break and continue keywords

The break keyword exits a flow control loop, such as a for loop. It basically breaks the loop.

In the code section 3.34, the loop would print out all the numbers from 1 to 10, but we have a check for when i equals 5. When the loop reaches its fifth iteration, it will be cut short by the break statement, at which point it will exit the loop.

Example Code section 3.34: An interrupted for loop.
  1. for (int i = 1; i <= 10; i++) {
    
  2.     System.out.println(i);
    
  3.     if (i == 5) {
    
  4.        System.out.println("STOP!");
    
  5.        break;
    
  6.     }
    
  7. }
    
Standard input or output Output for code section 3.34
1
2
3
4
5
STOP!

The continue keyword jumps straight to the next iteration of a loop and evaluates the boolean expression controlling the loop. The code section 3.35 is an example of the continue statement in action:

Example Code section 3.35: A for loop with a skipped iteration.
  1. for (int i = 1; i <= 10; i++) {
    
  2.     if (i == 5) {
    
  3.         System.out.println("Caught i == 5");
    
  4.         continue;
    
  5.     }
    
  6.     System.out.println(i);
    
  7. }
    
Standard input or output Output for code section 3.35
1
2
3
4
Caught i == 5
6
7
8
9
10

As the break and continue statements reduce the readability of the code, it is recommended to reduce their use or replace them with the use of if and while blocks. Some IDE refactoring operations will fail because of such statements.

Test your knowledge

Question 3.2: Consider the following code:

Example Question 3.2: Loops and conditions.
  1. int numberOfItems = 5;
    
  2. int currentItems = 0;
    
  3. int currentCandidate = 1;
    
  4.  
    
  5. while (currentItems < numberOfItems) {
    
  6.   currentCandidate = currentCandidate + 1;
    
  7.   System.out.println("Test with integer: " + currentCandidate);
    
  8.  
    
  9.   boolean found = true;
    
  10.   for (int i = currentCandidate - 1; i > 1; i--) {
    
  11.  
    
  12.     // Test if i is a divisor of currentCandidate
    
  13.     if ((currentCandidate % i) == 0) {
    
  14.       System.out.println("Not matching...");
    
  15.       found = false;
    
  16.       break;
    
  17.     }
    
  18.  
    
  19.   }
    
  20.  
    
  21.   if (found) {
    
  22.     System.out.println("Matching!");
    
  23.     currentItems = currentItems + 1;
    
  24.   }
    
  25. }
    
  26.  
    
  27. System.out.println("Find the value: " + currentCandidate);
    

What will be printed in the standard output?

Answer
Standard input or output Output for Question 3.2
Test with integer: 2
Matching!
Test with integer: 3
Matching!
Test with integer: 4
Not matching...
Test with integer: 5
Matching!
Test with integer: 6
Not matching...
Test with integer: 7
Matching!
Test with integer: 8
Not matching...
Test with integer: 9
Not matching...
Test with integer: 10
Not matching...
Test with integer: 11
Matching!
Find the value: 11

The snippet is searching the 5th prime number, that is to say: 11. It iterates on each positive integer from 2 (2, 3, 4, 5, 6, 7, 8, 9, 10, 11...), among them, it counts the prime numbers (2, 3, 5, 7, 11) and it stops at the 5th one.

So the snippet first iterates on each positive integer from 2 using the while loop:

Example Answer 3.2.1: while loop.
  1. int numberOfItems = 5;
    
  2. int currentItems = 0;
    
  3. int currentCandidate = 1;
    
  4.  
    
  5. while (currentItems < numberOfItems) {
    
  6.   currentCandidate = currentCandidate + 1;
    
  7.   System.out.println("Test with integer: " + currentCandidate);
    
  8.  
    
  9.   boolean found = true;
    
  10.   for (int i = currentCandidate - 1; i > 1; i--) {
    
  11.  
    
  12.     // Test if i is a divisor of currentCandidate
    
  13.     if ((currentCandidate % i) == 0) {
    
  14.       System.out.println("Not matching...");
    
  15.       found = false;
    
  16.       break;
    
  17.     }
    
  18.  
    
  19.   }
    
  20.  
    
  21.   if (found) {
    
  22.     System.out.println("Matching!");
    
  23.     currentItems = currentItems + 1;
    
  24.   }
    
  25. }
    
  26.  
    
  27. System.out.println("Find the value: " + currentCandidate);
    

For each iteration, the current number is either a prime number or not. If it is a prime number, the code at the left will be executed. If it is not a prime number, the code at the right will be executed.

Example Answer 3.2.2: A prime number.
  1. int numberOfItems = 5;
    
  2. int currentItems = 0;
    
  3. int currentCandidate = 1;
    
  4.  
    
  5. while (currentItems < numberOfItems) {
    
  6.   currentCandidate = currentCandidate + 1;
    
  7.   System.out.println("Test with integer: " + currentCandidate);
    
  8.  
    
  9.   boolean found = true;
    
  10.   for (int i = currentCandidate - 1; i > 1; i--) {
    
  11.  
    
  12.     // Test if i is a divisor of currentCandidate
    
  13.     if ((currentCandidate % i) == 0) {
    
  14.       System.out.println("Not matching...");
    
  15.       found = false;
    
  16.       break;
    
  17.     }
    
  18.  
    
  19.   }
    
  20.  
    
  21.   if (found) {
    
  22.     System.out.println("Matching!");
    
  23.     currentItems = currentItems + 1;
    
  24.   }
    
  25. }
    
  26.  
    
  27. System.out.println("Find the value: " + currentCandidate);
    
Example Answer 3.2.3: Not a prime number.
  1. int numberOfItems = 5;
    
  2. int currentItems = 0;
    
  3. int currentCandidate = 1;
    
  4.  
    
  5. while (currentItems < numberOfItems) {
    
  6.   currentCandidate = currentCandidate + 1;
    
  7.   System.out.println("Test with integer: " + currentCandidate);
    
  8.  
    
  9.   boolean found = true;
    
  10.   for (int i = currentCandidate - 1; i > 1; i--) {
    
  11.  
    
  12.     // Test if i is a divisor of currentCandidate
    
  13.     if ((currentCandidate % i) == 0) {
    
  14.       System.out.println("Not matching...");
    
  15.       found = false;
    
  16.       break;
    
  17.     }
    
  18.  
    
  19.   }
    
  20.  
    
  21.   if (found) {
    
  22.     System.out.println("Matching!");
    
  23.     currentItems = currentItems + 1;
    
  24.   }
    
  25. }
    
  26.  
    
  27. System.out.println("Find the value: " + currentCandidate);
    

The prime numbers are counted using currentItems. When currentItems is equal to numberOfItems (5), the program go out of the while loop. currentCandidate contains the last number, that is to say the 5th prime number:

Example Answer 3.2.4: End of the program.
  1. int numberOfItems = 5;
    
  2. int currentItems = 0;
    
  3. int currentCandidate = 1;
    
  4.  
    
  5. while (currentItems < numberOfItems) {
    
  6.   currentCandidate = currentCandidate + 1;
    
  7.   System.out.println("Test with integer: " + currentCandidate);
    
  8.  
    
  9.   boolean found = true;
    
  10.   for (int i = currentCandidate - 1; i > 1; i--) {
    
  11.  
    
  12.     // Test if i is a divisor of currentCandidate
    
  13.     if ((currentCandidate % i) == 0) {
    
  14.       System.out.println("Not matching...");
    
  15.       found = false;
    
  16.       break;
    
  17.     }
    
  18.  
    
  19.   }
    
  20.  
    
  21.   if (found) {
    
  22.     System.out.println("Matching!");
    
  23.     currentItems = currentItems + 1;
    
  24.   }
    
  25. }
    
  26.  
    
  27. System.out.println("Find the value: " + currentCandidate);
    

Labels

Labels can be used to give a name to a loop. The reason to do this is so we can break out of or continue with upper-level loops from a nested loop.

Here is how to label a loop:

label name:loop

To break out of or continue with a loop, use the break or continue keyword followed by the name of the loop.

For example:

Example Code section 3.36: A double for loop.
  1. int i, j;
    
  2. int[][] nums = {
    
  3.     {1, 2, 5},
    
  4.     {6, 9, 7},
    
  5.     {8, 3, 4}
    
  6. };
    
  7.  
    
  8. Outer:
    
  9. for (i = 0; i < nums.length; i++) {
    
  10.     for (j = 0; j < nums[i].length; j++) {
    
  11.         if (nums[i][j] == 9) {
    
  12.             System.out.println("Found number 9 at (" + i + ", " + j + ")");
    
  13.             break Outer;
    
  14.         }
    
  15.     }
    
  16. }
    
Standard input or output Output for code section 3.36
Found number 9 at (1, 1)

You needn't worry if you don't understand all the code, but look at how the label is used to break out of the outer loop from the inner loop. However, as such a code is hard to read and maintain, it is highly recommended not to use labels.

Try... catch blocks

See also Throwing and Catching Exceptions.

The try-catch blocks are used to catch any exceptions or other throwable objects within the code.

Here's what try-catch blocks looks like:

try {
statement1.1
statement1.2
...
statement1.n

} catch (exception1) {

statement2.1
...
statement2.n

}

The code listing 3.6 tries to print all the arguments that have been passed to the program. However, if there not enough arguments, it will throw an exception.

Computer code Code listing 3.6: Attempt.java
  1. public class Attempt {
    
  2.   public static void main(String[] args) {
    
  3.     try {
    
  4.       System.out.println(args[0]);
    
  5.       System.out.println(args[1]);
    
  6.       System.out.println(args[2]);
    
  7.       System.out.println(args[3]);
    
  8.     } catch (ArrayIndexOutOfBoundsException e) {
    
  9.       System.out.println("No enough arguments");
    
  10.     }
    
  11.   }
    
  12. }
    

In addition to the try and catch blocks, a finally block may be present. The finally block is always executed, even if an exception is thrown. It may appear with or without a catch block, but always with a try block.

Here is what a finally block looks like:

try {
statement1.1
statement1.2
...
statement1.n

} catch (exception1) {

statement2.1
...
statement2.n

} finally {

statement3.1
...
statement3.n

}

Examples

The code listing 3.7 recieves a number as parameter and print its binary representation.

Computer code Code listing 3.7: GetBinary.java
  1. public class GetBinary {
    
  2.     public static void main(String[] args) {
    
  3.         if (args.length == 0) {
    
  4.             // Print usage
    
  5.             System.out.println("Usage: java GetBinary <decimal integer>");
    
  6.             System.exit(0);
    
  7.         } else {
    
  8.             // Print arguments
    
  9.             System.out.println("Received " + args.length + " arguments.");
    
  10.             System.out.println("The arguments are:");
    
  11.             for (String arg : args) {
    
  12.                 System.out.println("\t" + arg);
    
  13.             }
    
  14.         }
    
  15.  
    
  16.         int number = 0;
    
  17.         String binary = "";
    
  18.  
    
  19.         // Get the input number
    
  20.         try {
    
  21.             number = Integer.parseInt(args[0]);
    
  22.         } catch (NumberFormatException ex) {
    
  23.             System.out.println("Error: argument must be a base-10 integer.");
    
  24.             System.exit(0);
    
  25.         }
    
  26.  
    
  27.         // Convert to a binary string
    
  28.         do {
    
  29.             switch (number % 2) {
    
  30.                 case 0: binary = '0' + binary; break;
    
  31.                 case 1: binary = '1' + binary; break;
    
  32.             }
    
  33.             number >>= 1;
    
  34.         } while (number > 0);
    
  35.  
    
  36.         System.out.println("The binary representation of " + args[0] + " is " + binary);
    
  37.     }
    
  38. }
    

The code listing 3.8 is a simulation of playing a game called Lucky Sevens. It is a dice game where the player rolls two dice. If the numbers on the dice add up to seven, he wins $4. If they do not, he loses $1. The game shows how to use control flow in a program as well as the fruitlessness of gambling.

Computer code Code listing 3.8: LuckySevens.java
  1. import java.util.*;
    
  2.  
    
  3. public class LuckySevens {
    
  4.     public static void main(String[] args) {
    
  5.         Scanner in = new Scanner(System.in);
    
  6.         Random random = new Random();
    
  7.         String input;
    
  8.         int startingCash, cash, maxCash, rolls, roll;
    
  9.  
    
  10.         // Loop until "quit" is input
    
  11.         while (true) {
    
  12.             System.out.print("Enter the amount of cash to start with (or \"quit\" to quit): ");
    
  13.  
    
  14.             input = in.nextLine();
    
  15.  
    
  16.             // Check if user wants to exit
    
  17.             if (input.toLowerCase().equals("quit")) {
    
  18.                 System.out.println("\tGoodbye.");
    
  19.                 System.exit(0);
    
  20.             }
    
  21.  
    
