My colleague Rod presented me with the following (paraphrased):

    public interface Executable<T> {
        T execute();

The question arose: What if an implementor doesn't have a meaningful value to return?

    public class MyExecutable implements Executable<____> {
        // ...

You could fill in the blank with Object and return null or this, perhaps. However, we agreed that this doesn't communicate as well to its clients the fact that there really isn't a return value and it shouldn't even be used. You can't use primitive void since it really isn't a type, and is a primitive anyhow (forbidden as a generic type parameter). What to do?

How about java.lang.Void?

    public class MyExecutable implements Executable<Void> {
        public Void execute() {
            // do stuff
            return null;

We felt that this clearly communicated that there's no useful return value for the functor. In fact, since java.lang.Void is uninstantiable, the only value the functor could return is null. Nifty, eh? Ever think you'd use java.lang.Void?

Jaggregate internally uses a number of generic functors that really don't need to return anything. I changed them to return java.lang.Void instead of Object, and I'm happy with the results.