A note of Polymorphism.

Polymorphism.java

abstract class Animal {
    abstract String talk();
}

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

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

static void doTalk(final Animal a) {
    println(a.talk());
}

static void main(String[] args) {
    doTalk(new Cat());
    doTalk(new Dog());
}