  22.             // Get number
    
  23.             try {
    
  24.                 startingCash = Integer.parseInt(input);
    
  25.             } catch (NumberFormatException ex) {
    
  26.                 System.out.println("\tPlease enter a positive integer greater than 0.");
    
  27.                 continue;
    
  28.             }
    
  29.  
    
  30.             // You have to start with some money!
    
  31.             if (startingCash <= 0) {
    
  32.                 System.out.println("\tPlease enter a positive integer greater than 0.");
    
  33.                 continue;
    
  34.             }
    
  35.  
    
  36.             cash = startingCash;
    
  37.             maxCash = cash;
    
  38.             rolls = 0;
    
  39.             roll = 0;
    
  40.  
    
  41.             // Here is the game loop
    
  42.             for (; cash > 0; rolls++) {
    
  43.                roll = random.nextInt(6) + 1;
    
  44.                roll += random.nextInt(6) + 1;
    
  45.  
    
  46.                 if (roll == 7)
    
  47.                     cash += 4;
    
  48.                 else
    
  49.                     cash -= 1;
    
  50.  
    
  51.                 if (cash > maxCash)
    
  52.                     maxCash = cash;
    
  53.             }
    
  54.  
    
  55.             System.out.println("\tYou start with $" + startingCash + ".\n"
    
  56.                     + "\tYou peak at $" + maxCash + ".\n"
    
  57.       + "\tAfter " + rolls + " rolls, you run out of cash.");
    
  58.         }
    
  59.     }
    
  60. }
    



Boolean expressions

Boolean values are values that evaluate to either true or false, and are represented by the boolean data type. Boolean expressions are very similar to mathematical expressions, but instead of using mathematical operators such as "+" or "-", you use comparative or boolean operators such as "==" or "!".

Comparative operators

Java has several operators that can be used to compare variables. For example, how would you tell if one variable has a greater value than another? The answer: use the "greater-than" operator.

Here is a list of the comparative operators in Java:

  • > : Greater than
  • < : Less than
  • >= : Greater than or equal to
  • <= : Less than or equal to
  • == : Equal to
  • != : Not equal to

To see how these operators are used, look at this example:

Example Code section 3.37: Comparisons.
  1. int a = 5, b = 3;
    
  2. System.out.println(a > b); // Value is true because a is greater than b
    
  3. System.out.println(a == b); // Value is false because a does not equal b
    
  4. System.out.println(a != b); // Value is true because a does not equal b
    
  5. System.out.println(b <= a); // Value is true because b is less than a
    
Computer code Output for code section 3.37
true
false
true
true

Comparative operators can be used on any primitive types (except boolean), but only the "equals" and "does not equal" operators work on objects. This is because the less-than/greater-than operators cannot be applied to objects, but the equivalency operators can.

Note Specifically, the == and != operators test whether both variables point to the same object. Objects will be covered later in the tutorial, in the "Classes, Objects, and Types" module.

Boolean operators

The Java boolean operators are based on the operations of the boolean algebra. The boolean operators operate directly on boolean values.

Here is a list of four common boolean operators in Java:

  • ! : Boolean NOT
  • && : Boolean AND
  • || : Boolean inclusive OR
  • ^ : Boolean exclusive OR

The boolean NOT operator ("!") inverts the value of a boolean expression. The boolean AND operator ("&&") will result in true if and only if the values on both sides of the operator are true. The boolean inclusive OR operator ("||") will result in true if either or both of the values on the sides of the operator is true. The boolean exclusive XOR operator ("^") will result in true if one and only of the values on the sides of the operator is true.

To show how these operators are used, here is an example:

Example Code section 3.38: Operands.
  1. boolean iMTrue = true;
    
  2. boolean iMTrueToo = true;
    
  3. boolean iMFalse = false;
    
  4. boolean iMFalseToo = false;
    
  5.  
    
  6. System.out.println("NOT operand:");
    
  7. System.out.println(!iMTrue);
    
  8. System.out.println(!iMFalse);
    
  9. System.out.println(!(4 < 5));
    
  10. System.out.println("AND operand:");
    
  11. System.out.println(iMTrue && iMTrueToo);
    
  12. System.out.println(iMFalse && iMFalseToo);
    
  13. System.out.println(iMTrue && iMFalse);
    
  14. System.out.println(iMTrue && !iMFalse);
    
  15. System.out.println("OR operand:");
    
  16. System.out.println(iMTrue || iMTrueToo);
    
  17. System.out.println(iMFalse || iMFalseToo);
    
  18. System.out.println(iMTrue || iMFalse);
    
  19. System.out.println(iMFalse || !iMTrue);
    
  20. System.out.println("XOR operand:");
    
  21. System.out.println(iMTrue ^ iMTrueToo);
    
  22. System.out.println(iMFalse ^ iMFalseToo);
    
  23. System.out.println(iMTrue ^ iMFalse);
    
  24. System.out.println(iMFalse ^ !iMTrue);
    
Computer code Output for code section 3.38
NOT operand:
false
true
false
AND operand:
true
false
false
true
OR operand:
true
false
true
false
XOR operand:
false
false
true
false

Here are the truth tables for the boolean operators:

a !a
true false
false true
a b a && b a || b a ^ b
true true true true false
true false false true true
false true false true true
false false false false false
For help on simplifying complex logic, see De Morgan's laws.

In Java, boolean logic has a useful property called short circuiting. This means that expressions will only be evaluated as far as necessary. In the expression (a && b), if a is false, then b will not be evaluated because the expression will be false no matter what. Here is an example that shows that the second expression is not automatically checked:

Example Code section 3.39: Short circuiting.
  1. System.out.println((4 < 5) || ((10 / 0) == 2));
    
Computer code Output for code section 3.39
true

To disable this property, you can use & instead of && and | instead of || but it's not recommended.

For the bitwise operations on & and |, see Arithmetic expressions.



Variables

In the Java programming language, the words field and variable are both one and the same thing. Variables are devices that are used to store data, such as a number, or a string of character data.

Variables in Java programming

Java is considered as a strongly typed programming language. Thus all variables in the Java programming language ought to have a particular data type. This is either declared or inferred and the Java language only allows programs to run if they adhere to type constraints.

If you present a numeric type with data that is not numeric, say textual content, then such declarations would violate Java’s type system. This gives Java the ability of type safety. Java checks if an expression or data is encountered with an incorrect type or none at all. It then automatically flags this occurrence as an error at compile time. Most type-related errors are caught by the Java compiler, hence making a program more secure and safe once compiled completely and successfully. Some languages (such as C) define an interpretation of such a statement and use that interpretation without any warning; others (such as PL/I) define a conversion for almost all such statements and perform the conversion to complete the assignment. Some type errors can still occur at runtime because Java supports a cast operation which is a way of changing the type of one expression to another. However, Java performs run time type checking when doing such casts, so an incorrect type cast will cause a runtime exception rather than succeeding silently and allowing data corruption.

On the other hand, Java is also known as a hybrid language. While supporting object oriented programming (OOP), Java is not a pure OO language like Smalltalk or Ruby. Instead, Java offers both object types and primitive types. Primitive types are used for boolean, character, and numeric values and operations. This allows relatively good performance when manipulating numeric data, at the expense of flexibility. For example, you cannot subclass the primitive types and add new operations to them.

Kinds of variables

In the Java programming language, there are four kinds of variables.

Computer code Code listing 3.9: ClassWithVariables.java
  1. public class ClassWithVariables {
    
  2.     public int id = 0;
    
  3.     public static boolean isClassUsed;
    
  4.  
    
  5.     public void processData(String parameter) {
    
  6.       Object currentValue = null;
    
  7.     }
    
  8. }
    

In the code listing 3.9, are examples of all four kinds of variables.

  • Instance variables: These are variables that are used to store the state of an object (for example, id). Every object created from a class definition would have its own copy of the variable. It is valid for and occupies storage for as long as the corresponding object is in memory.
  • Class variables: These variables are explicitly defined within the class-level scope with a static modifier (for example, isClassUsed). No other variables can have a static modifier attached to them. Because these variables are defined with the static modifier, there would always be a single copy of these variables no matter how many times the class has been instantiated. They live as long as the class is loaded in memory.
  • Parameters or Arguments: These are variables passed into a method signature (for example, parameter). Recall the usage of the args variable in the main method. They are not attached to modifiers (i.e. public, private, protected or static) and they can be used everywhere in the method. They are in memory during the execution of the method and can't be used after the method returns.
  • Local variables: These variables are defined and used specifically within the method-level scope (for example, currentValue) but not in the method signature. They do not have any modifiers attached to it. They no longer exist after the method has returned.
Test your knowledge

Question 3.5: Consider the following code:

Computer code Question 3.5: SomeClass.java
  1. public class SomeClass {
    
  2.   public static int c = 1;
    
  3.   public int a = c;
    
  4.   private int b;
    
  5.  
    
  6.   public void someMethod(int d) {
    
  7.     d = c;
    
  8.     int e;
    
  9.   }
    
  10. }
    

In the example above, we created five variables: a, b, c, d and e. All these variables have the same data type int (integer). However, can you tell what kind of variable each one is?

Answer
  • a and b are instance variables;
  • c is a class variable;
  • d is a parameter or argument; and,
  • e is a local variable.

Creating variables

A graphical representation of computer memory

Variables and all the information they store are kept in the computer's memory for access. Think of a computer's memory as a table of data — where each cell corresponds to a variable.

Upon creating a variable, we basically create a new address space and give it a unique name. Java goes one step further and lets you define what you can place within the variable — in Java parlance you call this a data type. So, you essentially have to do two things in order to create a variable:

  • Create a variable by giving it a unique name; and,
  • Define a data type for the variable.

The following code demonstrates how a simple variable can be created. This process is known as variable declaration.

Example Code section 3.40: A simple variable declaration.
  1. int a;
    

Assigning values to variables

Because we have provided a data type for the variable, we have a hint as to what the variable can and cannot hold. We know that int (integer) data type supports numbers that are either positive or negative integers. Therefore once a variable is created, we can provide it with any integer value using the following syntax. This process is called an assignment operation.

Example Code section 3.41: Variable declaration and assignment operation (on different lines).
  1. int a;
    
  2. a = 10;
    

Java provides programmers with a simpler way of combining both variable declaration and assignment operation in one line. Consider the following code:

Example Code section 3.42: Variable declaration and assignment operation (on the same line).
  1. int a = 10;
    

Grouping variable declarations and assignment operations

Consider the following code:

Example Code section 3.43: Ungrouped declarations.
  1. int a;
    
  2. int b;
    
  3. String c;
    
  4. a = 10;
    
  5. b = 20;
    
  6. c = "some text";
    

There are various ways by which you can streamline the writing of this code. You can group the declarations of similar data types in one statement, for instance:

Example Code section 3.44: Grouped declarations.
  1. int a, b;
    
  2. String c;
    
  3. a = 10;
    
  4. b = 20;
    
  5. c = "some text";
    

Alternatively, you can further reduce the syntax by doing group declarations and assignments together, as such:

Example Code section 3.45: Grouped declarations and assignments.
  1. int a = 10, b = 20;
    
  2. String c = "some text";
    

Identifiers

Although memory spaces have their own addresses — usually a hash number such as 0xCAD3, etc. — it is much easier to remember a variable's location in the memory if we can give it a recognizable name. Identifiers are the names we give to our variables. You can name your variable anything like aVariable, someVariable, age, someonesImportantData, etcetera. But notice: none of the names we described here has a space within it. Hence, it is pretty obvious that spaces aren't allowed in variable names. In fact, there are a lot of other things that are not allowed in variable names. The things that are allowed are:

  • Characters A to Z and their lower-case counterparts a to z.
  • Numbers 0 to 9. However, numbers should not come at the beginning of a variable's name.
  • And finally, special characters that include only $ (dollar sign) and _ (underscore).
Test your knowledge

Question 3.6: Which of the ones below are proper variable identifiers?

  1. f_name
  2. lastname
  3. someones name
  4. $SomeoneElsesName
  5. 7days
  6. TheAnswerIs42
Answer

I can tell you that 3 and 5 are not the right way to do things around here, the rest are proper identifiers.

Any valid variable names might be correct but they are not always what you should be naming your variables for a few reasons as listed below:

  • The name of the variable should reflect the value within them.
  • The identifier should be named following the naming guidelines or conventions for doing so. We will explain that in a bit.
  • The identifier shouldn't be a nonsense name like lname, you should always name it properly: lastName is the best way of naming a variable.

