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JavaWorld.com > Java Standard Edition >

Speaking on the Observer pattern

How can you use the Observer pattern in your Java design?

By Tony Sintes, JavaWorld.com, 05/25/01

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The benefit: it decouples the observer from the subject. The subject doesn't need to know anything special about its observers. Instead, the subject simply allows observers to subscribe. When the subject generates an event, it simply passes it to each of its observers.

(For more information on the Observer pattern please see Resources.)

Consider the following Java example:

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


In the code above, the Subject interface defines the methods that a Subject must implement in order for Observers to add and remove themselves from the Subject. 

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


The Observer interface (above) lists the methods that an Observer must implement so that a Subject can send an update notification to the Observer.

Let's consider a simple implementation of Subject -- an IntegerDataBag: 

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 );
            }
      }
}


IntegerDataBag holds onto Integer instances. The IntegerDataBag also allows Observers to add and remove themselves.

Consider these two implementations of Observer -- IntegerAdder and IntegerPrinter: 

import java.util.Iterator;
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 );
            }
      }
}
import java.util.Iterator;
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() );
                  }
            }
      }
}


IntegerAdder and IntegerPrinter add themselves to the integer bag as observers. When an IntegerAdder receives an update, it sums up the Integer values held in the bag and displays them. Likewise, when IntegerPrinter receives an update, it prints out the Integers held in the bag.

Here is a simple main() that exercises these classes: 

public class Driver {
      public static void main( String [] args ) {
            Integer i1 = new Integer( 1 ); Integer i2 = new Integer( 2 );
            Integer i3 = new Integer( 3 ); Integer i4 = new Integer( 4 );
            Integer i5 = new Integer( 5 ); Integer i6 = new Integer( 6 );
            Integer i7 = new Integer( 7 ); Integer i8 = new Integer( 8 );
            Integer i9 = new Integer( 9 );
            IntegerDataBag bag = new IntegerDataBag();
            bag.add( i1 ); bag.add( i2 ); bag.add( i3 ); bag.add( i4 );
            bag.add( i5 ); bag.add( i6 ); bag.add( i7 ); bag.add( i8 );
            IntegerAdder adder = new IntegerAdder( bag );
            IntegerPrinter printer = new IntegerPrinter( bag );
            // adder and printer add themselves to the bag
            System.out.println( "About to add another integer to the bag:" );
            bag.add( i9 );
            System.out.println("");
            System.out.println("About to remove an integer from the bag:");
            bag.remove( 0 );
      }
}


Upon running the main, you will see:

The results from running the main() method



The IntegerDataBag/IntegerAdder/IntegerPrinter is a simple example of the Observer pattern. Within Java itself there are a number of examples of the Observer pattern: the AWT/Swing event model, as well as the java.util.Observer and java.util.Observable interfaces serve as examples.

Read more about Core Java in JavaWorld's Core Java section.

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