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Jari,

The answer is to use interfaces as follows:

Interface Callbackable {

  void callBack();

}

// Note addition of implements clause to your Class A:

Class A extends Object implements Callbackable {
B b = new B();

void doIt(){
//start here and call a run()-method on
// the class B
  b.run(this);
  }
void callBack(){
  // do something
  // here
  }
}

Class B {

void run(Callbackable caller){
  }
}

Jari Nurminen wrote:

> Hey you Gurus there,
>
> To implement a callback I try to make a following
> cast-decision at runtime:
>
> Class A extends Object{
> B b = new B();
>
> void doIt(){
> //start here and call a run()-method on
> // the class B
>   b.run(this);
>   }
> void callBack(){
>   // do something
>   // here
>   }
> }
>
> Class B {
>
> void run(Object caller){
> // cast Object to A
>   ((A)caller).callBack();
> // the parameter in run-method is Object because the calling object could
> also
> // be any class class which implements the callBack() method (different
> // inheritance tree and/or package from class A), but if the
> // caller object is e.q. a class 'D' I should edit all the casting calls
> which might
> // be many.
> // caller.callBack() doesn't compile because Object doesn't implement
> callBack()
> // caller.getClass() returns runtime class of the object (type Class), but
> the
> //  caller.getClass().callBack() has same problem as the previous: it won't
>
> //   compile (because caller.getClass() doesn't return the instance of
> caller)
>   }
> }
>
> Is there a zero-admin solution to this when the caller object changes like:
>
> - A method which returns the instance of a class e.q. caller.getInstance()
>   (hard to believe, because what would be the return type of that
> function?)
> - Casting using getClass() method e.q.
> (caller.getClass()).caller.callBack()
>   (like this it doesn't work though). Since you can find out what is the
>   runtime class of an object you should be able to cast some other objects
>   to it, shouldn't you?
>
> I don't want any solutions how to implement it another way, the example was
> just to make clear what I'm talking about and the question is
> "is there this kind of runtime casting in Java?"
>
> cheers,
>                 Jari