Naming conventions for identifiers

When naming identifiers, you need to use the following guidelines which ensure that your variables are named accurately. As we discussed earlier, we should always name our variables in a way that tells us what they hold. Consider this example:

Example Code section 3.46: Unknown process.
  1. int a = 24;
    
  2. int b = 365;
    
  3. int c = a * b;
    

Do you know what this program does? Well, it multiplies two values. That much you guessed right. But, do you know what those values are? Exactly, you don't. Now consider this code:

Example Code section 3.47: Time conversion.
  1. int age = 24;
    
  2. int daysInYear = 365;
    
  3. int ageInDays = age * daysInYear;
    

Now you can tell what's happening, can't you? However, before we continue, notice the case of the variables. If a word contains CAPITAL LETTERS, it is in UPPER CASE. If a word has small letters, it is in lower case. Both cases in a word renders it as mIxEd CaSe.

The variables we studied so far had a mixed case. When there are two or more words making up the names of a variable, you need to use a special case called the camel-case. Just like the humps of a camel, your words need to stand out. Using this technique, the words first and name could be written as either firstName or FirstName.

The first instance, firstName is what we use as the names of variables. Remember though, firstName is not the same as FirstName because Java is case-sensitive. Case-sensitive basically implies that the case in which you wrote one word is the case you have to call that word in when using them later on. Anything other than that is not the same as you intended. You'll know more as you progress. You can hopefully tell now why the variables you were asked to identify weren't proper.

Literals (values)

Now that we know how variables should be named, let us look at the values of those variables. Simple values like numbers are called literals. This section shows you what literals are and how to use them. Consider the following code:

Example Code section 3.48: Literals.
  1. int age = 24;
    
  2. long bankBalance = 20000005L;
    

By now, we've only seen how numbers work in assignment statements. Let's look at data types other than numbers. Characters are basically letters of the English alphabet. When writing a single character, we use single quotes to encapsulate them. Take a look at the code below:

Example Code section 3.49: Character.
  1. char c = 'a';
    

Why, you ask? Well, the explanation is simple. If written without quotes, the system would think it's a variable identifier. That's the very distinction you have to make when differentiating between variables and their literal values. Character data types are a bit unusual. First, they can only hold a single character. What if you had to store a complete name within them, say John, would you write something like:

Example Code section 3.50: Character list.
  1. char firstChar = 'J';
    
  2. char secondChar = 'o';
    
  3. char thirdChar = 'h';
    
  4. char fourthChar = 'n';
    

Now, that's pathetic. Thankfully, there's a data type that handles large number of characters, it's called a String. A string can be initialized as follows:

Example Code section 3.51: String.
  1. String name = "John";
    

Notice, the use of double quotation marks instead of single quotation marks. That's the only thing you need to worry about.



Primitive Types

Primitive types are the most basic data types available within the Java language; these include boolean, byte, char, short, int, long, float and double. These types serve as the building blocks of data manipulation in Java. Such types serve only one purpose — containing pure, simple values of a kind. Because these data types are defined into the Java type system by default, they come with a number of operations predefined. You can not define a new operation for such primitive types. In the Java type system, there are three further categories of primitives:

  • Numeric primitives: short, int, long, float and double. These primitive data types hold only numeric data. Operations associated with such data types are those of simple arithmetic (addition, subtraction, etc.) or of comparisons (is greater than, is equal to, etc.)
  • Textual primitives: byte and char. These primitive data types hold characters (that can be Unicode alphabets or even numbers). Operations associated with such types are those of textual manipulation (comparing two words, joining characters to make words, etc.). However, byte and char can also support arithmetic operations.
  • Boolean and null primitives: boolean and null.

All the primitive types have a fixed size. Thus, the primitive types are limited to a range of values. A smaller primitive type (byte) can contain less values than a bigger one (long).

Category Types Size (bits) Minimum Value Maximum Value Precision Example
Integer byte 8 -128 127 From +127 to -128 byte b = 65;
char 16 0 216-1 All Unicode characters char c = 'A';
char c = 65;
short 16 -215 215-1 From +32,767 to -32,768 short s = 65;
int 32 -231 231-1 From +2,147,483,647 to -2,147,483,648 int i = 65;
long 64 -263 263-1 From +9,223,372,036,854,775,807 to -9,223,372,036,854,775,808 long l = 65L;
Floating-point float 32 2-149 (2-2-23)·2127 From 3.402,823,5 E+38 to 1.4 E-45 float f = 65f;
double 64 2-1074 (2-2-52)·21023 From 1.797,693,134,862,315,7 E+308 to 4.9 E-324 double d = 65.55;
Other boolean 1 -- -- false, true boolean b = true;
void -- -- -- -- --

Integer primitive types silently overflow:

Example Code section 3.52: Several operators.
  1. int i = Integer.MAX_VALUE;
    
  2. System.out.println(i);
    
  3. i = i + 1;
    
  4. System.out.println(i);
    
  5. System.out.println(Integer.MIN_VALUE);
    
Computer code Console for Code section 3.52
2147483647
-2147483648
-2147483648

As Java is strongly typed, you can't assign a floating point number (a number with a decimal point) to an integer variable:

Warning Code section 3.53: Setting a floating point number as a value to an int (integer) type.
  1. int age;
    
  2. age = 10.5;
    

A primitive type should be set by an appropriate value. The primitive types can be initialized with a literal. Most of the literals are primitive type values, except String Literals, which are instance of the String class.

Numbers in computer science

Programming may not be as trivial or boring as just crunching huge numbers any more. However, huge chunks of code written in any programming language today, let alone Java, obsessively deal with numbers. Be it churning out huge prime numbers,[1] or just calculating a cost of emission from your scooter. In 1965, Gemini V space mission escaped a near-fatal accident because of a programming error.[2] And again in 1979, a computer program calculated the ability of five nuclear reactors to withstand earthquakes as overestimated; this caused the plants to be shut down temporarily.[3] There is one thing common to both these programming errors: the subject data, being computed at the time the errors occurred, was numeric. Out of past experience, Java came bundled with revised type checking for numeric data and puts lots of emphasis on correctly identifying different types of it. So you must recognise the importance of numeric data when it comes to programming.

Numbers are stored in memory using a binary system. The memory is like a grid of cells:

                               

Each cell can contain a binary digit (shortened to bit), that is to say, zero or one:

 0   1   1   0   0   1   0   1 

Actually, each cell does contain a binary digit, as one bit is roughly equivalent to 1 and an empty cell in the memory signifies 0. A single binary digit can only hold two possible values: a zero or a one.

Memory state Value
                             0  0
                             1  1

Multiple bits held together can hold multiple permutations — 2 bits can hold 4 possible values, 3 can hold 8, and so on. For instance, the maximum number 8 bits can hold (11111111 in binary) is 255 in the decimal system. So, the numbers from 0 to 255 can fit within 8 bits.

Memory state Value
 0   0   0   0   0   0   0   0  0
 0   0   0   0   0   0   0   1  1
 0   0   0   0   0   0   1   0  2
 0   0   0   0   0   0   1   1  3

...
...
 1   1   1   1   1   1   1   1  255

It is all good, but this way, we can only host positive numbers (or unsigned integers). They are called unsigned integers. Unsigned integers are whole number values that are all positive and do not attribute to negative values. For this very reason, we would ask one of the 8 bits to hold information about the sign of the number (positive or negative). This leaves us with just 7 bits to actually count out a number. The maximum number that these 7 bits can hold (1111111) is 127 in the decimal system.

Positive numbers Negative numbers
Memory state Value
 0   0   0   0   0   0   0   0  0
 0   0   0   0   0   0   0   1  1
 0   0   0   0   0   0   1   0  2
 0   0   0   0   0   0   1   1  3

...

...
...
 0   1   1   1   1   1   1   1  127
Memory state Value
 1   0   0   0   0   0   0   0  -128
 1   0   0   0   0   0   0   1  -127
 1   0   0   0   0   0   1   0  -126
 1   0   0   0   0   0   1   1  -125

...

...
...
 1   1   1   1   1   1   1   1  -1

Altogether, using this method, 8 bits can hold numbers ranging from -128 to 127 (including zero) — a total of 256 numbers. Not a bad pay-off one might presume. The opposite to an unsigned integer is a signed integer that have the capability of holding both positive and negative values.

But, what about larger numbers. You would need significantly more bits to hold larger numbers. That's where Java's numeric types come into play. Java has multiple numeric types — their size dependant on the number of bits that are at play.

In Java, numbers are dealt with using data types specially formulated to host numeric data. But before we dive into these types, we must first set some concepts in stone. Just like you did in high school (or even primary school), numbers in Java are placed in clearly distinct groups and systems. As you'd already know by now, number systems includes groups like the integer numbers (0, 1, 2 ... ∞); negative integers (0, -1, -2 ... -∞) or even real and rational numbers (value of Pi, ¾, 0.333~, etcetera). Java simply tends to place these numbers in two distinct groups, integers (-∞ ... 0 ... ∞) and floating point numbers (any number with decimal points or fractional representation). For the moment, we would only look into integer values as they are easier to understand and work with.

Integer types in Java

With what we have learned so far, we will identify the different types of signed integer values that can be created and manipulated in Java. Following is a table of the most basic numeric types: integers. As we have discussed earlier, the data types in Java for integers caters to both positive and negative values and hence are signed numeric types. The size in bits for a numeric type determines what its minimum and maximum value would be. If in doubt, one can always calculate these values.

Lets see how this new found knowledge of the basic integer types in Java fits into the picture. Say, you want to numerically manipulate the days in a year — all 365 days. What type would you use? Since the data type byte only goes up to 127, would you risk giving it a value greater than its allowed maximum. Such decisions might save you from dreaded errors that might occur out of the programmed code. A much more sensible choice for such a numeric operation might be a short. Oh, why couldn't they make just one data type to hold all kinds of numbers? Wouldn't you ask that question? Well, let's explore why.

When you tell a program you need to use an integer, say even a byte, the Java program allocates a space in the memory. It allocates whole 8 bits of memory. Where it wouldn't seem to matter for today's memory modules that have place for almost a dozen trillion such bits, it matters in other cases. Once allocated that part of the memory gets used and can only be claimed back after the operation is finished. Consider a complicated Java program where the only data type you'd be using would be long integers. What happens when there's no space for more memory allocation jobs? Ever heard of the Stack Overflow errors. That's exactly what happens — your memory gets completely used up and fast. So, choose your data types with extreme caution.

Enough talk, let's see how you can create a numeric type. A numeric type begins with the type's name (short, int, etc.) and then provides with a name for the allocated space in the memory. Following is how it's done. Say, we need to create a variable to hold the number of days in a year.

Example Code section 3.54: Days in a year.
  1. short daysInYear = 365;
    

Here, daysInYear is the name of the variable that holds 365 as it's value, while short is the data type for that particular value. Other uses of integer data types in Java might see you write code such as this given below:

Example Code section 3.55: Integer data types in Java.
  1. byte maxByte = 127;
    
  2. short maxShort = 32767;
    
  3. int maxInt = 2147483647;
    
  4. long maxLong = 9223372036854775807;
    

Integer numbers and floating point numbers

The data types that one can use for integer numbers are byte, short, int and long but when it comes to floating point numbers, we use float or double. Now that we know that, we can modify the code in the code section 3.53 as:

Example Code section 3.56: Correct floating point declaration and assignment.
  1. double age = 10.5;
    

Why not float, you say? If we'd used a float, we would have to append the number with a f as a suffix, so 10.5 should be 10.5f as in:

Example Code section 3.57: The correct way to define floating point numbers of type float.
  1. float age = 10.5f;
    

Floating-point math never throws exceptions. Dividing a non-zero value by 0 equals infinity. Dividing a non-infinite value by infinity equals 0.

Test your knowledge

Question 3.7: Consider the following code:

Example Question 3.7: Primitive type assignments.
  1. ...
    
  2.  
    
  3. a = false;
    
  4. b = 3.2;
    
  5. c = 35;
    
  6. d = -93485L;
    
  7. e = 'q';
    

These are five variables. There are a long, a byte, a char, a double and a boolean. Retrieve the type of each one.

Answer
Example Answer 3.7: Primitive type assignments and declarations.
  1. boolean a;
    
  2. double b;
    
  3. byte c;
    
  4. long d;
    
  5. char e;
    
  6.  
    
  7. a = false;
    
  8. b = 3.2;
    
  9. c = 35;
    
  10. d = -93485L;
    
  11. e = 'q';
    
  • a can only be the boolean because only a boolean can handle boolean values.
  • e can only be the char because only a char can contain a character.
  • b can only be the double because only a double can contain a decimal number here.
  • d is the long because a byte can not contain such a low value.
  • c is the remaining one so it is the byte.

