An object may be observed by two or more observers. : Observable and Observer « Design Pattern « Java Tutorial

1. Java Tutorial
2. Design Pattern
3. Observable and Observer

import java.util.Observable;
import java.util.Observer;

class Watcher1 implements Observer {
  public void update(Observable obj, Object arg) {
    System.out.println("update() called, count is " + ((Integer) arg).intValue());
  }
}

class Watcher2 implements Observer {
  public void update(Observable obj, Object arg) {
    if (((Integer) arg).intValue() == 0)
      System.out.println("Done");
  }
}

class BeingWatched extends Observable {
  void counter(int period) {
    for (; period >= 0; period--) {
      setChanged();
      notifyObservers(new Integer(period));
      try {
        Thread.sleep(100);
      } catch (InterruptedException e) {
        System.out.println("Sleep interrupted");
      }
    }
  }
}

class MainClass {
  public static void main(String args[]) {
    BeingWatched observed = new BeingWatched();
    Watcher1 observing1 = new Watcher1();
    Watcher2 observing2 = new Watcher2();

    observed.addObserver(observing1);
    observed.addObserver(observing2);

    observed.counter(10);
  }
}