CODEWITHSUNDEEP
javalanguage-lawyer
csharpfolk
csharpfolk

Reputation: 4280

Extending private class in Java

I found the following program puzzling:

package package1;

public class Main {
    private static class A { }

    public static class B extends A { }
}

// ----------------
package package2;


public class Test {
    public static void main(String[] args) {
        Main.B b = new Main.B();
    }
}

Why Java allows public class B to extend private class A? In other languages e.g. C# class may only extend private class when it is private itself.

Can you provide any scenario when extending private class may be useful?

Upvotes: 4

Views: 8483

Answers (2)

ddarellis
ddarellis

Reputation: 3960

When you have functionality that is not really related to your class but cant stand alone, for example onclick events and so on... you create an inner class because the logic is only worth for this class. Beyond that you can extend and do whatever your program logic needs with that.

Also a very good OCP question.

Upvotes: 0

leeyuiwah
leeyuiwah

Reputation: 7152

This will allow A to be an implementation detail (not exposed to outside) yet reusable internally (be the basis of B). To users of B, what they care is B can fulfill all the contracts as exposed by its API. The fact that internally B uses A to fulfill those contracts should not bother any external users, right?.

Imagine that there can be many more sub-classes C, D, E, F, etc of A. Code of A can be reused by them.

Upvotes: 1

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