Data conversion (casting)

Data conversion (casting) can happen between two primitive types. There are two kinds of casting:

  • Implicit: casting operation is not required; the magnitude of the numeric value is always preserved. However, precision may be lost when converting from integer to floating point types
  • Explicit: casting operation required; the magnitude of the numeric value may not be preserved
Example Code section 3.58: Implicit casting (int is converted to long, casting is not needed).
  1. int  i = 65;
    
  2. long l = i;
    
Example Code section 3.59: Explicit casting (long is converted to int, casting is needed).
  1. long l = 656666L;
    
  2. int  i = (int) l;
    

The following table shows the conversions between primitive types, it shows the casting operation for explicit conversions:

from byte from char from short from int from long from float from double from boolean
to byte - (byte) (byte) (byte) (byte) (byte) (byte) N/A
to char - (char) (char) (char) (char) (char) N/A
to short (short) - (short) (short) (short) (short) N/A
to int - (int) (int) (int) N/A
to long - (long) (long) N/A
to float - (float) N/A
to double - N/A
to boolean N/A N/A N/A N/A N/A N/A N/A -

Unlike C, C++ and similar languages, Java can't represent false as 0 or null and can't represent true as non-zero. Java can't cast from boolean to a non-boolean primitive data type, or vice versa.

Notes

  1. As of edit (11 December 2013), the Great Internet Mersenne Prime Search project has so far identified the largest prime number as being 17,425,170 digits long. Prime numbers are valuable to cryptologists as the bigger the number, the securer they can make their data encryption logic using that particular number.
  2. Gemini 5 landed 130 kilometers short of its planned Pacific Ocean landing point due to a software error. The Earth's rotation rate had been programmed as one revolution per solar day instead of the correct value, one revolution per sidereal day.
  3. A program used in their design used an arithmetic sum of variables when it should have used the sum of their absolute values. (Evars Witt, "The Little Computer and the Big Problem", AP Newswire, 16 March 1979. See also Peter Neumann, "An Editorial on Software Correctness and the Social Process" Software Engineering Notes, Volume 4(2), April 1979, page 3)



Arithmetic expressions

In order to do arithmetic in Java, one must first declare at least one variable. Typically one declares a variable and assigns it a value before any arithmetic is done. Here's an example of declaring an integer variable:

Example Code section 3.59: Variable assignation.
  1. int x = 5;
    

After creating a variable, one can manipulate its value by using Java's operators: + (addition), - (subtraction), * (multiplication), / (integer division), % (modulo or remainder), ++ (pre- & postincrement by one), -- (pre- & postdecrement by one).

Computer code Code listing 3.10: Operators.java
  1. public class Operators {
    
  2.   public static void main(String[] args) {
    
  3.     int x = 5;
    
  4.     System.out.println("x = " + x);
    
  5.     System.out.println();
    
  6.  
    
  7.     System.out.println("--- Addition             ---");
    
  8.     x = 5;
    
  9.     System.out.println("x + 2 = " + (x + 2));
    
  10.     System.out.println("x = " + x);
    
  11.     System.out.println();
    
  12.  
    
  13.     System.out.println("--- Subtraction          ---");
    
  14.     x = 5;
    
  15.     System.out.println("x - 4 = " + (x - 4));
    
  16.     System.out.println("x = " + x);
    
  17.     System.out.println();
    
  18.  
    
  19.     System.out.println("--- Multiplication       ---");
    
  20.     x = 5;
    
  21.     System.out.println("x * 3 = " + (x * 3));
    
  22.     System.out.println("x = " + x);
    
  23.     System.out.println();
    
  24.  
    
  25.     System.out.println("--- (Integer) Division   ---");
    
  26.     x = 5;
    
  27.     System.out.println("x / 2 = " + (x / 2));
    
  28.     System.out.println("x = " + x);
    
  29.     System.out.println();
    
  30.  
    
  31.     System.out.println("--- Modulo (Remainder)   ---");
    
  32.     x = 5;
    
  33.     System.out.println("x % 2 = " + (x % 2));
    
  34.     System.out.println("x = " + x);
    
  35.     System.out.println();
    
  36.  
    
  37.     System.out.println("--- Preincrement by one  ---");
    
  38.     x = 5;
    
  39.     System.out.println("++x   = " + (++x  ));
    
  40.     System.out.println("x = " + x);
    
  41.     System.out.println();
    
  42.  
    
  43.     System.out.println("--- Predecrement by one  ---");
    
  44.     x = 5;
    
  45.     System.out.println("--x   = " + (--x  ));
    
  46.     System.out.println("x = " + x);
    
  47.     System.out.println();
    
  48.  
    
  49.     System.out.println("--- Postincrement by one ---");
    
  50.     x = 5;
    
  51.     System.out.println("x++   = " + (x++  ));
    
  52.     System.out.println("x = " + x);
    
  53.     System.out.println();
    
  54.  
    
  55.     System.out.println("--- Postdecrement by one ---");
    
  56.     x = 5;
    
  57.     System.out.println("x--   = " + (x--  ));
    
  58.     System.out.println("x = " + x);
    
  59.     System.out.println();
    
  60.   }
    
  61. }
    
Standard input or output Console for Code listing 3.10
x = 5

--- Addition             ---
x + 2 = 7
x = 5

--- Subtraction          ---
x - 4 = 1
x = 5

--- Multiplication       ---
x * 3 = 15
x = 5

--- (Integer) Division   ---
x / 2 = 2
x = 5

--- Modulo (Remainder)   ---
x % 2 = 1
x = 5

--- Preincrement by one  ---
++x   = 6
x = 6

--- Predecrement by one  ---
--x   = 4
x = 4

--- Postincrement by one ---
x++   = 5
x = 6

--- Postdecrement by one ---
x--   = 5
x = 4

The division operator rounds towards zero: 5/2 is 2, and -5/2 is -2. The remainder operator has the same sign as the left operand; it is defined such that ((a/b)*b) + (a%b) is always equal to a. The preincrement, predecrement, postincrement, and postdecrement operators are special: they also change the value of the variable, by adding or subtracting one. The only difference is that preincrement/decrement returns the new value of the variable; postincrement returns the original value of the variable.

Test your knowledge

Question 3.8: Consider the following code:

Computer code Question 3.8: Question8.java
  1. public class Question8 {
    
  2.   public static void main(String[] args) {
    
  3.     int x = 10;
    
  4.     x = x + 10;
    
  5.     x = 2 * x;
    
  6.     x = x - 19;
    
  7.     x = x / 3;
    
  8.     System.out.println(x);
    
  9.   }
    
  10. }
    

What will be printed in the standard output?

Answer
Standard input or output Output for Question 3.8
7

int x = 10; => 10
x = x + 10; => 20
x = 2 * x; => 40
x = x - 19; => 21
x = x / 3; => 7

When using several operators in the same expression, one must consider Java's order of precedence. Java uses the standard PEMDAS (Parenthesis, Exponents, Multiplication and Division, Addition and Subtraction) order. When there are multiple instances of the same precedence, Java reads from left to right. Consider what the output of the following code would be:

Example Code section 3.60: Several operators.
  1. System.out.println(10*5 + 100/10 - 5 + 7%2);
    
Standard input or output Console for Code section 3.60
56

The following chart shows how Java would compute this expression:


Figure 3.1: Computation of an arithmetic expression in the Java programming language
Java Computation of Arithmetic Expression.svg


Besides performing mathematical functions, there are also operators to assign numbers to variables (each example again uses the variable initialized as x = 5):

Computer code Code listing 3.11: Assignments.java
  1. public class Assignments {
    
  2.   public static void main(String[] args) {
    
  3.     int x = 5;
    
  4.     x = 3;
    
  5.     System.out.println("Assignment                                       (x = 3) : " + x);
    
  6.  
    
  7.     x = 5;
    
  8.     x += 5;
    
  9.     System.out.println("Assign x plus another integer to itself          (x += 5): " + x);
    
  10.  
    
  11.     x = 5;
    
  12.     x -= 4;
    
  13.     System.out.println("Assign x minus another integer to itself         (x -= 4): " + x);
    
  14.  
    
  15.     x = 5;
    
  16.     x *= 6;
    
  17.     System.out.println("Assign x multiplied by another integer to itself (x *= 6): " + x);
    
  18.  
    
  19.     x = 5;
    
  20.     x /= 5;
    
  21.     System.out.println("Assign x divided by another integer to itself    (x /= 5): " + x);
    
  22.   }
    
  23. }
    
Standard input or output Console for Code listing 3.11
Assignment                                       (x = 3) : 3
Assign x plus another integer to itself          (x += 5): 10
Assign x minus another integer to itself         (x -= 4): 1
Assign x multiplied by another integer to itself (x *= 6): 30
Assign x divided by another integer to itself    (x /= 5): 1

Using bitwise operators within Java

Java has besides arithmetic operators a set of bit operators to manipulate the bits in a number, and a set of logical operators. The bitwise logical operators are

Operator Function Value of
x before
Example
input
Example
output
Value of
x after
& Bitwise AND 7 x&27 3 7
| Bitwise OR 7 x|27 31 7
^ Bitwise XOR 7 x^27 28 7
~ Bitwise inversion 7 ~x -8 7

Besides these logical bitwise functions, there are also operators to assign numbers to variables (x = -5):

Operator Function Example
input
Example output
&= Assign x bitwisely ANDed with another value to itself x &= 3 3
|= Assign x bitwisely ORed with another value to itself x |= 3 -5
^= Assign x bitwisely XORed with another value to itself x ^= 3 -8
<<= Assign x divided by another integer to itself x <<= 1 -10
>>= Assign x bitwisely negated with another value to itself x >>= 1 -3
>>>= Assign x bitwisely negated with another value to itself x >>>= 1 2,305,843,009,213,693,949 (64 bit)

The shift operators are used to shift the bits to the left or right, which is also a quick way to multiply/divide by two:

Operator Function Value of
x before
Example
input
Example output Value of
x after
<< Logical shift left -15 x << 2 -60 -15
>> Arithmetic shift right -15 x >> 3 -2 -15
>>> Logical shift right -15 x >>> 3 2,305,843,009,213,693,937 (64 bit) -15



Literals

Java Literals are syntactic representations of boolean, character, numeric, or string data. Literals provide a means of expressing specific values in your program. For example, in the following statement, an integer variable named count is declared and assigned an integer value. The literal 0 represents, naturally enough, the value zero.

Example Code section 3.61: Numeric literal.
  1. int count = 0;
    

The code section 3.62 contains two number literals, followed by two boolean literals at line 1, one string literal, followed by one number literal at line 2 and one string literal, followed by one real number literal at line 3:

Example Code section 3.62: Literals.
  1. (2 > 3) ? true : false;
    
  2. "text".substring(2);
    
  3. System.out.println("Display a hard coded float: " + 37.19f);
    

Boolean Literals

There are two boolean literals

  • true represents a true boolean value
  • false represents a false boolean value

There are no other boolean literals, because there are no other boolean values!

Numeric Literals

There are three types of numeric literals in Java.

Integer Literals

In Java, you may enter integer numbers in several formats:

  1. As decimal numbers such as 1995, 51966. Negative decimal numbers such as -42 are actually expressions consisting of the integer literal with the unary negation operation -.
  2. As octal numbers, using a leading 0 (zero) digit and one or more additional octal digits (digits between 0 and 7), such as 077. Octal numbers may evaluate to negative numbers; for example 037777777770 is actually the decimal value -8.
  3. As hexadecimal numbers, using the form 0x (or 0X) followed by one or more hexadecimal digits (digits from 0 to 9, a to f or A to F). For example, 0xCAFEBABEL is the long integer 3405691582. Like octal numbers, hexadecimal literals may represent negative numbers.
  4. Starting in J2SE 7.0, as binary numbers, using the form 0b (or 0B) followed by one or more binary digits (0 or 1). For example, 0b101010 is the integer 42. Like octal and hex numbers, binary literals may represent negative numbers.

By default, the integer literal primitive type is int. If you want a long, add a letter el suffix (either the character l or the character L) to the integer literal. This suffix denotes a long integer rather than a standard integer. For example, 3405691582L is a long integer literal. Long integers are 8 bytes in length as opposed to the standard 4 bytes for int. It is best practice to use the suffix L instead of l to avoid confusion with the digit 1 (one) which looks like l in many fonts: 200l2001. If you want a short integer literal, you have to cast it.

Starting in J2SE 7.0, you may insert underscores between digits in a numeric literal. They are ignored but may help readability by allowing the programmer to group digits.

Floating Point Literals

Floating point numbers are expressed as decimal fractions or as exponential notation:

Example Code section 3.63: Floating point literals.
  1. double decimalNumber = 5.0;
    
  2. decimalNumber = 5d;
    
  3. decimalNumber = 0.5;
    
  4. decimalNumber = 10f;
    
  5. decimalNumber = 3.14159e0;
    
  6. decimalNumber = 2.718281828459045D;
    
  7. decimalNumber = 1.0e-6D;
    

Floating point numbers consist of:

  1. an optional leading + or - sign, indicating a positive or negative value; if omitted, the value is positive,
  2. one of the following number formats
    • integer digits (must be followed by either an exponent or a suffix or both, to distinguish it from an integer literal)
    • integer digits .
    • integer digits . integer digits
    • . integer digits
  3. an optional exponent of the form
    • the exponent indicator e or E
    • an optional exponent sign + or - (the default being a positive exponent)
    • integer digits representing the integer exponent value
  4. an optional floating point suffix:
    • either f or F indicating a single precision (4 bytes) floating point number, or
    • d or D indicating the number is a double precision floating point number (by default, thus the double precision (8 bytes) is default).

