inheritance in java does not work the way I expected

Question

I have the following class:

public class X {
    public void A() {
        B();
    }

    private static void B() {
        System.out.println("111111111");
    }
}

Now I have the following inherited class Z:

public class Z extends X {

    private static void B() {
        System.out.println("22222222");
    }
}

now if I do

Z myClass = new Z();
Z.A();

I will get: 111111111 as a result. (Also, eclipse tells me that B() in the inherited class is never called).

Why? And how can I run the inherited B method?

static methods are not called polymorphically. They are statically bound at compilation time. Use an instance method. And don't make it private either: private methods are private, and thus can't be overridden. https://docs.oracle.com/javase/tutorial/java/IandI/override.html — JB Nizet, Nov 01 '15 at 12:35

Avi · Accepted Answer · 2015-11-01T12:54:02.083

1

The B methods are static. When you call the method A it uses the implementation of class B (because that's where the method A is defined). Class B is not aware of the existence of class Z and cannot call a method of class Z.

Because the method is static, it's not overridden upon inheriting from B. Polymorphism only works with instances of a class. Static method does not play the polymorphism game and cannot be overridden.

edited Nov 01 '15 at 12:54

answered Nov 01 '15 at 12:49

Avi

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That was what I afraid of. Thanks for the explanation. – dsb Nov 01 '15 at 12:51
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It's also not overridden because the method is `private`. A private method is not even seen in a subclass, so it can't ever be overridden. If you give it a different access modifier, then the fact that it is also `static` comes into play. – Erwin Bolwidt Nov 01 '15 at 13:10

score 1 · Answer 2 · edited May 23 '17 at 12:23

1

Change access modifier of method from private static to protected

If you re-define base class non-static & non-private method in derived class, it's called overriding.

If you re-define base class static method in derived class, it's called method hiding or method shadowing.

You have done hiding rather overriding in your example.

Have a look at this SE Question

edited May 23 '17 at 12:23

Community

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answered Nov 01 '15 at 12:52

Ravindra babu

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score 0 · Answer 3 · answered Nov 01 '15 at 12:36

0

You're calling X's inherited A method, and this calls its private B method.

private methods and attributes are not inherited.

answered Nov 01 '15 at 12:36

Julen

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Thank you very much. I changes it from private to protected but still the same behaviour. Any idea why? – dsb Nov 01 '15 at 12:49
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That's not the reason. The reason is that the method is static. – Avi Nov 01 '15 at 12:49
@Avi Actually it is very much the reason, just as the fact that it is static is the reason. – Erwin Bolwidt Nov 01 '15 at 13:10

score 0 · Answer 4 · answered Nov 01 '15 at 13:08

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It looks like you are overriding method B() in class Z but method B() is not overridden here. Since B() is static so class A and class Z has there own method B(). Scope of all the Static methods and variables are class level not an object level.

answered Nov 01 '15 at 13:08

KP_JavaDev

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inheritance in java does not work the way I expected

4 Answers4