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();
    }
  }
}

public class ConcreteSubject extends Subject {
  private String subjectState;

  public String getSubjectState() {
    return subjectState;
  }
  public void setSubjectState(String subjectState) {
    this.subjectState = subjectState;
  }
}

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

public class ConcreteObserver extends Observer{
  private String name;
  private String observerState;
  private ConcreteSubject subject;

  public ConcreteObserver(ConcreteSubject subject , String name) {
    this.subject = subject;
    this.name = name;
  }

  public void update() {
    observerState = subject.getSubjectState();
    System.out.println("Observer " + name + "が新しい状態になりました");
  }


  public ConcreteSubject getSubject() {
    return subject;
  }
  public void setSubject(ConcreteSubject subject) {
    this.subject = subject;
  }
}

public class Client {
  public static void main(String[] args) {
    ConcreteSubject s = new ConcreteSubject();
    s.attatch(new ConcreteObserver(s , "Observer1"));
    s.attatch(new ConcreteObserver(s , "Observer2"));
    s.attatch(new ConcreteObserver(s , "Observer3"));
    s.attatch(new ConcreteObserver(s , "Observer4"));

    s.setSubjectState("NEW");
    s.notifyObservers();
  }
}

C:\JavaHello\Observer>java Client
Observer Observer1が新しい状態になりました
Observer Observer2が新しい状態になりました
Observer Observer3が新しい状態になりました
Observer Observer4が新しい状態になりました

C:\JavaHello\Observer>