abstract class Subject {
  private ArrayList<Observer> observersList = new ArrayList<Observer>();
  
  // add observers
  public void Attach(Observer ob) {
    observersList.add(ob);
  }
  
  // remove observers
  public void Detach(Observer ob) {
    observersList.remove(ob);
  }
  
  public void Notify()
  {
    for (Observer ob : observersList) {
      ob.Update();
    }
  }
}

abstract class Observer {
  public abstract void Update();
}

public class ConcreteObserver extends Observer {
  private String name;
  private String observerStatus;
  private ConcreteSubject subject;
  
  public ConcreteObserver(ConcreteSubject subject, String name)
  {
    this.subject = subject;
    this.name = name;
  }
  
  public void Update() 
  {
    observerStatus = subject.getSubjectStatus();
  }  
}

public class ConcreteSubject extends Subject {
  private String subjectStatus;

  public String getSubjectStatus() {
    return subjectStatus;
  }

  public void setSubjectStatus(String subjectStatus) {
    this.subjectStatus = subjectStatus;
  }

  public static void main(String[] args) {
    // TODO Auto-generated method stub
    ConcreteSubject s = new ConcreteSubject();

    s.Attach(new ConcreteObserver(s, "X"));
    s.Attach(new ConcreteObserver(s, "Y"));
    s.Attach(new ConcreteObserver(s, "Z"));
    
    s.setSubjectStatus("Ready");
    s.Notify();
  }
}