Here, integer digits represents one or more of the digits 0 through 9.

Starting in J2SE 7.0, you may insert underscores between digits in a numeric literal. They are ignored but may help readability by allowing the programmer to group digits.

Character Literals

Character literals are constant valued character expressions embedded in a Java program. Java characters are sixteen bit Unicode characters, ranging from 0 to 65535. Character literals are expressed in Java as a single quote, the character, and a closing single quote ('a', '7', '$', 'π'). Character literals have the type char, an unsigned integer primitive type. Character literals may be safely promoted to larger integer types such as int and long. Character literals used where a short or byte is called for must be cast to short or byte since truncation may occur.

String Literals

String literals consist of the double quote character (") (ASCII 34, hex 0x22), zero or more characters (including Unicode characters), followed by a terminating double quote character ("), such as: "Ceci est une string."

So a string literal follows the following grammar:

<STRING :
        "\""
        (    (~["\"","\\","\n","\r"])
        |("\\"
            ( ["n","t","b","r","f","\\","'","\""]
            |["0"-"7"](["0"-"7"])?
            |["0"-"3"]["0"-"7"]["0"-"7"]
            )
        )
        )*
        "\"">

Within string and character literals, the backslash character can be used to escape special characters, such as unicode escape sequences, or the following special characters:

Name Character ASCII hex
Backspace \b 8 0x08
TAB \t 9 0x09
NUL character \0 0 0x00
newline \n 10 0x0a
carriage control \r 13 0xd
double quote \" 34 0x22
single quote \' 39 0x27
backslash \\ 92 0x5c

String literals may not contain unescaped newline or linefeed characters. However, the Java compiler will evaluate compile time expressions, so the following String expression results in a string with three lines of text:

Example Code section 3.64: Multi-line string.
  1. String text = "This is a String literal\n"
    
  2.             + "which spans not one and not two\n"
    
  3.             + "but three lines of text.\n";
    

null

null is a special Java literal which represents a null value: a value which does not refer to any object. It is an error to attempt to dereference the null value — Java will throw a NullPointerException. null is often used to represent uninitialized state.

Mixed Mode Operations

In concatenation operations, the values in brackets are concatenated first. Then the values are concatenated from the left to the right. Be careful when mixing character literals and integers in String concatenation operations:

Example Code section 3.65: Concatenation operations.
  1. int one = '1';
    
  2. int zero = '0';
    
  3.  
    
  4. System.out.println("120? " + one + '2' + zero);
    
Standard input or output Console for Code section 3.66
120? 49248

The unexpected results arise because '1' and '0' are converted twice. The expression is concatenated as such:

"120? " + one + '2' + zero
"120? " + 49 + '2' + 48
"120? 49" + '2' + 48
"120? 492" + 48
"120? 49248"
  1. one and zero are integers. So they store integer values. The integer value of '1' is 49 and the integer value of '0' is 48.
  2. So the first concatenation concatenates "120? " and 49. 49 is first converted into String, yielding "49" and the concatenation returns the string "120? 49".
  3. The second concatenation concatenates "120? 49" and '2'. '2' is converted into the String "2" and the concatenation returns the string "120? 492".
  4. The concatenation between "120? 492" and '0' returns the string "120? 49248".

The code section 66 yields the desired result:

Example Code section 3.66: Correct primitive type.
  1. char one = '1';
    
  2. char zero = '0';
    
  3.  
    
  4. System.out.println("120? " + one + '2' + zero);
    
Standard input or output Console for Code section 3.66
120? 120
Test your knowledge

Question 3.9: Consider the following code:

Example Question 3.9: New concatenation operations.
  1. int one = '1';
    
  2. int zero = '0';
    
  3.  
    
  4. System.out.println("  3? " + (one + '2' + zero));
    
  5. System.out.println("102? " + 100 + '2' + 0);
    
  6. System.out.println("102? " + (100 + '2' + 0));
    
Standard input or output Console for Question 3.9
  3? 147
102? 10020
102? 150

Explain the results seen.

Answer

For the first line:

" 3? " + (one + '2' + zero)
" 3? " + (49 + '2' + 48)
" 3? " + (99 + 48)
" 3? " + 147
" 3? 147"

For the second line:

"102? " + 100 + '2' + 0
"102? 100" + '2' + 0
"102? 1002" + 0
"102? 10020"

For the last line:

"102? " + (100 + '2' + 0)
"102? " + (150 + 0)
"102? " + 150
"102? 150"



Methods

Methods are how we communicate with objects. When we invoke or call a method we are asking the object to carry out a task. We can say methods implement the behaviour of objects. For each method we need to give a name, we need to define its input parameters and we need to define its return type. We also need to set its visibility (private, protected or public). If the method throws an Exception, that needs to be declared as well. It is called a method definition. The syntax of method definition is: class MyClass {

 ...
 public ReturnType methodName(ParamOneType parameter1,
             ParamTwoType parameter2)
   throws ExceptionName {
     ReturnType returnType;
     ...
     return returnType;
   }
 ...

}

We can declare that the method does not return anything using the void Java keyword. For example:

Example Code section 3.67: Method without returned data.
  1. private void methodName(String parameter1, String parameter2) {
    
  2.   ...
    
  3.   return;
    
  4. }
    

When the method returns nothing, the return keyword at the end of the method is optional. When the execution flow reaches the return keyword, the method execution is stopped and the execution flow returns to the caller method. The return keyword can be used anywhere in the method as long as there is a way to execute the instructions below:

Warning Code section 3.68: return keyword location.
  1. private void aMethod(int a, int b) {
    
  2.   int c = 0;
    
  3.   if (a > 0) {
    
  4.     c = a;
    
  5.     return;
    
  6.   }
    
  7.   int c = c + b;
    
  8.   return;
    
  9.   int c = c * 2;
    
  10. }
    

In the code section 3.68, the return keyword at line 5 is well placed because the instructions below can be reached when a is negative or equal to 0. However, the return keyword at line 8 is badly placed because the instructions below can't be reached.

Test your knowledge

Question 3.9: Consider the following code:

Example Question 3.9: Compiler error.
  1. private int myMethod(int a, int b, boolean c) {
    
  2.   b = b + 2;
    
  3.   if (a > 0) {
    
  4.     a = a + b;
    
  5.     return a;
    
  6.   } else {
    
  7.     a = 0;
    
  8.   }
    
  9. }
    

The code above will return a compiler error. Why?

Answer
Example Answer 3.9: Compiler error.
  1. private int myMethod(int a, int b, boolean c) {
    
  2.   b = b + 2;
    
  3.   if (a > 0) {
    
  4.     a = a + b;
    
  5.     return a;
    
  6.   } else {
    
  7.     a = 0;
    
  8.   }
    
  9. }
    

The method is supposed to return a int but when a is negative or equal to 0, it returns nothing.

Parameter passing

We can pass any primitive data types or objects to a method but the two are not processed the same way.

Primitive type parameter

The primitive types are passed in by value. It means that as soon as the primitive type is passed in, there is no more link between the value inside the method and the source variable:

Example Code section 3.69: A method modifying a variable.
  1. private void modifyValue(int number) {
    
  2.   number += 1;
    
  3. }
    
Example Code section 3.70: Parameter by value.
  1. int i = 0;
    
  2. modifyValue(i);
    
  3. System.out.println(i);
    
Standard input or output Output for Code section 3.70
0  

As you can see in code section 3.70, the modifyValue() method has not modified the value of i.

Object parameter

The object references are passed by value. It means that:

  • There is no more link between the reference inside the method and the source reference,
  • The source object itself and the object itself inside the method are still the same.

You must understand the difference between the reference of an object and the object itself. A object reference is the link between a variable name and an instance of object:

Object objectnew Object()

An object reference is a pointer, an address to the object instance.

The object itself is the value of its attributes inside the object instance:

object.firstName "James"
object.lastName "Gosling"
object.birthDay "May 19"

Take a look at the example above:

Example Code section 3.71: A method modifying an object.
  1. private void modifyObject(FirstClass anObject) {
    
  2.   anObject.setName("Susan");
    
  3. }
    
Example Code section 3.72: Parameter by reference.
  1. FirstClass object = new FirstClass();
    
  2. object.setName("Christin");
    
  3.  
    
  4. modifyObject(object);
    
  5.  
    
  6. System.out.println(object.getName());
    
Standard input or output Output for Code section 3.72
Susan

The name has changed because the method has changed the object itself and not the reference. Now take a look at the other example:

Example Code section 3.73: A method modifying an object reference.
  1. private void modifyObject(FirstClass anObject) {
    
  2.   anObject = new FirstClass();
    
  3.   anObject.setName("Susan");
    
  4. }
    
Example Code section 3.74: Parameter by reference.
  1. FirstClass object = new FirstClass();
    
  2. object.setName("Christin");
    
  3.  
    
  4. modifyObject(object);
    
  5.  
    
  6. System.out.println(object.getName());
    
Standard input or output Output for Code section 3.74
Christin

The name has not changed because the method has changed the reference and not the object itself. The behavior is the same as if the method was in-lined and the parameters were assigned to new variable names:

Example Code section 3.75: In-lined method.
  1. FirstClass object = new FirstClass();
    
  2. object.setName("Christin");
    
  3.  
    
  4. // Start of the method
    
  5. FirstClass anObject = object;
    
  6. anObject = new FirstClass();
    
  7. anObject.setName("Susan");
    
  8. // End of the method
    
  9.  
    
  10. System.out.println(object.getName());
    
Standard input or output Output for Code section 3.75
Christin

Variable argument list

Java SE 5.0 added syntactic support for methods with variable argument list, which simplifies the typesafe usage of methods requiring a variable number of arguments. Less formally, these parameters are called varargs[2]. The last parameter can be followed with ..., and Java will box all the arguments into an array. Vararg parameter must always be the last method parameter:

Example Code section 3.76: A method using vararg parameters.
  1. public void drawPolygon(java.awt.Point... points) {
    
  2.   //…
    
  3. }
    

When calling the method, a programmer can simply separate the points by commas, without having to explicitly create an array of Point objects. Within the method, the points can be referenced as points[0], points[1], etc. If no points are passed, the array has a length of zero. To require the programmer to use a minimum number of parameters, those parameters can be specified before the variable argument:

Example Code section 3.77: Variable arguments.
  1. // A polygon needs at least three points.
    
  2. public void drawPolygon(Point p1, Point p2, Point p3, Point... otherPoints) {}
    

Return parameter

So as we can see, a method may or may not return a value. If the method does not return a value we use the void Java keyword.

Same as the parameter passing, the method can return a primitive type or an object reference. So a method can return only one value. What if you want to return more than one value from a method. You can always pass in an object reference to the method, and let the method modify the object properties. The modified values can be considered as an output value from the method. However you can also create an Object array inside the method, assign the return values and return the array to the caller. You could have a problem however, if you want to mix primitive data types and object references as the output values from the method.

There is a better approach. Defines special return object with the needed return values. Create that object inside the method, assign the values and return the reference to this object. This special object is "bound" to this method and used only for returning values, so do not use a public class. The best way is to use a nested class, see example below:

Computer code Code listing 3.12: Multiple returned variables.
  1. public class MyObject {
    
  2.   ...
    
  3.  
    
  4.   /** Nested object is for return values from getPersonInfoById method */
    
  5.   public static class ReturnObject {
    
  6.       private int age;
    
  7.       private String name;
    
  8.  
    
  9.       public void setAge(int age) {
    
  10.           this.age = age;
    
  11.       }
    
  12.  
    
  13.       public int getAge() {
    
  14.           return age;
    
  15.       }
    
  16.  
    
  17.       public void setName(String name) {
    
  18.           name = name;
    
  19.       }
    
  20.  
    
  21.       public String getName() {
    
  22.           return name;
    
  23.       }
    
  24.   } // End of nested class definition
    
  25.  
    
  26.   /** Method using the nested class to return values */
    
  27.   public ReturnObject getPersonInfoById(int id) {
    
  28.     int    age;
    
  29.     String name;
    
  30.     ...
    
  31.     // Get the name and age based on the ID from the database
    
  32.     ...
    
  33.     ReturnObject result = new ReturnObject();
    
  34.     result.setAge(age);
    
  35.     result.setName(name);
    
  36.  
    
  37.     return result;
    
  38.   }
    
  39. }
    

In the above example the getPersonInfoById method returns an object reference that contains both values of the name and the age. See below how you may use that object:

Example Code section 3.78: Retrieving the values.
  1. MyObject object = new MyObject();
    
  2. MyObject.ReturnObject person = object.getPersonInfoById(102);
    
  3.  
    
