Dear class,

It seems I've managed to mess up another example.  In class on Thursday (4/17), we made the following change to B's h method:  Instead of h taking a B argument, we changed it to take an Object argument.  While this is valid and classes A and B and C will compile, the call x.h(w) is now ambiguous.  The ambiguity arises when a subclass overloads a method with a "more lenient" argument.  In this case, we overloaded h with the more lenient Object.  (More lenient means Object is higher on the branch than A).  To the compiler, both A's h(A) and B's h(Object) match.  The compiler cannot tell which one to run.  Do not worry; I will not have a trace problem with ambiguities on the exam.  Again, you can assume on the exam that the entire trace problem compiles.  Overloaded methods on the exam will only have "more strict" arguments, never "more lenient" arguments.  If you stick to the algorithm we discussed in class, you'll be fine.

class A {
  public void f(A o) { System.out.print("A-f"); }
  public void h(A o) { System.out.print("A-h");  }
  public void m() { System.out.println("  A-m"); }
}

class B extends A {
  public void f(A o) { System.out.print("B-f"); }
  public void h(Object o) { System.out.print("B-h"); }    // altered method
  public void m() { System.out.println("  B-m"); }
}

class C extends B {
  public void h(C o) { System.out.print("C-h"); }
}

public class Untitled2 {
  public static void main(String[] args) {
    A w = new A();
    B x = new B();
    A y = x;
    C z = new C();
    x.h(w);                // ambiguous call
  }
}

Note that this ambiguity does not occur when a subclass overloads with a more strict argument.  (i.e. try doing the call x.h(z) on the original problem (when B's h took a B argument).  The ambiguity also does not occur when you overload in the same class.  (Try putting an h(Object) in the A class...no errrors).

For those of you not in class, the algorithm I'm talking about is here:

http://www.cs.ucf.edu/courses/cop3330/spr2003/methods.html  Follow this algorithm on the final exam.

While I've got your attention, THE FINAL EXAM IS FRIDAY APRIL 25 AT 7PM IN OUR REGULAR CLASSROOM.

Feel free to email me about this issue.  I'll will be available by appointment up until Friday.

Mat