Demonstrate the Observable class and the Observer interface.

Here is a Java Program to Demonstrate the Observable class and the Observer interface.

import java.util.*;

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

// This is the class being observed.
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 ObserverDemo {
  public static void main(String args[]) {
    BeingWatched observed = new BeingWatched();
    Watcher observing = new Watcher();

    /* Add the observing to the list of observers for
       observed object.  */
    observed.addObserver(observing);

    observed.counter(10);
  }
}

Output of Above Java Program
update() called, count is 10
update() called, count is 9
update() called, count is 8
update() called, count is 7
update() called, count is 6
update() called, count is 5
update() called, count is 4
update() called, count is 3
update() called, count is 2
update() called, count is 1
update() called, count is 0

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