  4. System.out.println("Person Name=" + person.getName());
    
  5. System.out.println("Person Age =" + person.getAge());
    
Test your knowledge

Question 3.10: Consider the following code:

Example Question 3.10: Compiler error.
  1. private int myMethod(int a, int b, String c) {
    
  2.   if (a > 0) {
    
  3.     c = "";
    
  4.     return c;
    
  5.   }
    
  6.   int b = b + 2;
    
  7.   return b;
    
  8. }
    

The code above will return a compiler error. Why?

Answer
Example Answer 3.10: Compiler error.
  1. private int myMethod(int a, int b, String c) {
    
  2.   if (a > 0) {
    
  3.     c = "";
    
  4.     return c;
    
  5.   }
    
  6.   int b = b + 2;
    
  7.   return b;
    
  8. }
    

The method is supposed to return a int but at line 4, it returns c, which is a String.

Special method, the constructor

The constructor is a special method called automatically when an object is created with the new keyword. Constructor does not have a return value and its name is the same as the class name. Each class must have a constructor. If we do not define one, the compiler will create a default so called empty constructor automatically.

Computer code Code listing 3.13: Automatically created constructor.
  1. public class MyClass {
    
  2.   /**
    
  3.   * MyClass Empty Constructor
    
  4.   */
    
  5.   public MyClass() {
    
  6.   }
    
  7. }
    

Static methods

A static method is a method that can be called without an object instance. It can be called on the class directly. For example, the valueOf(String) method of the Integer class is a static method:

Example Code section 3.79: Static method.
  1. Integer i = Integer.valueOf("10");
    

As a consequence, it cannot use the non-static methods of the class but it can use the static ones. The same way, it cannot use the non-static attributes of the class but it can use the static ones:

Example Code section 3.80: Static attribute.
  1. private static int count = 0;
    
  2.  
    
  3. public static int getNewInteger() {
    
  4.   return count++;
    
  5. }
    

You can notice that when you use System.out.println(), out is a static attribute of the System class. A static attribute is related to a class, not to any object instance, so there is only one value for all the object instances. This attribute is unique in the whole Java Virtual Machine. All the object instances use the same attribute:

Computer code Code listing 3.14: A static attribute.
  1. public class MyProgram {
    
  2.  
    
  3.     public static int count = 0;
    
  4.  
    
  5.     public static void main (String[] args) {
    
  6.         MyProgram.count++;
    
  7.  
    
  8.         MyProgram program1 = new MyProgram();
    
  9.         program1.count++;
    
  10.  
    
  11.         MyProgram program2 = new MyProgram();
    
  12.         program2.count++;
    
  13.  
    
  14.         new MyProgram().count++;
    
  15.         System.out.println(MyProgram.count);
    
  16.     }
    
  17. }
    
Standard input or output Output for Code listing 3.14
4
Test your knowledge

Question 3.11: Visit the Oracle JavaDoc of the class java.lang.Integer.

How many static fields this class has?

Answer

4.

  • int MAX_VALUE,
  • int MIN_VALUE,
  • int SIZE and
  • Class<Integer> TYPE.
To learn how to overload and override a method, see Overloading Methods and Constructors.



API/java.lang.String

String is a class built into the Java language defined in the java.lang package. It represents character strings. Strings are ubiquitous in Java. Study the String class and its methods carefully. It will serve you well to know how to manipulate them skillfully. String literals in Java programs, such as "abc", are implemented as instances of this class like this:

Example Code section 3.81: String example.
  1. String str = "This is string literal";
    

On the right hand side a String object is created represented by the string literal. Its object reference is assigned to the str variable.

Immutability

Strings are immutable; that is, they cannot be modified once created. Whenever it looks as if a String object was modified actually a new String was created. For instance, the String.trim() method returns the string with leading and trailing whitespace removed. Actually, it creates a new trimmed string and then returns it. Pay attention on what happens in Code section 3.82:

Example Code section 3.82: Immutability.
  1. String badlyCutText = "     Java is great.     ";
    
  2. System.out.println(badlyCutText);
    
  3.  
    
  4. badlyCutText.trim();
    
  5. System.out.println(badlyCutText);
    
Standard input or output Output for Code section 3.82
     Java is great.    
     Java is great.    

The trim() method call does not modify the object so nothing happens. It creates a new trimmed string and then throws it away.

Example Code section 3.83: Assignment.
  1. String badlyCutText = "     Java is great.     ";
    
  2. System.out.println(badlyCutText);
    
  3.  
    
  4. badlyCutText = badlyCutText.trim();
    
  5. System.out.println(badlyCutText);
    
Standard input or output Output for Code section 3.83
     Java is great.    
Java is great.

The returned string is assigned to the variable. It does the job as the trim() method has created a new String instance.

Concatenation

The Java language provides special support for the string concatenation with operator +:

Example Code section 3.84: Examples of concatenation.
  1. System.out.println("First part");
    
  2. System.out.println(" second part");
    
  3. String str = "First part" + " second part";
    
  4. System.out.println(str);
    
Standard input or output Output for Code section 3.84
First part
 second part
First part second part

The concatenation is not always processed at the same time. Raw string literals concatenation is done at compile time, hence there is a single string literal in the byte code of the class. Concatenation with at least one object is done at runtime.

+ operator can concatenate other objects with strings. For instance, integers will be converted to strings before the concatenation:

Example Code section 3.85: Concatenation of integers.
  1. System.out.println("Age=" + 25);
    
Standard input or output Output for Code section 3.85
Age=25

Each Java object has the String toString() inherited from the Object class. This method provides a way to convert objects into Strings. Most classes override the default behavior to provide more specific (and more useful) data in the returned String:

Example Code section 3.86: Concatenation of objects.
  1. System.out.println("Age=" + new Integer(31));
    
Standard input or output Output for Code section 3.86
Age=31

Using StringBuilder/StringBuffer to concatenate strings

Remember that String objects are immutable objects. Once a String is created, it can not be modified, takes up memory until garbage collected. Be careful of writing a method like this:

Example Code section 3.87: Raw concatenation.
  1. public String convertToString(Collection<String> words) {
    
  2.   String str = "";
    
  3.   // Loops through every element in words collection
    
  4.   for (String word : words) {
    
  5.     str = str + word + " ";
    
  6.   }
    
  7.   return str;
    
  8. }
    

On the + operation a new String object is created at each iteration. Suppose words contains the elements ["Foo", "Bar", "Bam", "Baz"]. At runtime, the method creates thirteen Strings:

  1. ""
  2. "Foo"
  3. " "
  4. "Foo "
  5. "Foo Bar"
  6. " "
  7. "Foo Bar "
  8. "Foo Bar Bam"
  9. " "
  10. "Foo Bar Bam "
  11. "Foo Bar Bam Baz"
  12. " "
  13. "Foo Bar Bam Baz "

Even though only the last one is actually useful.

To avoid unnecessary memory use like this, use the StringBuilder class. It provides similar functionality to Strings, but stores its data in a mutable way. Only one StringBuilder object is created. Also because object creation is time consuming, using StringBuilder produces much faster code.

Example Code section 3.88: Concatenation with StringBuilder.
  1. public String convertToString(Collection<String> words) {
    
  2.   StringBuilder buf = new StringBuilder();
    
  3.   // Loops through every element in words collection
    
  4.   for (String word : words) {
    
  5.     buf.append(word);
    
  6.     buf.append(" ");
    
  7.   }
    
  8.   return buf.toString();
    
  9. }
    

As StringBuilder isn't thread safe (see the chapter on Concurrency). You can't use it in more than one thread. For multi-thread environment, use StringBuffer instead, which does the same and is thread safe. However, as StringBuffer is slower, so only use it when it is required. Moreover, only StringBuffer existed before Java 5.

Comparing Strings

Comparing strings is not as easy as it may first seem. Be aware of what you are doing when comparing String's using ==:

Example Code section 3.89: Dangerous comparison.
  1. String greeting = "Hello World!";
    
  2. if (greeting == "Hello World!") {
    
  3.     System.out.println("Match found.");
    
  4. }
    
Standard input or output Output for Code section 3.89
Match found.

The difference between the above and below code is that the above code checks to see if the String's are the same objects in memory which they are. This is as a result of the fact that String's are stored in a place in memory called the String Constant Pool. If the new keyword is not explicitly used when creating the String it checks to see if it already exists in the Pool and uses the existing one. If it does not exist, a new Object is created. This is what allows Strings to be immutable in Java. To test for equality, use the equals(Object) method inherited by every class and defined by String to return true if and only if the object passed in is a String containing the exact same data:

Example Code section 3.90: Right comparison.
  1. String greeting = "Hello World!";
    
  2. if (greeting.equals("Hello World!")) {
    
  3.     System.out.println("Match found.");
    
  4. }
    
Standard input or output Output for Code section 3.90
Match found.

Remember that the comparison is case sensitive.

Example Code section 3.91: Comparison with lowercase.
  1. String greeting = "Hello World!";
    
  2. if (greeting.equals("hello world!")) {
    
  3.     System.out.println("Match found.");
    
  4. }
    
Standard input or output Output for Code section 3.91

To order String objects, use the compareTo() method, which can be accessed wherever we use a String datatype. The compareTo() method returns a negative, zero, or positive number if the parameter is less than, equal to, or greater than the object on which it is called. Let's take a look at an example:

Example Code section 3.92: Order.
  1. String person1 = "Peter";
    
  2. String person2 = "John";
    
  3. if (person1.compareTo(person2) > 0) {
    
  4.     // Badly ordered
    
  5.     String temp = person1;
    
  6.     person1 = person2;
    
  7.     person2 = temp;
    
  8. }
    

The code section 3.92 is comparing the String variable person1 to person2. If person1 was to be different, even in the slightest manner we will get a value above or below 0 depending on the exact difference. The result is negative if this String object lexicographically precedes the argument string. The result is a positive integer if this String object lexicographically follows the argument string. Take a look at the Java API for more details.

Splitting a String

Sometimes it is useful to split a string into separate strings, based on a regular expressions. The String class has a split() method, since Java 1.4, that will return a String array:

Example Code section 3.93: Order.
  1. String person = "Brown, John:100 Yonge Street, Toronto:(416)777-9999";
    
  2. ...
    
  3. String[] personData = person.split(":");
    
  4. ...
    
  5. String name    = personData[0];
    
  6. String address = personData[1];
    
  7. String phone   = personData[2];
    

Another useful application could be to split the String text based on the new line character, so you could process the text line by line.

Substrings

It may also be sometimes useful to create substrings, or strings using the order of letters from an existing string. This can be done in two methods.

The first method involves creating a substring out of the characters of a string from a given index to the end:

Example Code section 3.94: Truncating string.
  1. String str = "coffee";
    
  2. System.out.println(str.substring(3));
    
Standard input or output Output for Code section 3.94
fee

The index of the first character in a string is 0.

c o f f e e
0 1 2 3 4 5

By counting from there, it is apparent that the character in index 3 is the second "f" in "coffee". This is known as the beginIndex. All characters from the beginIndex until the end of the string will be copied into the new substring.

The second method involves a user-defined beginIndex and endIndex:

Example Code section 3.95: Extraction of string.
  1. String str = "supporting";
    
  2. System.out.println(str.substring(3, 7));
    
Standard input or output Output for Code section 3.95
port

The string returned by substring() would be "port".

s u p p o r t i n g
0 1 2 3 4 5 6 7 8 9

Please note that the endIndex is not inclusive. This means that the last character will be of the index endIndex-1. Therefore, in this example, every character from index 3 to index 6, inclusive, was copied into the substring.

Note It is easy to mistake the method substring() for subString() (which does not exist and would return with a syntax error on compilation). Substring is considered to be one word. This is why the method name does not seem to follow the common syntax of Java. Just remember that this style only applies to methods or other elements that are made up of more than one word.

String cases

The String class also allows for the modification of cases. The two methods that make this possible are toLowerCase() and toUpperCase().

Example Code section 3.96: Case modification.
  1. String str = "wIkIbOoKs";
    
  2. System.out.println(str.toLowerCase());
    
  3. System.out.println(str.toUpperCase());
    
Standard input or output Output for Code section 3.96
wikibooks
WIKIBOOKS

These methods are useful to do a search which is not case sensitive:

Example Code section 3.97: Text search.
  1. String word = "Integer";
    
  2. String text = "A number without a decimal part is an integer."
    
  3.   + " Integers are a list of digits.";
    
  4.  
    
  5. ...
    
  6.  
    
  7. // Remove the case
    
  8. String lowerCaseWord = word.toLowerCase();
    
  9. String lowerCaseText = text.toLowerCase();
    
  10.  
    
