/** File name: Subject.java **/


package observerpattern;

public interface Subject {
      public void addObserver( Observer o );
      public void removeObserver( Observer o );    
      public void notifyObservers() ; 
}



/** File name: Observer.java **/

package observerpattern;

public interface Observer {
      public void update( Subject o );
}




/** File name: ConcreteObserver.java **/

package observerpattern;

public class ConcreteObserver implements Observer {

    public ConcreteObserver() {
      
    }

     public void update( Subject o ) {
         System.out.println("Subject CHANGED");
     }
}





/** File name: IntegerDataBag.java **/

package observerpattern;

import java.util.ArrayList;
import java.util.Iterator;

public class IntegerDataBag implements Subject {
    
      private ArrayList list = new ArrayList();
      private ArrayList observers = new ArrayList();

      public void add( Integer i ) {
            list.add( i );
            notifyObservers();
      }

      public Iterator iterator() {
            return list.iterator();
      }

      public Integer remove( int index ) {
            if( index < list.size() ) {
                  Integer i = (Integer) list.remove( index );
                  notifyObservers();
                  return i;
            }
            return null;
      }

      public void addObserver( Observer o ) {
            observers.add( o );
      }

      public void removeObserver( Observer o ) {
            observers.remove( o );
      }

      private void notifyObservers() {
            // loop through and notify each observer
            Iterator i = observers.iterator();
            while( i.hasNext() ) {
                  Observer o = ( Observer ) i.next();
                  o.update( this );
            }
      }

}




/** File name: Main.java **/

package observerpattern;

public class Main implements Observer {
    

    public Main() {
        IntegerDataBag i = new IntegerDataBag();
        Observer concreteObserver = new ConcreteObserver();
        i.addObserver(concreteObserver);
        
        System.out.println("\nAdding 3");
        i.add(new Integer(3)) ;
        System.out.println("\nAdding 4");
        i.add(new Integer(4)) ;
        System.out.println("\nAdding 4");
        i.add(new Integer(5)) ;
        
    }
    
     public void update( Subject o ) {
         System.out.println("Subject CHANGED");
     }
  
     public static void main(String[] args) {
         new Main() ;
     }
    
}




Following output is generated:

Adding 3
Subject CHANGED

Adding 4
Subject CHANGED

Adding 4
Subject CHANGED