Demonstration of subtype polymorphism

SubtypePolymorphism.java

import java.util.Random;

public class SubtypePolymorphism {

    public static abstract class Animal {
        public abstract String talk();

        public void write() {
            System.out.println(this.talk());
        }

    }

    public static class Cat extends Animal {
        public String talk() {
            return "Meow!";
        }
    }

    public static class Dog extends Animal {
        public String talk() {
            return "Woof!";
        }
    }

    public static class UsingPolymorphism {

        public static void main() {
            Animal a = randomAnimal();
            a.write();
        }

    }

    public static class NotUsingPolymorphism {

        public static void main() {
            Animal a = randomAnimal();
            // 'Leaky abstraction'! maintenance pain!
            if(a instanceof Dog) {
                System.out.println("Woof!");
            } else if (a instanceof Cat) {
                System.out.println("Meow!");
            } else {
                // This is a big problem!!!
                System.out.println("I don't know what you are.");
            }
        }

    }

    // See how much simpler the code is?. It also is 
    // future proof since any type that impelemements Animal
    // will already be supported. Also, details about the behavior 
    // of animal implementations stays encapsulated in the implementations.

    public static Animal randomAnimal() {
        int r = new Random().nextInt();
        if(r % 2 == 0) {
            return new Cat();
        } else {
            return new Dog();
        }
    }

}