// Carsten Schuermann 
// Observer Pattern in Java
// Someplace on the web 

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

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

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

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 );
            }
      }
}

public class IntegerAdder implements Observer {
      private IntegerDataBag bag;
      public IntegerAdder( IntegerDataBag bag ) {
            this.bag = bag;               
            bag.addObserver( this );
      }
      public void update( Subject o ) {
            if( o == bag ) {
                  System.out.println( "The contents of the IntegerDataBag have changed." );
                  int counter = 0;
                  Iterator i = bag.iterator();
                  while( i.hasNext() ) {
                        Integer integer = ( Integer ) i.next();
                        counter+=integer.intValue();
                  }
                  System.out.println( "The new sum of the integers is: " + counter );
            }
      }
}

public class IntegerPrinter implements Observer {
      private IntegerDataBag bag;
      public IntegerPrinter( IntegerDataBag bag ) {
            this.bag = bag;               
            bag.addObserver( this );
      }
      public void update( Subject o ) {
            if( o == bag ) {
                  System.out.println( "The contents of the IntegerDataBag have changed." );
                  System.out.println( "The new contents of the IntegerDataBag contains:" );
                  Iterator i = bag.iterator();
                  while( i.hasNext() ) {
                        System.out.println( i.next() );
                  }
            }
      }
}