Polymorphism and private methods

Question

In the following example, why does show() invoke TestA.m3 instead of TestC.m3 when the target object is an instance of TestC?
package lib;
public class TestA {
public void show() { m1(); m2(); m3(); }
void m1() { System.out.println("A1"); }
void m2() { System.out.println("A2"); }
void m3() { System.out.println("A3"); }
package pkg;
public class TestB extends lib.TestA {
void m1() { System.out.println("B1"); }
void m2() { System.out.println("B2"); }
void m3() { System.out.println("B3"); }
package lib;
public class TestC extends pkg.TestB {
void m1() { System.out.println("C1"); }
int m2() { System.out.println("C2"); return 0; }
private void m3() { System.out.println("C3"); }
public void test() {
((TestA)this).m3(); //verify accessibility of TestA.m3
//default package
public class Test {
public static void main(String[] args) {
new lib.TestC().show();
new lib.TestC().test();
Output from Test:
C1
A2
A3
A3
m3 in TestA is accessible from TestC (JLS 6.6.1). Since TestC is a subclass of TestA and TestA.m3 is non-private and accessible from TestC, TestC.m3 overrides TestA.m3 (JLS 8.4.6.1). Since TestC.m3 overrides TestA.m3, TestC.m3 is the method to be invoked (JLS 15.12.4.4).

Answer 1

Hi,
m3 in TestA is accessible from TestC (JLS 6.6.1). Since TestC is a
subclass of TestA and TestA.m3 is non-private and accessible from
TestC, TestC.m3 overrides TestA.m3 (JLS 8.4.6.1). Since TestC.m3
overrides TestA.m3, TestC.m3 is the method to be invoked (JLS
15.12.4.4). The problem is the nasty package visibility (I hate it):
Let's begin ;-)
TestB in fact does not see any of the methods of it's super class,
so it can't override them. It is in a different package.
TestC does see all methods of TestA but none of the methods of TestB.
So it overrides m1(). (TestB does not play any role in this example!!)
m2() and m3() have package visibility, too. Usually you can only
open visibility wider but not restrict visibility. Furthermore
you must not redefine methods with a different return type (m2() of TestC seems to do so, but only seems ;-))).
In fact m2 and m3 of TestC are completely new methods, TestA doesn't
know about.
Clear?
If not, just for fun mark a method of TestA protected or public.
The compiler will cry ...
Regards.
Karl