Tricky question about polymorphism

[doni klauz]

Hi,

I've encounter the following suprising behaviuor with polymorphism and I need someone to explain it to me.

This is simple example :

abstract Class A
{
void test(A a)
{
System.out.println("You are in A");
}
}

Class B extends A
{
void test(B b)
{
System.out.println("You are in B");
}
}



A b1 = new B();
A b2 = new B();
b1.test(b1);

result:

You are in A

[Ernest Friedman-Hill]

Hi Doni,

Welcome to JavaRanch!

The short answer is that there's no polymorphism here. The two different "test" methods have different argument lists, so they are not polymorphic; they're said to be overloaded. It's up to the compiler to decide which of a set of overloaded methods is called, and the compiler looks only at the type of the variable on which the method is called. In contrast, the choice among polymorphic methods is made at runtime based on the actual type of the object.

In any event, here, the compiler sees only that the variable is of type "A", and it looks for a method "test" in "A" that it can call with an argment of type "B", and finds one, and calls it.

Now, if you change the argument type of B.test() to be "A", then you'd see polymorphic behavior. Try it!

[Corey Hollaway]

/// ****************************
/// POLYMORPH: Testing polymorphism
/// ****************************
public class Polymorph{
public static void main(String[] args){
System.out.println("I'm beaming you into B!");
B myB = new B();
myB.test(myB);
 
System.out.println("I'm beaming you into A!");
A myA = new B();
myA.test(myA);// But the instance of myA is B!
// doni klauz is right
// this is messed up
 
// Because the test in myA should only accept (new A()'s)
// Instances of the A class should be the only objects
// that could enter the A.test(A a) method  :(  
 
System.out.println("I'm beaming you into B!");
myB.test();
 
System.out.println("I'm beaming you into A!");// NNOOO Scotty!
myA.test();
}
}
 
/// ****************************
/// A: Superclass
/// ****************************
abstract class A{
public void test(A a){
System.out.println("In A!");
}
public void test(){
System.out.println("AAA!");
}
}
 
/// ****************************
/// B: Subclass
/// ****************************
class B extends A{
public void test(B b){
System.out.println("In B!");
}
public void test(){
System.out.println("BBB!");
}
}

[Ernest Friedman-Hill]

// Because the test in myA should only accept (new A()'s)
// Instances of the A class should be the only objects
// that could enter the A.test(A a) method

But note that class B extends class A, and therefore, by definition, a B is an A. An instance of B can correctly be passed to either A.test(A) or B.test(B); an instance of A could only be passed to A.test(A).
[ July 22, 2004: Message edited by: Ernest Friedman-Hill ]

[Corey Hollaway]

I see now, thank you
[ July 22, 2004: Message edited by: Corey Hollaway ]

[doni klauz]

Thanks