Generic observable pattern

Observable.java

package com.dougkeen.util;

import java.util.WeakHashMap;

import org.apache.commons.lang3.ObjectUtils;

public class Observable<T> {
	private T value;
	private WeakHashMap<Observer<T>, Boolean> listeners = new WeakHashMap<Observer<T>, Boolean>();

	public Observable() {
		super();
	}

	public Observable(T value) {
		super();
		this.value = value;
	}

	public T getValue() {
		return value;
	}

	public void setValue(T value) {
		if (!ObjectUtils.equals(this.value, value)) {
			this.value = value;
			notifyOfChange(value);
		}
	}

	public void registerObserver(Observer<T> observer) {
		listeners.put(observer, true);
	}

	public void unregisterObserver(Observer<T> observer) {
		listeners.remove(observer);
	}

	protected void notifyOfChange(T value) {
		for (Observer<T> listener : listeners.keySet()) {
			if (listener != null) {
				listener.onUpdate(value);
			}
		}
	}
}

Observer.java

package com.dougkeen.util;

public interface Observer<T> {
	void onUpdate(final T newValue);
}