Re: Please explain this polymorphism twist to me.

From:

Owen Jacobson <angrybaldguy@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 24 Apr 2008 09:47:34 -0700 (PDT)

Message-ID:

<09a58be2-a230-400b-81e0-6c8c14891d70@c58g2000hsc.googlegroups.com>

On Apr 24, 12:05 pm, Martin <nim...@cs.tu-berlin.de> wrote:

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) {
            System.out.println("Visitor.visit(Super)");
        }

        public void visit(Sub s) {
            System.out.println("Visitor.visit(Sub)");
        }
    }

    public class Super {
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

    public class Sub extends Super {
    }

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

}

-------------- The answer is: (scroll down)
-
-
-
-
-
-
-
-
-
-
-
"Visitor.visit(Super)"
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
"Visitor.visit(Sub)".

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.

In

    public class Super {
        public void accept(Visitor visitor) {
            visitor.visit(this);
        }
    }

the static (compile-time) type of the expression 'this' is Super. You
will have to override accept in Sub (you can use identical code),
where the static type of 'this' is Sub.

Static types are *always* used to select methods to dispatch to, in
Java.

-o