Why do I have to write this code? ListDataEvent has the type. Why are there three different methods to call on the listener? The *listener* should unpack the event!

TaskQueue.java

    private void notifyListeners(ListDataEvent lde) {
        synchronized (_listeners) {
            for (ListDataListener ldl : _listeners) {
                switch (lde.getType()) {
                    case ListDataEvent.CONTENTS_CHANGED:
                        ldl.contentsChanged(lde);
                        break;
                    case ListDataEvent.INTERVAL_ADDED:
                        ldl.intervalAdded(lde);
                        break;
                    case ListDataEvent.INTERVAL_REMOVED:
                        ldl.intervalRemoved(lde);
                        break;
                }
            }
        }
    }