Question

Consider the following example:

public class Sandbox {
    public interface Listener<T extends JComponent> {
        public void onEvent(T event);
    }

    public interface AnotherInterface extends Listener<JPanel>, Listener<JLabel> {
    }
}

This fails with the following error

/media/PQ-WDFILES/programming/Sandbox/src/Sandbox.java:20: Sandbox.Listener cannot be inherited with different arguments: <javax.swing.JPanel> and <javax.swing.JLabel>
        public interface AnotherInterface extends Listener<JPanel>, Listener<JLabel> {
               ^
1 error

Why though? There is no overlap in the generated methods. As a matter of fact, that essentially means

public interface AnotherInterface {
    public void onEvent(JPanel event);
    public void onEvent(JLabel event);
}

No overlap there. So why is it failing?

---

In case your wondering what I’m doing and have a better solution: I have a bunch of Events and a Listener interface that’s almost exactly like the Listener class above. I’m wanting to create an adapter and an adapter interface, and for that I need to extend all the Listener interfaces with a specific event. Is this possible? Is there a better way to do this?

Answer 1

No. You cant.
It’s because generics are supported only at compiler level. So you can’t do thinks like

public interface AnotherInterface {
    public void onEvent(List<JPanel> event);
    public void onEvent(List<JLabel> event);
}

or implements interface with several parameters.

upd

I think workaround will be like this:

public class Sandbox {
//    ....
    public final class JPanelEventHandler implements Listener<JPanel> {
        AnotherInterface target;
        JPanelEventHandler(AnotherInterface target){this.target = target;}
        public final void onEvent(JPanel event){
             target.onEvent(event);
        }
    }
///same with JLabel
}