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 mentioned in other answers, the canonical typical usage of instanceof is for checking if an identifier is referring to a more specific type. Example:

Object someobject = ... some code which gets something that might be a button ...
if (someobject instanceof Button) {
    // then if someobject is in fact a button this block gets executed
} else {
    // otherwise execute this block
}

Note however, that the type of the left-hand expression must be a parent type of the right hand expression (see JLS 15.20.2 and Java Puzzlers, #50, pp114). For example, the following will fail to compile:

public class Test {
    public static void main(String [] args) {
        System.out.println(new Test() instanceof String); // will fail to compile
    }
}

This fails to compile with the message:

Test.java:6: error: inconvertible types
        System.out.println(t instanceof String);
                       ^
  required: String
  found:    Test
1 error

As Test is not a parent class of String. OTOH, this compiles perfectly and prints false as expected:

public class Test {
    public static void main(String [] args) {
        Object t = new Test();
        // compiles fine since Object is a parent class to String
        System.out.println(t instanceof String); 
    }
}