Generics == bashing your head against a wall

Well, I've been spending a few hours generifying commons-collections recently. The jobs only just started, and my head hurts from all the times I've banged it against the wall. Is there anybody listening who might actually be able to communicate to the powers that be in Sun that generics are just way messed up?

This time, my problem is in CompositeCollection. It holds a collection of collections, and makes it look like a single collection. Simple enough use case. So here is a simple generification (showing the methods that add a composited collection to the collection of composited collections):

public class CompositeCollection<E> implements Collection<E> {

  private Collection<Collection<E>> all;

  public void addComposited(Collection<E>);
  public void addComposited(Collection<E>, Collection<E>);
  public void addComposited(Collection<E>[]);

}

Thats great! It works and we're all happy, right? Well, not really, because those three addComposited() methods can be simplified using varargs of course:

public class CompositeCollection<E> implements Collection<E> {

  private Collection<Collection<E>> all;

  public void addComposited(Collection<E>...);

}

Now isn't that a much simpler API. We can pass in an array, one, two, or any number of collections we like, right? Wrong.

The problem is that calling this method is totally screwed due to the messed up generics implementation:

public class MyClass {

  private void process() {
    Collection<String> a = new ArrayList<String>()
    Collection<String> b = new ArrayList<String>()
    CompositeCollection<String> c = new CompositeCollection<String>;

    c.addComposited(a, b);    // compiler warning!!!!!!!!
  }

}

Why is there a compiler warning? Because in order to pass a and b to the method the varargs implementation needs to create an array - Collection<String>[]. But the generics implementation can't create a generified array, yet thats what the method is demanding. Arrrgh!!

So, instead of being helpful, Java creates an ungenerified Collection[], and then the generics system warns us that it is an unchecked cast to make it a Collection<String>[]. But this conversion is all internal to the compiler! As far as the developer is concerned, there is nothing even vaguely type unsafe about what s/he is doing.

Now this is just headbangingly stupid. There is absolutely no risk of breaking the generics model here. No risk of a ClassCastException or ArrayStoreException. No, this is really just another case of generics being nearly completely broken with regards to arrays. And the varargs group didn't manage to realise/coordinate and get a decent implementation that works.

The problem for me is serious though. I am writing an API (commons-collections) that will be widely used, and will have to retain backwards binary compatibility. If I go with the varargs option (the logically correct choice), then every client of this method will get a useless java warning - not very friendly. The alternative is to go back to the original design and have three methods, an array, one-arg and two arg. And really thats just crap.