What is Observer Design Pattern in Java ?.
The Gang of Four book (Design Patterns: Elements of Reusable Object-Oriented Software, 1995, Pearson Education, Inc. Publishing as Pearson Addison Wesley) says that the Observer design pattern should
"Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically."
Example : Let's say there is Weather Broadcasting company which broadcast weather changes to its subscribers. The subscribers or observers such as News Channels , News Paper Agencies and Weather Information Provider websites etc ... gets updated as soon as weather changes by the Weather Broadcasting company. Let's see how we can implement this Subject - Observer model in Java.
Program to demonstrate how to implement Observer Design Pattern in Java.
1. Subject.java
package com.hubberspot.designpattern.behavioral.observer;
public interface Subject {
public void addObserver(Observer observer);
public void removeObserver(Observer observer);
public void notifyObservers();
}
2. Observer.java
package com.hubberspot.designpattern.behavioral.observer;
public interface Observer {
public void update(int temperature);
}
3. WeatherBroadcast.java
package com.hubberspot.designpattern.behavioral.observer;
import java.util.ArrayList;
import java.util.List;
public class WeatherBroadcast implements Subject {
private List< Observer > listOfObservers;
private int temperature;
public WeatherBroadcast() {
listOfObservers = new ArrayList< Observer >();
}
@Override
public void addObserver(Observer observer) {
listOfObservers.add(observer);
}
@Override
public void removeObserver(Observer observer) {
listOfObservers.remove(observer);
}
@Override
public void notifyObservers() {
for(Observer observer : listOfObservers) {
observer.update(temperature);
}
}
public void temperatureChanged(int newTemperature) {
temperature = newTemperature;
notifyObservers();
}
}
4. NewsPaperAgency.java
package com.hubberspot.designpattern.behavioral.observer;
public class NewsPaperAgency implements Observer {
@Override
public void update(int temperature) {
System.out.println("News Paper Agencies gets current temperature as: "
+ temperature + " degrees");
}
}
5. NewsChannel.java
package com.hubberspot.designpattern.behavioral.observer;
public class NewsChannel implements Observer {
@Override
public void update(int temperature) {
System.out.println("News Channel gets current temperature as: "
+ temperature + " degrees");
}
}
6. WeatherInfoWebsite.java
package com.hubberspot.designpattern.behavioral.observer;
public class WeatherInfoWebsite implements Observer {
@Override
public void update(int temperature) {
System.out.println("Weather Info Websites gets current temperature as: "
+ temperature + " degrees");
}
}
7. ObserverDesignPatternTest.java
package com.hubberspot.designpattern.behavioral.observer;
public class ObserverDesignPatternTest {
public static void main(String[] args) throws InterruptedException {
WeatherBroadcast weatherBroadcast = new WeatherBroadcast();
Observer newsPaperAgency = new NewsPaperAgency();
Observer newsChannel = new NewsChannel();
Observer weatherInfoWebsite = new WeatherInfoWebsite();
weatherBroadcast.addObserver(newsPaperAgency);
weatherBroadcast.addObserver(newsChannel);
weatherBroadcast.addObserver(weatherInfoWebsite);
weatherBroadcast.temperatureChanged(30);
System.out.println("\nAfter 2 minutes temperature changes to 33 degrees...\n");
Thread.sleep(120000);
weatherBroadcast.temperatureChanged(33);
}
}
Output of the program :
Video tutorial to demonstrate how to implement Observer Design Pattern in Java.
