Question

I’m getting several compiler warnings about my use of generics in one class and could use some help. I have the following setup:

Map<EventType, List<? extends Listener>> map;

I have several different event types and for each one I want to keep a list of the corresponding listener type. Listener is an interface and there are several interfaces that extend it.

Later on I add a List to this map like:

List<StateListener> list = new LinkedList<StateListener>();
list.add(someStateListener);
map.put(Events.STATE, list);

And then I get warnings later on …

List<StateListener> list = map.get(Events.STATE);

for unchecked conversion.

I read through some other posts but I didn’t really see anything that helped. Thanks any comments

Answer 1

There’s no guarantee that the List coming from map.get(Events.STATE) is a List<StateListener> since you declared the map with the value type of List<? extends Listener>

You need to cast it, or declare accordingly;

List<? extends Listener> list = map.get(Events.STATE);