13.4.23 Method Overriding

If an instance method is added to a subclass and it overrides a method in a superclass, then the subclass method will be found by method invocations in pre-existing binaries, and these binaries are not impacted. If a class method is added to a class, then this method will not be found, because the invocation of a class method is resolved at compile time to use the fully qualified name of the class where the method is declared. Thus if the example:

class Hyper {
	void hello() { System.out.print("Hello, "); }
	static void world() { System.out.println("world!"); }
}
class Super extends Hyper { }

class Test {
	public static void main(String[] args) {
		Super s = new Super();
		s.hello();
		s.world();
	}
}

is compiled and executed, it produces the output:

Hello, world!

Suppose that a new version of class Super is produced:

class Super extends Hyper {
	void hello() { System.out.print("Goodbye, cruel "); }
	static void world() { System.out.println("earth!"); }
}

If Super is recompiled but not Hyper or Test, then running the new binary with the existing binaries for Hyper and Test will produce the output:

Goodbye, cruel world!

This example demonstrates that the invocation in:

s.world();

in the method main is resolved, at compile time, to a symbolic reference to the class containing the class method world, as though it had been written:

Hyper.world();

This is why the world method of Hyper rather than Super is invoked in this example. Of course, recompiling all the classes to produce new binaries will allow the output:

Goodbye, cruel earth!

to be produced.