  11. // Search
    
  12. int index = lowerCaseText.indexOf(lowerCaseWord);
    
  13. while (index != -1) {
    
  14.   System.out.println(word
    
  15.     + " appears at column "
    
  16.     + (index + 1)
    
  17.     + ".");
    
  18.   index = lowerCaseText.indexOf(lowerCaseWord, index + 1);
    
  19. }
    
Standard input or output Output for Code section 3.97
Integer appears at column 38.
Integer appears at column 47.
Test your knowledge

Question 3.12: You have mail addresses in the following form: <firstName>.<lastName>@<companyName>.org

Write the String getDisplayName(String) method that receives the mail string as parameter and returns the readable person name like this: LASTNAME Firstname

Answer
Example Answer 3.12: getDisplayName()
  1. public static String getDisplayName(String mail) {
    
  2.   String displayName = null;
    
  3.  
    
  4.   if (mail != null) {
    
  5.     String[] mailParts = mail.split("@");
    
  6.     String namePart = mailParts[0];
    
  7.     String[] namesParts = namePart.split("\\.");
    
  8.  
    
  9.     // The last name
    
  10.     String lastName = namesParts[1];
    
  11.     lastName = lastName.toUpperCase();
    
  12.  
    
  13.     // The first name
    
  14.     String firstName = namesParts[0];
    
  15.  
    
  16.     String firstNameInitial = firstName.substring(0, 1);
    
  17.     firstNameInitial = firstNameInitial.toUpperCase();
    
  18.  
    
  19.     String firstNameEnd = firstName.substring(1);
    
  20.     firstNameEnd = firstNameEnd.toLowerCase();
    
  21.  
    
  22.     // Concatenation
    
  23.     StringBuilder displayNameBuilder = new StringBuilder(lastName).append(" ").append(firstNameInitial).append(firstNameEnd);
    
  24.     displayName = displayNameBuilder.toString();
    
  25.   }
    
  26.  
    
  27.   return displayName;
    
  28. }
    
  1. We only process non null strings,
  2. We first split the mail into two parts to separate the personal information from the company information and we keep the name data,
  3. Then we split the name information to separate the first name from the last name. As the split() method use regular expression and . is a wildcard character, we have to escape it (\.). However, in a string, the \ is also a special character, so we need to escape it too (\\.),
  4. The last name is just capitalized,
  5. As the case of all the first name characters will not be the same, we have to cut the first name. Only the first name initial will be capitalized,
  6. Now we can concatenate all the fragments. We prefer to use a StringBuilder to do that.

See also



Classes, Objects and Types

An object is composed of fields and methods. The fields, also called data members, characteristics, attributes, or properties, describe the state of the object. The methods generally describe the actions associated with a particular object. Think of an object as a noun, its fields as adjectives describing that noun, and its methods as the verbs that can be performed by or on that noun.

For example, a sports car is an object. Some of its fields might be its height, weight, acceleration, and speed. An object's fields just hold data about that object. Some of the methods of the sports car could be "drive", "park", "race", etc. The methods really don't mean much unless associated with the sports car, and the same goes for the fields.

The blueprint that lets us build our sports car object is called a class. A class doesn't tell us how fast our sports car goes, or what color it is, but it does tell us that our sports car will have a field representing speed and color, and that they will be say, a number and a word (or hex color code), respectively. The class also lays out the methods for us, telling the car how to park and drive, but these methods can't take any action with just the blueprint — they need an object to have an effect.

In Java, a class is located in a file similar to its own name. If you want to have a class called SportsCar, its source file needs to be SportsCar.java. The class is created by placing the following in the source file:

Computer code Code listing 3.13: SportsCar.java
  1. public class SportsCar {
    
  2.    /* Insert your code here */
    
  3. }
    

The class doesn't do anything yet, as you will need to add methods and field variables first.

The objects are different from the primitive types because:

  1. The primitive types are not instantiated.
  2. In the memory, only their value is stored, directly. No reference to an instance is stored.
  3. In the memory, the allocated space is fixed, whatever their value. The allocated space of an object vary, for instance either the object is intantiated or not.
  4. The primitive types don't have methods callable on them.
  5. A primitive type can't be inherited.

Instantiation and constructors

In order to get from class to object, we "build" our object by instantiation. Instantiation simply means to create an instance of a class. Instance and object are very similar terms and are sometimes interchangeable, but remember that an instance refers to a specific object, which was created from a class.

This instantiation is brought about by one of the class's methods, called a constructor. As its name implies, a constructor builds the object based on the blueprint. Behind the scenes, this means that computer memory is being allocated for the instance, and values are being assigned to the data members.

In general there are four constructor types: default, non-default, copy, and cloning.

A default constructor will build the most basic instance. Generally, this means assigning all the fields values like null, zero, or an empty string. Nothing would stop you, however, from your default sports car color from being red, but this is generally bad programming style. Another programmer would be confused if your basic car came out red instead of say, colorless.

Example Code section 3.79: A default constructor.
  1. SportsCar car = new SportsCar();
    

A non-default constructor is designed to create an object instance with prescribed values for most, if not all, of the object's fields. The car is red, goes from 0-60 in 12 seconds, tops out at 190mph, etc.

Example Code section 3.80: A non-default constructor.
  1. SportsCar car = new SportsCar("red", 12, 190);
    

A copy constructor is not included in the Java language, however one can easily create a constructor that do the same as a copy constructor. It's important to understand what it is. As the name implies, a copy constructor creates a new instance to be a duplicate of an already existing one. In Java, this can be also accomplished by creating the instance with the default constructor, and then using the assignment operator to equivocate them. This is not possible in all languages though, so just keep the terminology under your belt.

Java has the concepts of cloning object, and the end results are similar to copy constructor. Cloning an object is faster than creation with the new keyword, because all the object memory is copied at once to destination cloned object. This is possible by implementing the Cloneable interface, which allows the method Object.clone() to perform a field-by-field copy.

Example Code section 3.81: Cloning object.
  1. SportsCar car = oldCar.clone();
    

Type

When an object is created, a reference to the object is also created. The object can not be accessed directly in Java, only through this object reference. This object reference has a type assigned to it. We need this type when passing the object reference to a method as a parameter. Java does strong type checking.

Type is basically a list of features/operations, that can be performed through that object reference. The object reference type is basically a contract that guarantees that those operations will be there at run time.

When a car is created, it comes with a list of features/operations listed in the user manual that guarantees that those will be there when the car is used.

When you create an object from a class by default its type is the same as its class. It means that all the features/operations the class defined are there and available, and can be used. See below:

Example Code section 3.82: Default type.
  1. (new ClassName()).operations();
    

You can assign this to a variable having the same type as the class:

Example Code section 3.83: A variable having the same type as the class.
  1. ClassName objRefVariable = new ClassName();
    
  2. objRefVariable.operations();
    

You can assign the created object reference to the class, super class, or to an interface the class implements:

Example Code section 3.84: Using the super class.
  1. SuperClass objectRef = new ClassName(); // features/operations list are defined by the SuperClass class
    
  2. ...
    
  3. Interface inter = new ClassName(); // features/operations list are defined by the interface
    

In the car analogy, the created car may have different Type of drivers. We create separate user manuals for them, Average user manual, Power user manual, Child user manual, or Handicapped user manual. Each type of user manual describes only those features/operations appropriate for the type of driver. The Power driver may have additional gears to switch to higher speeds, that are not available to other type of users...

When the car key is passed from an adult to a child we replacing the user manuals, that is called Type Casting.

In Java, casts can occur in three ways:

  • up casting going up in the inheritance tree, until we reach the Object
  • up casting to an interface the class implements
  • down casting until we reach the class the object was created from

Autoboxing/unboxing

Autoboxing and unboxing, language features since Java 1.5, make the programmer's life much easier when it comes to working with the primitive wrapper types. Consider this code fragment:

Example Code section 3.85: Traditional object creation.
  1. int age = 23;
    
  2. Integer ageObject = new Integer(age);
    

Primitive wrapper objects were Java's way of allowing one to treat primitive data types as though they were objects. Consequently, one was expected to wrap one's primitive data type with the corresponding primitive wrapper object, as shown above.

Since Java 1.5, one may write as below and the compiler will automatically create the wrap object. The extra step of wrapping the primitive is no longer required. It has been automatically boxed up on your behalf:

Example Code section 3.86: Autoboxing.
  1. int age = 23;
    
  2. Integer ageObject = age;
    
Note Keep in mind that the compiler still creates the missing wrapper code, so one doesn't really gain anything performance-wise. Consider this feature a programmer convenience, not a performance booster.

Each primitive type has a class wrapper:

Primitive type Class wrapper
byte java.lang.Byte
char java.lang.Character
short java.lang.Short
int java.lang.Integer
long java.lang.Long
float java.lang.Float
double java.lang.Double
boolean java.lang.Boolean
void java.lang.Void

Unboxing uses the same process in reverse. Study the following code for a moment. The if statement requires a boolean primitive value, yet it was given a Boolean wrapper object. No problem! Java 1.5 will automatically unbox this.

Example Code section 3.87: Unboxing.
  1. Boolean canMove = new Boolean(true);
    
  2.  
    
  3. if (canMove) {
    
  4.   System.out.println("This code is legal in Java 1.5");
    
  5. }
    
Test your knowledge

Question 3.11: Consider the following code:

Example Question 3.11: Autoboxing/unboxing.
  1. Integer a = 10;
    
  2. Integer b = a + 2;
    
  3. System.out.println(b);
    

How many autoboxings and unboxings are there in this code?

Answer
Example Answer 3.11: Autoboxing/unboxing.
  1. Integer a = 10;
    
  2. Integer b = a + 2;
    
  3. System.out.println(b);
    

3

  • 1 autoboxing at line 1 to assign.
  • 1 unboxing at line 2 to do the addition.
  • 1 autoboxing at line 2 to assign.
  • No autoboxing nor unboxing at line 3 as println() supports the Integer class as parameter.

Methods in the Object class

Methods in the java.lang.Object class are inherited, and thus shared in common by all classes.

The clone method

The java.lang.Object.clone() method returns a new object that is a copy of the current object. Classes must implement the marker interface java.lang.Cloneable to indicate that they can be cloned.

The equals method

The java.lang.Object.equals(java.lang.Object) method compares the object to another object and returns a boolean result indicating if the two objects are equal. Semantically, this method compares the contents of the objects whereas the equality comparison operator "==" compares the object references. The equals method is used by many of the data structure classes in the java.util package. Some of these data structure classes also rely on the Object.hashCode method—see the hashCode method for details on the contract between equals and hashCode. Implementing equals() isn't always as easy as it seems, see 'Secrets of equals()' for more information.

The finalize method

The java.lang.Object.finalize() method is called exactly once before the garbage collector frees the memory for object. A class overrides finalize to perform any clean up that must be performed before an object is reclaimed. Most objects do not need to override finalize.

There is no guarantee when the finalize method will be called, or the order in which the finalize method will be called for multiple objects. If the JVM exits without performing garbage collection, the OS may free the objects, in which case the finalize method doesn't get called.

The finalize method should always be declared protected to prevent other classes from calling the finalize method.

protected void finalize() throws Throwable { ... }

The getClass method

The java.lang.Object.getClass() method returns the java.lang.Class object for the class that was used to instantiate the object. The class object is the base class of reflection in Java. Additional reflection support is provided in the java.lang.reflect package.

The hashCode method

The java.lang.Object.hashCode() method returns an integer (int) that is used as a hash code for storing the object in an associative array. Classes that implement the java.util.Map interface provide associative arrays and rely on the hashCode method. A good hashCode implementation will return a hash code that is stable (does not change) and evenly distributed (the hash codes of unequal objects tend to be unequal and the hash codes are evenly distributed across integer values).

Because associative arrays depend on both the equals and hashCode methods, there is an important contract between these two methods that must be maintained if the objects are to be inserted into a Map:

For two objects a and b
  • a.equals(b) == b.equals(a)
  • if a.equals(b) then a.hashCode() == b.hashCode()

In order to maintain this contract, a class that overrides the equals method must also override the hashCode method, and vice versa, so that hashCode is based on the same properties (or a subset of the properties) as equals.

A further contract that the map has with the object is that the results of the hashCode and equals methods will not change once the object has been inserted into the map. For this reason, it is generally a good practice to base the hash function on immutable properties of the object.

Two equal objects must have the same hashcode. However, 2 different objects are NOT required to have different hashcodes.

The toString method

The java.lang.Object.toString() method returns a java.lang.String that contains a text representation of the object. The toString method is implicitly called by the compiler when an object operand is used with the string concatenation operators (+ and +=).

The wait and notify thread signaling methods

Every object has two wait lists for threads associated with it. One wait list is used by the synchronized keyword to acquire the mutex lock associated with the object. If the mutex lock is currently held by another thread, the current thread is added to the list of blocked threads waiting on the mutex lock. The other wait list is used for signaling between threads accomplished through the wait and notify and notifyAll methods.

