Question

I’m trying to declare an interface that contains a method which will return a list of things that implement both Comparator<Object> and Action, i.e.

<T extends Comparator<Object> & Action> List<T> getThings();

This compiles fine, but the problem comes when I try to call this method. I want to be able to do this:

List<Action> things = getThings();
List<Comparator<Object>> things = getThings();

When I try to do this I get the following compilation error:

 incompatible types; no instance(s) of type variable(s) T exist so that
 java.util.List<T> conforms to java.util.List<javax.swing.Action>
 found   : <T>java.util.List<T>
 required: java.util.List<javax.swing.Action>

The following doesn’t work either:

List<? extends Action> things = getThings();
List<? extends Comparator<Object>> things = getThings();

Another way to achieve this effect is to create an empty interface that extends both Comparator<Object> and Action and use that as the return type, i.e.

public interface ComparatorAction extends Comparator<Object>, Action { }
List<ComparatorAction> getThings();

But I don’t want to have to do this. There’s got to be a way to do what I want, right? Any ideas?

Thanks!

P.S. I’m having a tough time coming up with a good title for this post so feel free to change it.

Answer 1

You could also parameterize the method(s) from which you call getThings(). For instance:

public static <U extends Comparator<Object> & Action> void main(String[] args) {
    List<U> l = getThings();
    Action a = l.get(0);
    Comparator<Object> c = l.get(0);
}