Dan's Mock #4 Q. 25

[Maria Garcia]

  
class A {
  void m1(A a) {System.out.print("A");}
}
class B extends A {
  void m1(B b) {System.out.print("B");}
}
class C extends B {
  void m1(C c) {System.out.print("C");}
}
class D {
  public static void main(String[] args) {
    A c1 = new C();
    A c2 = new C();
    A c3 = new C();
    C c4 = new C();
    c1.m1(c4);
    c2.m1(c4);
    c3.m1(c4);
  }
}

Why is the output AAA and not CCC ?

[Reid M. Pinchback]

The short answer is "because this is Java, not C++".
The long answer is that you have to keep track of what type of reference you are using at any given time.
Consider the line "c1.m1(c4)".
c4 is a C reference.
c1 is an A reference.
A objs have method m1 that takes an A argument
c4 gets converted into an A reference
the c1 method gets invoked
This isn't like C++ where you invoke according to the most resolved type, one of those subtle language differences. If class A had an overloaded method that also took a C reference as an arg, then you'd get the behaviour you expected. Resolution is done in terms of information available at compile-time, and so the definition of A (the reference type) determines what gets invoked.
Much the same argument for the other two statements.

[Tom Adams]

Here is my take on it...
In the C class m1 is an overloaded method not an overridden method. Overrides must have the exact same signature as the original. Thus the call from the A object reference call can only invoke the method in class A.
If the m1 method in class C was called p1 it wouldn't be confusing...but with a different signature it might as well have a different name.
Any method call from the A object reference not defined within that class = you can't get there from here.
Alternate version of the class with comments...took out some of the stuff that didn't seem? to matter.

class A {  
void m1(A a) {
System.out.print("A");
}
void m2(A a) {
System.out.print("A");
}
}
class C extends A {
void m1(C c) {// Overload (signature not =)
System.out.print("C");
}
void m2(A a) {// Overridden (signature =)
System.out.print("C");
}
}
class D {
public static void main(String[] args) {
A c1 = new C();
C c4 = (C)c1;
c1.m1(c4);// Overloaded Method = A
c1.m2(c4);// Overridden Method = C
}
}

"...the reference type (not the object type) determines which overloaded method is invoked"
- Sun Certified Programmer & Developer for Java 2 Study Guide (Exam 310-035 & 310-027)

[Mellihoney Michael]

polymorphism and overloading,overriding is the key exam point here!!!