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

Most people have correctly explained the "What" of this question but no one explained "How" correctly.

So here's a simple illustration:

String s = new String("Hello");
if (s instanceof String) System.out.println("s is instance of String"); // True
if (s instanceof Object) System.out.println("s is instance of Object"); // True
//if (s instanceof StringBuffer) System.out.println("s is instance of StringBuffer"); // Compile error
Object o = (Object)s;
if (o instanceof StringBuffer) System.out.println("o is instance of StringBuffer"); //No error, returns False
else System.out.println("Not an instance of StringBuffer"); // 
if (o instanceof String) System.out.println("o is instance of String"); //True

Outputs:

s is instance of String
s is instance of Object
Not an instance of StringBuffer
o is instance of String

The reason for compiler error when comparing s with StringBuffer is well explained in docs:

You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

which implies the LHS must either be an instance of RHS or of a Class that either implements RHS or extends RHS.

How to use use instanceof then?
Since every Class extends Object, type-casting LHS to object will always work in your favour:

String s = new String("Hello");
if ((Object)s instanceof StringBuffer) System.out.println("Instance of StringBuffer"); //No compiler error now :)
else System.out.println("Not an instance of StringBuffer");

Outputs:

Not an instance of StringBuffer