public class Animals {

public static abstract class Animal {

public abstract void speak();

}

public static class Mammal extends Animal {

public void speak() {

System.out.println("Hello!");

}

}

public static class Dog extends Mammal {

public void speak() {

System.out.println("Woof, woof!");

}

public void wagTail() {

System.out.println("(wags tail)");

}

}

public static class Cat extends Mammal {

public void speak() {

System.out.println("Meow!");

}

}

public static void main(String[] args) {

Mammal fido = new Dog();

fido.speak();

// Dog fido2 = fido; // Won't compile: Java won't implicitly downcast

((Mammal) fido).speak();

Mammal mittens = (Mammal) new Cat(); // Safe

Mammal sparky = new Mammal();

// Compiles, but will cause a ClassCastException at run-time,

Dog huh = (Dog) sparky;

// so we won't even get here.

huh.wagTail();

}

}