Casting to a recursively bounded type

Question

I defined a Java interface to represent the ability of an object to copy itself (no, I don't want to use Cloneable, but thanks for the suggestion ;-). It's generic:

public interface Copiable<T> {
    T copy();
}

An object will only make copies of its own type:

public class Foo implements Copiable<Foo> {
    Foo copy() { return new Foo(); }
}

Now, I'm reading Class<?> objects from a non-generic API (Hibernate metadata, if that is of interest to anyone), and, if they are Copiable, I want to register them with one of my components. The register method has the following signature:

<T extends Copiable<T>> register(Class<T> clazz);

My problem is, how do I cast the class before passing it to the method? What I'm currently doing is:

Class<?> candidateClass = ...
if (Copiable.class.isAssignableFrom(candidateClass)) {
    register(candidateClass.asSubclass(Copiable.class));
}

This does what I want, but with a compiler warning. And I don't think I'll be able to express the recursive bound in a runtime cast.

Is there a way to rework my code to make it typesafe?

Thanks

Answer 1

There's no way to cast non-generic API to generic and not get a warning. You can use @SuppressWarnings("unchecked") annotation to suppress it.
Class<?> is rather pointless in this case, too - you can use non-generic Class candidateClass instead.

Answer 2

A runtime cast to <? extends Copiable<?> fails (in the register() call) because there's no way to declare that the first ? and the second ? are the same (unknown) thing.

The asSubclass() call gives you a compiler warning for pretty much the same reason -- because the generics have been erased, there's no way at runtime to prove that a Copiable is a Copiable<AnythingInParticular>. You've proved that it's a Copiable, but you haven't proved that it's a Copiable<Itself>.

I think the best you can hope for is to localize the warnings to one messy method, and add @SuppressWarnings("unchecked") and some comments.

Do you know for sure this will work at runtime? I get the feeling the compiler's telling the truth here. Do the classes you're getting from Hibernate actually extend Copiable, or do they just happen to declare methods that match the Copiable interface? If it's the latter, then -- Java being a strongly typed language -- your isAssignableFrom() is always going to return false anyway.

Answer 3

Maybe you could apply the super type token paradigm somehow? Why do you need to constrain your register()? I think the problem is that you use Class.forName and it is too much a runtime thing.