instantiation of an extended type with baseclass

[Mike Nightsky]

I hope i did not dublicat a question, but i didn't know how formulate may search query to find my anser. By the way i searched but didn't come up with a n answer.

My question.
I have a base class -> A
and i have a class thats extends A -> Ab (its extends it only in methods not in data)

if i get a A and want a Ab is there a simpler way to acompolish this than
crate an constructor in Ab that takes an A and pulls out the values and puts it into the super()-constructor?

Maybe its a intermediate question, but i didn't came up with a solution.
thanx in advance

[Paul Sturrock]

You could just cast your object to the type you want.

[Mike Nightsky]

Thanx Mike

[Stan James]

I'm not sure what you're trying to do. Maybe this?

A one = new A();
Ab two = (Ab)one;

That won't work because one is an instance of A which IS NOT an Ab. If you really need an Ab you might do what you suggested:

Ab two = new Ab(one);

This is a good time to ask yourself if "extends" was the right way to define Ab. An alternative is to make Ab just extend Object, hold a reference to an A instance, and pass any method calls through to that instance. For polymorphism you could make A and Ab implement the same interface for the methods on A.

public class Ab implements InterfaceA
{
   Private A a;
   public Ab( A a )
   {
      this.a = a;
   }
   public int anAMethod()
   {
      return a.anAMethod();
   }
   public int aNewAbMethod()
   {
      return 1;
   }
}

Does that fit the problem?