Java Inheritance Puzzle

Question

I have created the following puzzle for inheritance in Java:

Animal.java

public class Animal {
    private String sound;

    public void roar() {
        System.out.println(sound);
    }

    public void setSound(String sound) {
        this.sound = sound;
    }
}  

Tiger.java

public class Tiger extends Animal {
    public String sound;

    public Tiger() {
        sound = "ROAR";
    }
}

Jungle.java

public class Jungle {
    public static void main(String[] args) {
        Tiger diego = new Tiger();

        diego.roar();
        diego.sound = "Hust hust";
        diego.roar();
        diego.setSound("bla");
        diego.roar();
        System.out.println(diego.sound);
    }
}

Output:

null
null
bla
Hust hust

I guess this weird behaviour is taking place, because sound in Animal is private while sound in Tiger is public. But can you explain (and tell me the relevant parts of the JLS) why this happens?

Answer 1

Fields are not polymorphic, methods are polymorphic.

 diego.roar();

calls roar() method in Animal and prints sound from Animal.

diego.sound = "Hust hust";

Sets sound value in Tiger class sound variable

diego.roar();

returns null; because prints sound from Animal, which is still null. Above sound assignment reflects on Tiger class variable, not Animal class.

diego.setSound("bla");

sets Animal sound to bla

diego.roar();

prints bla because setSound update sound variable of Animal class with bla.

System.out.println(diego.sound);

prints Hust hust due to the fact that diego is of type Tiger and you have accessed field sound of Tiger and fields are not polymorphic.

Please refer java language specification 8.3 for more details.