Base class for Java Observable

BaseObservable.java

public abstract class BaseObservable<LISTENER_CLASS> {

    private final Object MONITOR = new Object();

    private final Set<LISTENER_CLASS> mListeners = new HashSet<>();

    public void registerListener(LISTENER_CLASS listener) {
        synchronized (MONITOR) {
            boolean hadNoListeners = mListeners.size() == 0;
            mListeners.add(listener);
            if (hadNoListeners && mListeners.size() == 1) {
                onFirstListenerRegistered();
            }
        }
    }

    public void unregisterListener(LISTENER_CLASS listener) {
        synchronized (MONITOR) {
            boolean hadOneListener = mListeners.size() == 1;
            mListeners.remove(listener);
            if (hadOneListener && mListeners.size() == 0) {
                onLastListenerUnregistered();
            }
        }
    }

    protected Set<LISTENER_CLASS> getListeners() {
        synchronized (MONITOR) {
            return Collections.unmodifiableSet(new HashSet<>(mListeners));
        }
    }

    protected void onFirstListenerRegistered() {

    }

    protected void onLastListenerUnregistered() {

    }

}

https://gist.github.com/techyourchance/6602815188294c1c58779d3e8d16f12b#file-baseobservable-java-L10
Could instead do:

boolean added = mListeners.add(listener);
           if (hadNoListeners && added) {
               onFirstListenerRegistered();
           }

https://gist.github.com/techyourchance/6602815188294c1c58779d3e8d16f12b#file-baseobservable-java-L20
Similiarly:

boolean removed = mListeners.remove(listener);
            if (hadOneListener && removed) {
                onLastListenerUnregistered();
            }