mergeWith(X) in ... cannot be applied to X

[HK]

Well, in fact the message reads:

Machine.java:19: mergeWith(Action<capture of ? super CDTA>) in
Action<capture of ? super CDTA> cannot be applied to (Action<capture of
? super CDTA>)
a1.mergeWith(a2);

So this is seriously strange, isn't it. I guess two
`capture of's are generally not compatible, but why?

Harald.

 

[E.Otter]

Looking at the Java lang spec v3 (free download from sun and covers v5) and
trying to translate its super-techno-babble on pgs 89-91 on Capture
Conversions into something understandable, I would say basically the
compiler must gaurantee that the parameter types of one thing be safely
assignable to the other thing without a cast in all situations. In the case
of two <? super CDTA> that can't be gauranteed. I think the logic goes like
this, if CDTA extends ClassB and ClassB extends ClassA, then ClassB could
used with one Action and ClassA with the other, but a ClassA object can't be
assigned to a ClassB reference.

You might be able to redo the class with one ? super CDTA and the other ?
extends CDTA or do something like this:
public class Action<T super CDTA>> {

public void mergeWith(Action<T>) { }

}

Ouch my head hurts, it really hurts...
E.Otter