Java – contains type inference for return types, wildcards, and cross types

I'm trying to declare an interface that contains a method that will return an implementation comparator < Object > and action, that is

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

This compilation is good, but when I try to call this method, a problem occurs I want to do this:

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

When I try to do this, I get the following compilation errors:

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 are also invalid:

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

Another way to achieve this effect is to create an empty interface that uses comparator < Object > and action as return types, that is

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

But I don't want to There must be a way to do what I want, right? Any ideas?

thank you!

PS: I have a hard time proposing a good title for this position, so change it at any time

Solution

You can also parameterize the method that calls getthings () For example:

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);
}