Observer Design Pattern (with and without Java API)

It’s a behavioral pattern and can be understood as publish-subscribe model. An object notifies to other objects about his state changes.

“This pattern talks about one-to-many dependency b/w objects to provide a behavior where change of one object state would be notified to other objects and those objects would be updated accordingly as well.”

An object which is being watched known as ‘Subject’. An object which is watching the state is known as ‘Observer’ or ‘Listener’.

Example:

               1) Using my own implementation as below:

Observer.java

interface Observer {      

               void update(Object newsContent);

}

Subject.java

import java.util.ArrayList;

public abstract class Subject {

               protected ArrayList<Observer> observers = new ArrayList<Observer>();  

               public void registerObserver(Observer observer) {         

                              observers.add(observer);

               }

               public void removeObserver(Observer observer) {         

                              observers.remove(observer);

               }

 

               public abstract void notifyObservers();

               public void notifyObservers(Object obj){

                              for (Observer ob : observers) {                  

                                             System.out.println("Notifying Observers on change...");             

                                             ob.update(obj);           

                              }

               }

}

News.java

public class News extends Subject {      

               private String newsContent;     

               private String advertisement;     

               private int noOfPages;

               public News(String newsContent, String advertisement, int noOfPages) {

                              this.newsContent = newsContent;            

                              this.advertisement = advertisement;          

                              this.noOfPages = noOfPages;     

               }

               @Override

               public void notifyObservers() {   

                              for (Observer ob : observers) {                  

                                             System.out.println("Notifying Observers on change...");             

                                             ob.update(this.newsContent);           

                              }

               }

               public String getNewsContent() {

                              return newsContent;

               }

               public void setNewsContent(String newsContent) {

                              this.newsContent = newsContent;

                              notifyObservers();

               }

               public String getAdvertisement() {

                              return advertisement;

               }

               public void setAdvertisement(String advertisement) {

                              this.advertisement = advertisement;

                              notifyObservers(this.advertisement);

               }

               public int getNoOfPages() {

                              return noOfPages;

               }

               public void setNoOfPages(int noOfPages) {

                              this.noOfPages = noOfPages;

               }

}

ElectronicNewsPaper.java

public class ElectronicNewsPaper implements Observer {  

               @Override     

               public void update(Object newsContent) {   

                              System.out.println("ElectronicNewsPaper Today's News Content : " + (String)newsContent);    

                              //Business-logic to update the Electronic News-Paper

               }

}

PrintNewsPaper.java

public class PrintNewsPaper implements Observer {     

               @Override    

               public void update(Object newsContent) {   

                              System.out.println("PrintNewsPaper Today's News Content : " + (String)newsContent);   

                              //Business-logic to update the Print News-Paper

               }

}

ObserverClient.java

public class ObserverClient {

               public static void main(String args[]) {

                              PrintNewsPaper printMedia = new PrintNewsPaper();             

                              ElectronicNewsPaper electronicMedia = new ElectronicNewsPaper();

                              News todayNews = new News("today's News........................", "today's Advertisement...........s", 12);    

                              todayNews.registerObserver(printMedia);          

                              todayNews.registerObserver(electronicMedia);     

                              todayNews.setNewsContent("today's News got updated...................");

                               System.out.println("=================================================");

                              todayNews.removeObserver(electronicMedia);

                              todayNews.setNewsContent("today's News again got updated...................");

                              todayNews.setAdvertisement("today's Advertisement got updated...................");

               }

}

Output:-

Notifying Observers on change...

PrintNewsPaper Today's News Content : today's News got updated...................

Notifying Observers on change...

ElectronicNewsPaper Today's News Content : today's News got updated...................

===================================================

Notifying Observers on change...

PrintNewsPaper Today's News Content : today's News again got updated...................

Notifying Observers on change...

PrintNewsPaper Today's News Content : today's Advertisement got updated...................

2) Using Java built-in API as below:

The java API has one Interface called as ‘java.util.Observer’ and one class called as ‘java.util.Observable’ to provide pub-sub mechanism.

News.java

import java.util.Observable;

public class News extends Observable{   

               private String newsContent;     

               private String advertisement;     

               private int noOfPages;

               public News(String newsContent, String advertisement, int noOfPages) {

                              this.newsContent = newsContent;            

                              this.advertisement = advertisement;          

                              this.noOfPages = noOfPages;     

               }

               public String getNewsContent() {

                              return newsContent;

               }

               public void setNewsContent(String newsContent) {

                              this.newsContent = newsContent;

                              setChanged();

                              notifyObservers(newsContent);

               }

               public String getAdvertisement() {

                              return advertisement;

               }

               public void setAdvertisement(String advertisement) {

                              this.advertisement = advertisement;

               }

               public int getNoOfPages() {

                              return noOfPages;

               }

               public void setNoOfPages(int noOfPages) {

                              this.noOfPages = noOfPages;

               }

}

ElectronicNewsPaper.java

import java.util.Observable;

import java.util.Observer;

public class ElectronicNewsPaper implements Observer {  

               @Override

               public void update(Observable arg0, Object newsContent) {

                              System.out.println("ElectronicNewsPaper Today's News Content : " + (String)newsContent);    

                              //Business-logic to update the Electronic News-Paper

               }

}

PrintNewsPaper.java

import java.util.Observable;

import java.util.Observer;

public class PrintNewsPaper implements Observer {     

               @Override

               public void update(Observable arg0, Object newsContent) {

                              System.out.println("PrintNewsPaper Today's News Content : " + (String)newsContent);   

                              //Business-logic to update the Print News-Paper

               }

}

ObserverClient.java

public class ObserverClient {

               public static void main(String args[]) {

                              PrintNewsPaper printMedia = new PrintNewsPaper();             

                              ElectronicNewsPaper electronicMedia = new ElectronicNewsPaper();

                              News todayNews = new News("today's News........................", "today's Advertisement...........s", 12);    

                              todayNews.addObserver(printMedia);          

                              todayNews.addObserver(electronicMedia);     

                              todayNews.setNewsContent("today's News got updated...................");

                             

                              System.out.println("===================================================");

                              todayNews.deleteObserver(electronicMedia);

                             

                              todayNews.setNewsContent("today's News again got updated...................");

               }

}

Output:-

ElectronicNewsPaper Today's News Content : today's News got updated...................

PrintNewsPaper Today's News Content : today's News got updated...................

===================================================

PrintNewsPaper Today's News Content : today's News again got updated...................