Design Patterns in Java (Creational) - Observe Pattern

Observe Pattern

• One to Many
• Publisher / Subscriber

Observer using java implementation

public class Main {



 public static void main(String[] args) {

  TwitterStream ts = new TwitterStream();

  Client c1 = new Client("Pal");

  Client c2 = new Client("Rod");

  ts.addObserver(c1);

  ts.addObserver(c2);

  ts.someoneTweeted();

 }

}



class TwitterStream extends Observable{

 public void someoneTweeted(){

  setChanged();

  notifyObservers();

 }

}



class Client implements Observer{

 private String name;

 

 public Client(String name){

  this.name = name;

 }

 

 @Override

 public void update(Observable o, Object arg) {

  System.out.println("Updating " + name + " someone has tweeted something");



 } 

}

Observer using own implementation

Main

public static void main(String[] args) {

Subject subject = new MessageStream();

PhoneClient pc = new PhoneClient(subject);

TabletClient tc = new TabletClient(subject);

pc.addMessage("Message from phone");

tc.addMessage("Message from tablet");

}

Subject

public abstract class Subject {

private List<Observer> observers = new ArrayList<Observer>();

abstract void setState(String state);

abstract String getState();

public void attach(Observer obs){

observers.add(obs);

}

public void detach(Observer obs){

observers.remove(obs);

}

public void notifyObservers(){

for (Observer observer : observers) {

observer.update();

}

}

}

Observer

public abstract class Observer {

protected Subject subject;

abstract void update();

}

PhoneClient

public class PhoneClient extends Observer {

public PhoneClient(Subject subject){

this.subject = subject;

this.subject.attach(this);

}

public void addMessage(String message){

this.subject.setState(message + " - sent from phone");

}

@Override

void update() {

System.out.println("Phone Stream: " + subject.getState() );

}

}

TabletClient

public class TabletClient extends Observer {

public TabletClient(Subject subject){

this.subject = subject;

this.subject.attach(this);

}

public void addMessage(String message){

this.subject.setState(message + " - sent from phone");

}

@Override

void update() {

System.out.println("Tablet Stream: " + subject.getState() );

}

}