What is the 'instanceof' operator used for in Java?

Question

What is the instanceof operator used for? I\'ve seen stuff like

if (source instanceof Button) {
    //...
} else {
    //...
}

Answer 1

As described on this site:

The instanceof operator can be used to test if an object is of a specific type...

if (objectReference instanceof type)

A quick example:

String s = "Hello World!"
return s instanceof String;
//result --> true

However, applying instanceof on a null reference variable/expression returns false.

String s = null;
return s instanceof String;
//result --> false

Since a subclass is a 'type' of its superclass, you can use the instanceof to verify this...

class Parent {
    public Parent() {}
}

class Child extends Parent {
    public Child() {
        super();
    }
}

public class Main {
    public static void main(String[] args) {
        Child child = new Child();
        System.out.println( child instanceof Parent );
    }
}
//result --> true

I hope this helps!