Please explain this polymorphism twist to me.

Martin <>
Thu, 24 Apr 2008 09:05:54 -0700 (PDT)
Hi there,

I discovered a strange twist in polymorphism that leaves me in the
dark. If you can figure out what the following output will be, then
please explain to me why.

public class OverloadTest {

    public class Visitor {
    public void visit(Super s) {

    public void visit(Sub s) {

    public class Super {
    public void accept(Visitor visitor) {

    public class Sub extends Super {

    public static void main(String[] args) {
    OverloadTest o = new OverloadTest();
    Sub s = Sub();
    s.accept( Visitor());

-------------- The answer is: (scroll down)
That means, the call "visitor.visit(this);" in the type Super is
handled by "Visitor.visit(Super)" instead of "Visitor.visit(Sub)".
This occurs as especially strange to me because a look in the debugger
confirms that the actual (dynamic) type of "this" is Sub! In my
opinion, the call should therefore be dynamically handled by

I hope I didn't confuse you with Super-Sub, now, and you can tell me
where I need to review the JLS to turn on my light again.


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