Interface Container.ItemSetChangeNotifier

The interface for adding and removing ItemSetChangeEvent listeners. By implementing this interface a class explicitly announces that it will generate a ItemSetChangeEvent when its contents are modified. Note that the general Java convention is not to explicitly declare that a class generates events, but to directly define the addListener and removeListener methods. That way the caller of these methods has no real way of finding out if the class really will send the events, or if it just defines the methods to be able to implement an interface.