Why can’t we override a base class method with private extended class method?

Question

class One {
    void foo() { }
}
class Two extends One {
    private void foo() { /* more code here */ }
}

Why is the above snippet of code wrong?<

Answer 1

The given answers give you the technical explanation why you cannot extend One by Two. I would like to give you an understanding why this is not possible due to the object oriented pattern and not because of the language itself.

Usually the class One is a general definition of a class with its accessors, the methods, to the outer world. Sublcasses which extend this class must provide the same accessors to the outer world. Two extends One in your example, what means, that Two provides the same accessors to the outer world like One does. If you would change the visibility of the accessors of One, the outer world could no longer access your class as they are used to do it with an object of type One.