Java Inheritance issue

Question

While exploring for scjp questions, I came across this behaviour which I found strange.

I have declared two classes Item and Bolt as follows:

class Item {
    int cost = 20;

    public int getCost() {
        return cost;
    }
}

class Bolt extends Item {
    int cost = 10;

    public int getCost() {
        return cost;
    }
}

and tried to access the value of cost twice

public class Test {
    public static void main(String[] args) {
        Item obj = new Bolt();
        System.out.println(obj.cost);
        System.out.println(obj.getCost());
    }
}

The output I get is 20 10. I can't understand how this happens.

Answer 1

obj is a reference of type Item hence the first 20 since the value of cost field of Item is 20. The second value is 10 because the runtime type of obj is Bolt and hence getCost() invokes getCost of Bolt class (since Bolt extends Item).

In short, runtime polymorphism is applicable to only instance members (method overriding) and not instance fields.

Answer 2

Class fields do not participate in polymorphism game. The methods do.

So, when you access the field you go to one that is defined in base class because you object's type is Item. When you call method you get the actual value because you invoke method using polymorphism.

Conclusion:

Fields are always private. If you want to access field write method.