Java Programming/Syntax/Comments

Comments

Comments can appear anywhere in the source code where whitespace is allowed. Java comments can be written in two ways: slash-slash (// ... ) comments and slash-star (/* ... */) comments

Slash-slash comments

// This comment extends to the end of the line.

// Two forward slashes (with no whitespace between them) begin such comments.
// This type of comment is called a "slash-slash" comment

Slash-star comments

/* This comment, a "slash-star" comment, spans multiple lines.
 * It begins with the slash-star sequence (with no space between 
 * the '/' and '*' characters) and extends to the star-slash sequence.
 * By convention, subsequent lines of slash-star comments
 * begin with a star aligned under the star in the open comment 
 * sequence, but this is not required.
 */

/* This also works; slash-star comments may be on a single line */

/* So does this.
   There are no stars on the subsequent lines
 */

Slash-star comments may also be placed between any Java tokens:

int i = /* maximum integer */ Integer.MAX_VALUE;

Javadoc comments

Javadoc comments are a special case of slash-star comments.

/** 
 * Comments which start with slash-star-star are JavaDoc comments. 
 * These are used to extract documentation from the Java source. 
 * More on javadoc will be covered later.
 */

Comments may not be nested

Java slash-star comments may not be nested.

/* This comment appears to contain /* a nested comment. */ 
 * The comment ends after the first star-slash and
 * everything after the star-slash sequence is parsed
 * as non-comment source.
 */

If you accidentally nest such comments, you will probably get a syntax error from the compiler soon after the first star-slash sequence. If you need to have the sequence */ inside a comment you can use html numeric entities: */.

Comments in String Literals

Comments are not parsed as comments when they occur in String literals.

 String text = "/* Ceci n’est pas un commentaire. */";

results in a 33 character string.

Unicode Characters in Comments

Be aware that Java still interprets Unicode sequences within comments. For example, the Unicode sequence \u002a\u002f (whose codepoints correspond to */) is processed early in the Java compiler's lexical scanning of the source file, even before comments are processed, so this is a valid star-slash comment in Java:

/* Ceci est un commentaire. \u002a\u002f
String statement = "Ceci n’est pas un commentaire.";

and is lexically equivalent to

/* Ceci est un commentaire. */
String statement = "/* Ceci n’est pas un commentaire. */";

(The '*' character is Unicode 002A and the '/' character is Unicode 002F).

Similar caveats apply to newline characters in slash-slash comments.

For example:

//This is a single line comment \u000a This is code

That is because the \u000a is the Unicode for a new line, making the compiler think that you have added a new line when you haven't.
This may be useful when declaring more than one thing on a line and you still wish to use // type comments

int x = 0; //X is the value of the carr \u000a int y=0; //Y is the interest

It is recommended that you use the /* and */ style comments instead or just separate them into two lines, it won't hurt and it will make your code more readable.