import java.util.*;

abstract class Subject {
  private ArrayList observers = new ArrayList();

  public void attatch(Observer observer) {
    observers.add(observer);
  }

  public void detatch(Observer observer) {
    observers.remove(observer);
  }

  public void notifyObservers() {
    for (int i=0;i<observers.size();i++) {
      Observer observer = (Observer)observers.get(i);
      observer.update();
    }
  }
}