How instanceof will work on an interface

instanceof can be used to test if an object is a direct or descended instance of a given class. instanceof can also be used with interfaces even though interfaces can't be instantiated like classes. Can anyone explain how instanceof works?

First of all, we can store instances of classes that implements a particular interface in an interface reference variable like this.

package com.test;

public class Test implements Testable {

	public static void main(String[] args) {

		Testable testable = new Test();

		// OR

		Test test = new Test();

		if (testeable instanceof Testable)
			System.out.println("instanceof succeeded");
		if (test instanceof Testable)
			System.out.println("instanceof succeeded");
	}
}

interface Testable {

}

ie, any runtime instance that implements a particular interface will pass the instanceof test

EDIT

and the output

instanceof succeeded
instanceof succeeded

@RohitJain

You can create instances of interfaces by using anonymous inner classes like this

Runnable runnable = new Runnable() {
	
	public void run() {
		System.out.println("inside run");
	}
};

and you test the instance is of type interface, using instanceof operator like this

System.out.println(runnable instanceof Runnable);

and the result is 'true'

object instanceof object_interface will yield true.

You do an instanceof check of a reference against an instance, and it checks the type of instance that particular reference is pointing to.

Now since you can create a reference of an interface, which points to an instance of implementing class (same concept as, Super class reference pointing to subclass instance). So, you can do an instanceof check on it.

For e.g :-

public interface MyInterface {
}

class ImplClass implements MyInterface {

    public static void main(String[] args) {
        MyInterface obj = new ImplClass();
  
        System.out.println(obj instanceof ImplClass);   // Will print true.
    }
}

- First of all instanceof is used to compare whether the Object Reference Variable holding the object is of certain type or not.

Eg:

public void getObj(Animal a){       // a is an Object Reference Variable of type Animal

    if(a instanceof Dog){


       }

}

- In the case of interface, the class which implements it can be used with instanceof.

Eg:

public interface Brush{

  public void paint();
}

public class Strokes implements Brush{

       public void paint(){

          System.out.println("I am painting");

    }


}

public class Test{

  
  public static void main(String[] args){
    
          Brush b = new Strokes();

         if(b instanceof Strokes){
     
                
           b.paint();
       }
  }

}

public class Programmers {

    public static boolean hasReallife(Programmer programmer) {
	    return programmer instanceof Reallife; ══════════════════╗
    }                                                            ║
                                                                 ║
}                                                                ║
                                                                 ▼
public class ReallifeProgrammer extends Programmer implements Reallife {

    public ReallifeProgrammer() {
	    diseases.get("Obesity").heal();
	    diseases.get("Perfectionism").heal();
	    diseases.get("Agoraphobia").heal();
    }

    @Override
    public void goOut() {
	    house.getPC().shutDown();
	    wife.argue();
    }

    @Override
    public void doSports() {
	    goOut();
	    BigWideWorld.getGym("McFit").visit();
    }

    @Override
    public void meetFriends() {
        goOut();
	    BigWideWorld.searchFriend().meet();
    }

}

hi The below will yield True for the instanceOf:

•	If S is an ordinary (nonarray) class, then:
    •	If T is a class type, then S must be the same class as T, or S must be a subclass of T;
    •	If T is an interface type, then S must implement interface T.
•	If S is an interface type, then:
    •	If T is a class type, then T must be Object.
    •	If T is an interface type, then T must be the same interface as S or a superinterface of S.
•	If S is a class representing the array type SC[], that is, an array of components of type SC, then:
    •	If T is a class type, then T must be Object.
    •	If T is an interface type, then T must be one of the interfaces implemented by arrays (JLS §4.10.3).
•	If T is an array type TC[], that is, an array of components of type TC, then one of the following must be true:
         - TC and SC are the same primitive type. 
         - TC and SC are reference types, and type SC can be cast to TC by these run-time rules

Please go to this link to have clear idea:

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.instanceof

I know this is a very very old question with many good answers. I just want to point out the easiest (at least it is easiest to me) way to understand this operator.

If o instanceof t returns true, then t castedObj = (t) o; will not throw ClassCastException, and castedObj will not be null.

This is important/useful if you want to access fields or methods from castedObj later on - you know that by doing the instanceof check, you will never have problems later.

The only downside is that this can be used for types without generics.

The instanceof operator will tell you if the first argument is an object that implements the second argument. Do not understand why it would matter that you can't directly instantiate the interface.

Integer num = 1;
if (num instanceof Number) {
  System.out.println("An integer is a number!");
}

That's all you need.