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 problem with this code is that if it were legal, Java wouldn't be able to respect the private modifier of foo if you accessed it indirectly through the One base class. For example, if I were to write

One obj = new Two();
obj.foo();

Then we'd be in trouble because we'd be calling the private method foo of Two indirectly, since when the compiler checks the line obj.foo() it looks at One to determine if foo is accessible, not at Two. The reason for this is that the compiler can't always tell what obj could be pointing at - if, for example, I write something like

One obj = Math.random() < 0.5? new One() : new Two();
obj.foo();

Then the compiler can't know whether obj points at a One or a Two. Consequently, it defers to One when checking access specifiers. If we were indeed allowed to mark foo private in Two, then the compiler would incorrectly allow us to call it through obj, which has type One, bypassing the guarantee that only the object itself can call private methods.