Need ideas - pattern using generics

Question

Recently I tried to implement sth. simillar to ActionListeners pattern in Swing.
(this is common pattern for handling actions etc.)
... I encountered few problems (compiler errors, warnings).
I prepared sample code that highlights it.
---- Main.java ----
package genericstest;
import java.util.ArrayList;
class Event {
public String toString() {
return "base event";
interface GenericListener<T extends Event> {
public void doAction(T ev);
class MyEvent extends Event {
public String toString() {
return "my event";
class MyListener implements GenericListener<MyEvent> {
//this method implements interface method
//- as far as I know it is legal according to spec
//and sth I really like in java generics
public void doAction( MyEvent e) {
System.out.println("in my listener:"+e.toString());
public class Main {
public static void main(String[] args) {
new Main().run();
private void run() {
Event me = new MyEvent();
//solution that I chosen - with one cast - works, compiles w/o warnings
ArrayList<GenericListener<Event> > listns =
new ArrayList< GenericListener<Event> >();
//here is the cast I would like to avoid
GenericListener<Event> el = (GenericListener<Event>) new MyListener();
listns.add( el);
for (GenericListener<Event> l : listns)
l.doAction( me );
//my: first solution - compiler gives warning [unchecked]
//use -Xlint:unchecked flag
/*ArrayList<GenericListener > listns = new ArrayList<GenericListener >();
listns.add( new MyListener());
for (GenericListener l : listns) {
l.doAction( me );//here is warning
//one of not compiling solutions - I tried to go with "? extends" and "? super"
//but got no sensible results
ArrayList<GenericListener<? extends Event> > listns = new ArrayList<GenericListener<? extends Event> >();
listns.add( new MyListener());
for (GenericListener<? extends Event> l : listns) {
l.doAction( me //this produces compiler error
---- End of file -----
Code above works (as expected) but contains one small cast that I would like to avoid.
I was trying to rewrite it but never got anything satysfying (see in sample).
After reading topic:
http://forum.java.sun.com/thread.jsp?thread=512445&forum=316&message=2541231
I understood all errors, warnings...problem... but finally I realised that "probably" I am not able to write my code without using casts (or having warnings). (The problem is that in one place I need both contrvariance and covariance (if aI am calling things correctly - adding to generic collection and retriving from same collection) ).
So the question is:
Does anybody have any idea how to rewrite code above ? I am particullary interested in using
this new small language feature that allows me to write my own ActionListeners without using casts (see MyListener).

Answer 1

First, it is good to understand what you want to do, otherwise there's no chance to convience compiler that you are doing right :)
From this code: Event me = new MyEvent(); It is obvious that you want to deliver MyEvent to some listeners. If you are delivering MyEvent, then you should have a list of MyEvent listeners (listeners for any other Event are not good -- they cannot process MyEvent anyway, because they expect arguments of different type). So, why in your code you are trying to declare a List<GenericListener<Event>>? If you want to have a list of MyEvent listeners, then you shall have List<GenericListener<MyEvent>>.