Use of wait/notify allows efficient coordination of tasks between threads. When one thread needs to wait for another thread to complete an operation, or needs to wait until an event occurs, the thread can suspend its execution and wait to be notified when the event occurs. This is in contrast to polling, where the thread repeatedly sleeps for a short period of time and then checks a flag or other condition indicator. Polling is both more computationally expensive, as the thread has to continue checking, and less responsive since the thread won't notice the condition has changed until the next time to check.

The wait methods

There are three overloaded versions of the wait method to support different ways to specify the timeout value: java.lang.Object.wait(), java.lang.Object.wait(long) and java.lang.Object.wait(long, int). The first method uses a timeout value of zero (0), which means that the wait does not timeout; the second method takes the number of milliseconds as a timeout; the third method takes the number of nanoseconds as a timeout, calculated as 1000000 * timeout + nanos.

The thread calling wait is blocked (removed from the set of executable threads) and added to the object's wait list. The thread remains in the object's wait list until one of three events occurs:

  1. another thread calls the object's notify or notifyAll method;
  2. another thread calls the thread's java.lang.Thread.interrupt method; or
  3. a non-zero timeout that was specified in the call to wait expires.

The wait method must be called inside of a block or method synchronized on the object. This insures that there are no race conditions between wait and notify. When the thread is placed in the wait list, the thread releases the object's mutex lock. After the thread is removed from the wait list and added to the set of executable threads, it must acquire the object's mutex lock before continuing execution.

The notify and notifyAll methods

The java.lang.Object.notify() and java.lang.Object.notifyAll() methods remove one or more threads from an object's wait list and add them to the set of executable threads. notify removes a single thread from the wait list, while notifyAll removes all threads from the wait list. Which thread is removed by notify is unspecified and dependent on the JVM implementation.

The notify methods must be called inside of a block or method synchronized on the object. This insures that there are no race conditions between wait and notify.



Keywords

Keywords are special tokens in the language which have reserved use in the language. Keywords may not be used as identifiers in Java — you cannot declare a field whose name is a keyword, for instance.

Examples of keywords are the primitive types, int and boolean; the control flow statements for and if; access modifiers such as public, and special words which mark the declaration and definition of Java classes, packages, and interfaces: class, package, interface.

Below are all the Java language keywords:

In addition, the identifiers null, true, and false denote literal values and may not be used to create identifiers.

abstract

abstract is a Java keyword. It can be applied to a class and methods. An abstract class cannot be directly instantiated. It must be placed before the variable type or the method return type. It is recommended to place it after the access modifier and after the static keyword. A non-abstract class is a concrete class. An abstract class cannot be final.

Only an abstract class can have abstract methods. An abstract method is only declared, not implemented:

Computer code Code listing 1: AbstractClass.java
  1. public abstract class AbstractClass {
    
  2.     // This method does not have a body; it is abstract.
    
  3.     public abstract void abstractMethod();
    
  4.  
    
  5.     // This method does have a body; it is implemented in the abstract class and gives a default behavior.
    
  6.     public void concreteMethod() {
    
  7.         System.out.println("Already coded.");
    
  8.     }
    
  9. }
    

An abstract method cannot be final, static nor native. Either you instantiate a concrete sub-class, either you instantiate the abstract class by implementing its abstract methods alongside a new statement:

Example Code section 1: Abstract class use.
  1. AbstractClass myInstance = new AbstractClass() {
    
  2.     public void abstractMethod() {
    
  3.         System.out.println("Implementation.");
    
  4.     }
    
  5. };
    

A private method cannot be abstract.


assert

assert is a Java keyword used to define an assert statement. An assert statement is used to declare an expected boolean condition in a program. If the program is running with assertions enabled, then the condition is checked at runtime. If the condition is false, the Java runtime system throws a AssertionError.

Assertions may be declared using the following syntax:

Computer code
assert expression1 [: expression2];

expression1 is a boolean that will throw the assertion if it is false. When it is thrown, the assertion error exception is created with the parameter expression2 (if applicable).

An example:

Computer code
assert list != null && list.size() > 0 : "list variable is null or empty";
Object value = list.get(0);

Assertions are usually used as a debugging aid. They should not be used instead of validating arguments to public methods, or in place of a more precise runtime error exception.

Assertions are enabled with the Java -ea or -enableassertions runtime option. See your Java environment documentation for additional options for controlling assertions.

boolean

boolean is a keyword which designates the boolean primitive type. There are only two possible boolean values: true and false. The default value for boolean fields is false.

The following is a declaration of a private boolean field named initialized, and its use in a method named synchronized.

Computer code
private boolean initialized = false;
public synchronized void {
   if ( !initialized ) {
      connection = connect();
      initialized = true;
   }
}


Note that there is no automatic conversion between integer types (such as int) to boolean as is possible in some languages like C. Instead, one must use an equivalent expression such as (i != 0) which evaluates to true if i is not zero.

break

break is a Java keyword.

Jumps (breaks) out from a loop. Also used at switch statement.

For example:

Computer code
for ( int i=0; i < maxLoopIter; i++ ) {
   System.out.println("Iter=" +i);
   if ( i == 5 ) {
      break;  // -- 5 iteration is enough --
   }
}


See also:

byte

byte is a keyword which designates the 8 bit signed integer primitive type.

The java.lang.Byte class is the nominal wrapper class when you need to store a byte value but an object reference is required.

Syntax:

byte <variable-name> = <integer-value>;

For example:

Computer code
byte b = 65;

or

Computer code
byte b = 'A'

The number 65 is the code for 'A' in ASCII.

See also:

case

case is a Java keyword.

This is part of the switch statement, to find if the value passed to the switch statement matches a value followed by case.

For example:

Computer code
int i = 3;
switch(i) {
case 1:
   System.out.println("The number is 1.");
   break;
case 2:
   System.out.println("The number is 2.");
   break;
case 3:
   System.out.println("The number is 3."); // this line will print
   break;
case 4:
   System.out.println("The number is 4.");
   break;
case 5:
   System.out.println("The number is 5.");
   break;
default:
  System.out.println("The number is not 1, 2, 3, 4, or 5.");
}

catch

catch is a keyword.

It's part of a try block. If an exception is thrown inside a try block, the exception will be compared to any of the catch part of the block. If the exception match with one of the exception in the catch part, the exception will be handled there.

For example:

Computer code
try {
   //...
     throw new MyException_1();
   //...
} catch ( MyException_1 e ) {
   // --- Handle the Exception_1 here --
} catch ( MyException_2 e ) {
   // --- Handle the Exception_2 here --
}


See also:

char

char is a keyword. It defines a character primitive type. char can be created from character literals and numeric representation. Character literals consist of a single quote character (') (ASCII 39, hex 0x27), a single character, and a close quote ('), such as 'w'. Instead of a character, you can also use unicode escape sequences, but there must be exactly one.

Syntax:

char variable name1 = 'character1';
Example Code section 1: Three examples.
  1. char oneChar1 = 'A';
    
  2. char oneChar2 = 65;
    
  3. char oneChar3 = '\u0041';
    
  4. System.out.println(oneChar1);
    
  5. System.out.println(oneChar2);
    
  6. System.out.println(oneChar3);
    
Computer code Output for Code section 1
A
A
A

65 is the numeric representation of character 'A' , or its ASCII code.

The nominal wrapper class is the java.lang.Character class when you need to store a char value but an object reference is required.

Example Code section 2: char wrapping.
  1. char aCharPrimitiveType = 'A';
    
  2. Character aCharacterObject = aCharPrimitiveType;
    

See also:

class

class is a Java keyword which begins the declaration and definition of a class.

The general syntax of a class declaration, using Extended Backus-Naur Form, is

class-declaration ::= [access-modifiers] class identifier
                      [extends-clause] [implements-clause] 
                      class-body

extends-clause ::= extends class-name
implements-clause ::= implements interface-names 
interface-names ::= interface-name [, interface-names]
class-body ::= { [member-declarations] }
member-declarations = member-declaration [member-declarations]
member-declaration = field-declaration 
                     | initializer 
                     | constructor 
                     | method-declaration
                     | class-declaration

The extends word is optional. If omitted, the class extends the Object class, as all Java classes inherit from it.

See also:

const

const is a reserved keyword, presently not being used.

In other programming languages, such as C, const is often used to declare a constant. However, in Java, final is used instead.

continue

continue is a Java keyword. It skips the remainder of the loop and continues with the next iteration.

For example:

Computer code
int maxLoopIter = 7;

for (int i = 0; i < maxLoopIter; i++ ) {
   if (i == 5) {
      continue;  // -- 5 iteration is skipped --
   }
   System.println("Iteration = " + i);
}


results in

0
1
2
3
4
6
7

See also

default

default is a Java keyword.

This is an optional part of the switch statement, which only executes if none of the above cases are matched.

See also:

do

do is a Java keyword.

It starts a do-while looping block. The do-while loop is functionally similar to the while loop, except the condition is evaluated after the statements execute

Syntax:

Computer code
do {
    //statements;
} while (condition);

For example:

Computer code
do {
    i++;
} while ( i < maxLoopIter );

See also:

double

double is a keyword which designates the 64 bit float primitive type.

The java.lang.Double class is the nominal wrapper class when you need to store a double value but an object reference is required.

Syntax:

double <variable-name> = <float-value>;

For example:

Computer code
double d = 65.55;


See also:

else

else is a Java keyword. It is an optional part of a branching statement. It starts the 'false' statement block.

The general syntax of a if, using Extended Backus-Naur Form, is

branching-statement ::= if condition-clause 
                                    single-statement | block-statement
                       [ else 
                                    single-statement | block-statement ]
 
condition-clause    ::= ( Boolean Expression )
single-statement    ::= Statement
block-statement     ::= { Statement [ Statement ] }

For example:

Computer code
if ( expression ) {
   System.out.println("'True' statement block");
} else {
   System.out.println("'False' statement block");
}


See also:

enum

enum is a keyword since Java 1.5.

This keyword defines a group of constants.

For example:

Computer code
/** Grades of courses */
  enum Grade { A, B, C, D, F };
  // ...
  private Grade gradeA = Grade.A;


This enumeration constant then can be passed in to methods:

Computer code
student.assignGrade(gradeA);
  /**
   * Assigns the grade for this course to the student
   * @param GRADE  Grade to be assigned
   */

  public void assignGrade(final Grade GRADE) {
    grade = GRADE;
  }


An enumeration may also have parameters:

Computer code
public enum DayOfWeek {
  /** Enumeration constants */
  MONDAY(1), TUESDAY(2), WEDNESDAY(3), THURSDAY(4), FRIDAY(5), SATURDAY(6), SUNDAY(0);

  /** Code for the days of the week */
  private byte dayCode = 0;

  /**
   * Private constructor
   * @param VALUE  Value that stands for a day of the week.
   */

  private DayOfWeek(final byte VALUE) {
    dayCode = java.lang.Math.abs(VALUE%7);
  }
 
  /**
   * Gets the day code
   * @return  The day code
   */

  public byte getDayCode() {
    return dayCode;
  }
}


It is also possible to let an enumeration implement interfaces other than java.lang.Comparable and java.io.Serializable, which are already implicitly implemented by each enumeration:

Computer code
public enum DayOfWeek implements Runnable {
  MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
 
  /**
   * Run method prints all elements
   */

  public void run() {
    System.out.println("name() = " + name() +
      ", toString() = \"" + toString() + "\"");
  }
}

extends

extends is a Java keyword.

Used in class and interface definition to declare the class or interface that is to be extended.

Syntax:

Computer code
public class MyClass extends SuperClass
{
  //...
}
 
public interface MyInterface extends SuperInterface
{
  //...
}


In Java 1.5 and later, the "extends" keyword is also used to specify an upper bound on a type parameter in Generics.

Computer code
class Foo<T extends Number> { /*...*/ }


See also:

final

final is a keyword. Beware! It has distinct meanings depending whether it is used for a class, a method, or for a variable. It must be placed before the variable type or the method return type. It is recommended to place it after the access modifier and after the static keyword.

Example Code section 1: Keyword order.
  1. private static final long serialVersionUID = -5437975414336623381L;
    

For a variable

The final keyword only allows a single assignment for the variable. That is to say, once the variable has been assigned, its value is in read-only. If the variable is a primitive type, its value will no longer change. If it is an object, only its reference will no longer change. Keep in mind that its value can still be changed.

Warning Code section 2: Forbidden double assignment.
  1. final int a = 1;
    
  2. a = 2;
    
Example Code section 3: Only modify the value of the object.
  1. final ArrayList list = new ArrayList();
    
  2. System.out.println(list.size());
    
  3. list.add("One item");
    
  4. System.out.println(list.size());
    
Standard input or output Console for Code section 3
0
1

A final variable is often used for universal constants, such as pi:

Example Code section 4: Pi constant.
  1. static final double PI = 3.1415