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...................

              

Prototype Design Pattern

This is a part of Creational pattern, which helps to create objects as per request. This pattern uses philosophy of cloning to create objects, and we know that two way of cloning is possible, one is shallow cloning, and another is deep cloning. Behavior of both cloning types is different so choose wisely that which type of cloning you need.

You can learn more about cloning here:

               http://javawitharun.blogspot.hk/2013/05/java-cloning.html

When to use Prototype pattern?

               i) If creation of object is complex or costly

               If addition or removal of objects are expected on runtime

               If client should be unaware of the object creation

               If similar object is required as the existing one

One scenario where prototype pattern is used?

               When creating a bank application, and we know that bank transactions need expensive database queries. Transactions would be linked with bank account which may be an individual or group and having their profile with bank. So in this case once the actual object would be created (exactly single object), it may be required further in number of places, so prototype can be used to get the copy of object and finally the modified object can replace the old object.

Example:-

PrototypeClient.java

import java.io.ByteArrayInputStream;

import java.io.ByteArrayOutputStream;

import java.io.IOException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

import java.io.Serializable;

import java.util.HashMap;

import java.util.Map;

public class PrototypeClient {

               public static void main( String[] args ) {

                              Shoes xyz = Factory.makeObject("Sport");

                              xyz.wear();

               }

}

Shoes.java

abstract class Shoes implements Serializable, Cloneable{  

               private static final long serialVersionUID = -1937047519230746677L;

               abstract void wear();

               public Object shallowCloning() throws CloneNotSupportedException    {

                              return this.clone();

               }

               public Shoes deepCloning() throws IOException, ClassNotFoundException{

                              //Serialization of object

                              ByteArrayOutputStream bos = new ByteArrayOutputStream();

                              ObjectOutputStream out = new ObjectOutputStream(bos);

                              out.writeObject(this);

                              //De-serialization of object

                              ByteArrayInputStream bis = new   ByteArrayInputStream(bos.toByteArray());

                              ObjectInputStream in = new ObjectInputStream(bis);

                              Shoes clonedShoes = (Shoes) in.readObject();

                              return clonedShoes;

               }

}

SportShoes.java

class SportShoes extends Shoes {

               private static final long serialVersionUID = 440347043243434494L;

               public String toString() {

                              return "SportShoes";

               }

               @Override

               public void wear() {

                              //some SportShoes specific code to wear

                              System.out.println("I am wearing " + toString());

               }

}

CasualShoes.java

class CasualShoes extends Shoes {

               private static final long serialVersionUID = -6493171299609719559L;

               public String toString() {

                              return "CasualShoes";

               }

               @Override

               public void wear() {

                              //some CasualShoes specific code to wear

                              System.out.println("I am wearing " + toString());

               }

}

FormalShoes.java

class FormalShoes extends Shoes {

               private static final long serialVersionUID = -5670125744755511170L;

               public String toString() {

                              return "FormalShoes";

               }

               @Override

               public void wear() {

                              //some FormalShoes specific code to wear

                              System.out.println("I am wearing " + toString());

               }

}

Factory.java

class Factory {

               private static Map<String, Object> prototypes = new HashMap<String, Object>();

               static {

                              prototypes.put( "Sport",   new SportShoes() );

                              prototypes.put( "Casual",  new CasualShoes() );

                              prototypes.put( "Formal", new FormalShoes() );

               }

               public static Shoes makeObject(String shoesType) {

                              Shoes shoes = (Shoes)prototypes.get(shoesType);

                              //deep cloning (using in-memory)

                              try {

                                             Shoes deepClonedShoes = shoes.deepCloning();

                                             System.out.println("Deep-Cloned Shoes ========== "+deepClonedShoes);

                              } catch (IOException e) {

                                             e.printStackTrace();

                              } catch (ClassNotFoundException e) {

                                             e.printStackTrace();

                              }

                              //Shallow cloning

                              Shoes shallowClonedShoes = null;

                              try {

                                             shallowClonedShoes = (Shoes) shoes.shallowCloning();

                              } catch (CloneNotSupportedException e) {

                                             e.printStackTrace();

                              }

                              System.out.println("Shallow-Cloned Shoes ========== " +shallowClonedShoes );

                              return shallowClonedShoes;

               }

}

Output:

 Deep-Cloned Shoes ========== SportShoes

Shallow-Cloned Shoes ========== SportShoes

I am wearing SportShoes