Overriding private methods in (non-)static classes

Question

I have this test code example:

public class Test {

    private static class Test3 {

        private void print1() {
            System.out.println("1");
        }
    }
    private static class Test4 extends Test3 {

        private void print1() {
            System.out.println("2");
        }
    }

    public static void main(String[] args) {
        System.out.println("Overriden call to private method ----------------");
        OuterTest.Test1 test1 = new OuterTest.Test1();
        OuterTest.Test2 test2 = new OuterTest.Test2();
        OuterTest.Test1 test12 = new OuterTest.Test2();

        test1.invokeOverriden();
        test2.invokeOverriden();
        test12.invokeOverriden();

        System.out.println("Call to private method from parent class ----------------");

        test1.invokeNotOverriden();
        test2.invokeNotOverriden();
        test12.invokeNotOverriden();

        System.out.println(" Some magic ----------------");

        Test3 test3 = new Test3();
        Test4 test4 = new Test4();
        Test3 test34 = new Test4();

        test3.print1();
        test4.print1();
        test34.print1();
    }
}

class OuterTest {

    public static class Test1 {
        public void invokeOverriden() {
            print1();
        }
        public void invokeNotOverriden() {
            print1();
        }

        private void print1() {
            System.out.println("1");
        }
    }

    public static class Test2 extends Test1 {

        @Override
        public void invokeOverriden() {
            print1();
        }
        private void print1() {
            System.out.println("2");
        }
    }
}

First, all works as I think it should:

Overriden call to private method ----------------
1
2
2

Then, inherited class's private method dissapears, if I called non-implemented parent method. It could be explained as "All private methods are final and hidden from derived classes", so invokeNotOverriden() doesn't know anything about methods in Test2 class:

Call to private method from parent class ----------------
1
1
1

Finally, in static class some magic suddenly appears when I call non-static private method:

Some magic ----------------
1
2
1

I expected 1 2 2 here. Why am I wrong?

I know. And I don't override private methods. I created class object as Base b = new Inherited() and called its private method. I think, b is an Inherited now. Why is it not true? — Aguinore

Victor Sorokin Victor Sorokin · Accepted Answer · 2014-10-23T08:26:02

You've got 1 2 1 in some magic section, because private methods are resolved w/o polymorphism, compiler creates call to method contained in type variable declared with, in your case Test3. Declare print1 as non-private in Test3 (and, consequently, in Test4 as it's prohibited to tighten access modifiers of methods) and see polymorphism in action, so you'll get expected 1 2 2.

Consider shorter example:

class Test {

    private static class Test3 {
        private void print1() {
            System.out.println("non-polymorphic 1");
        }

        void polymorphic() {
            System.out.println("polymorphic 1");
        }
    }

    private static class Test4 extends Test3 {
        private void print1() {
            System.out.println("non-polymorphic 2");
        }

        void polymorphic() {
            System.out.println("polymorphic 2");
        }
    }

    public static void main(String[] args) {
        Test4 t4 = new Test4();
        t4.print1();
        t4.polymorphic();

        System.out.println("======");

        Test3 t34 = new Test4();
        t34.print1();
        t34.polymorphic();
    }
}

CLARIFICATION ... to my comment to this answer: polymorphism isn't effective for access to data fields, only for methods. Consider:

private static class Test3 {
    int i = 1;
}

private static class Test4 extends Test3 {
    int i = 2;
}

public static void main(String[] args) {
    Test4 t4 = new Test4();
    System.out.println(t4.i);

    System.out.println("======");

    Test3 t34 = new Test4();
    System.out.println(t34.i);
}

Despite i declared non-private, t34.i value is 1.

Overriding private methods in (non-)static classes

3 Answers