Java Programming/Keywords/instanceof

instanceof is a keyword.

It checks if an object reference is an instance of a type, and returns a boolean value;

The <object-reference> instanceof Object will return true for all non-null object references, since all Java objects are inherited from Object. instanceof will always return false if <object-reference> is null.

Syntax:

<object-reference> instanceof TypeName

For example:

class Fruit  {   //...  }  class Apple extends Fruit  {   //...  }  class Orange extends Fruit  {   //...  }  public class Test  {     public static void main(String[] args)     {        Collection<Object> coll = new ArrayList<Object>();          Apple app1 = new Apple();        Apple app2 = new Apple();        coll.add(app1);        coll.add(app2);          Orange or1 = new Orange();        Orange or2 = new Orange();        coll.add(or1);        coll.add(or2);          printColl(coll);     }       private static String printColl( Collection<?> coll )     {        for (Object obj : coll)        {           if ( obj instanceof Object )           {              System.out.print("It is a Java Object and");           }           if ( obj instanceof Fruit )           {              System.out.print("It is a Fruit and");           }           if ( obj instanceof Apple )           {              System.out.println("it is an Apple");           }           if ( obj instanceof Orange )           {              System.out.println("it is an Orange");           }        }     }  }

Run the program:

java Test

The output:

"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Orange"
"It is a Java Object and It is a Fruit and it is an Orange"

Note that the instanceof operator can also be applied to interfaces. For example, if the example above was enhanced with the interface

interface Edible {  //... }

and the the classes modified such that they implemented this interface

class Orange extends Fruit implements Edible  {   ...  }

we could ask if our object were edible.

if ( obj instanceof Edible )  {    System.out.println("it is edible");  }