Observer Pattern
Author: Deron Eriksson
Description: This Java tutorial describes the observer pattern, a behavioral object pattern.
Tutorial created using: Windows Vista || JDK 1.6.0_11 || Eclipse JEE Ganymede SR1 (Eclipse 3.4.1)

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The observer pattern is a behavioral object design pattern. In the observer pattern, an object called the subject maintains a collection of objects called observers. When the subject changes, it notifies the observers. Observers can be added or removed from the collection of observers in the subject. The changes in state of the subject can be passed to the observers so that the observers can change their own state to reflect this change.

The subject has an interface that defines methods for attaching and detaching observers from the subject's collection of observers. This interface also features a notification method. This method should be called when the state of the subject changes. This notifies the observers that the subject's state has changed. The observers have an interface with a method to update the observer. This update method is called for each observer in the subject's notification method. Since this communication occurs via an interface, any concrete observer implementing the observer interface can be updated by the subject. This results in loose coupling between the subject and the observer classes.

Now we'll look at an example of the observer pattern. We'll start by creating an interface for the subject called WeatherSubject. This will declare three methods: addObserver(), removeObserver(), and doNotify().


package com.cakes;

public interface WeatherSubject {

	public void addObserver(WeatherObserver weatherObserver);

	public void removeObserver(WeatherObserver weatherObserver);

	public void doNotify();


We'll also create an interface for the observers called WeatherObserver. It features one method, a doUpdate() method.


package com.cakes;

public interface WeatherObserver {

	public void doUpdate(int temperature);

The WeatherStation class implements WeatherSubject. It is our subject class. It maintains a set of WeatherObservers which are added via addObserver() and removed via removeObserver(). When WeatherSubject's state changes via setTemperature(), the doNotify() method is called, which contacts all the WeatherObservers with the temperature via their doUpdate() methods.


package com.cakes;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class WeatherStation implements WeatherSubject {

	Set<WeatherObserver> weatherObservers;
	int temperature;

	public WeatherStation(int temperature) {
		weatherObservers = new HashSet<WeatherObserver>();
		this.temperature = temperature;

	public void addObserver(WeatherObserver weatherObserver) {

	public void removeObserver(WeatherObserver weatherObserver) {

	public void doNotify() {
		Iterator<WeatherObserver> it = weatherObservers.iterator();
		while (it.hasNext()) {
			WeatherObserver weatherObserver = it.next();

	public void setTemperature(int newTemperature) {
		System.out.println("\nWeather station setting temperature to " + newTemperature);
		temperature = newTemperature;


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