// duynotes blog
public class Subject {
  private ArrayList<Observer> observers = new ArrayList<Observer>();
  public void registerObserver(Observer observer) {
    observers.add(observer);
  }
  public void unregisterObserver(Observer observer){  
  observers.remove(observer);
}
  public void notify() {
    for (Observer o : observers) {
      o.update();
    }
  }
}