Observer design pattern

Observer design pattern

This works on the principle of many-to-one subscriber: publisher concept.

public interface Subject {

      public void addObserver(Observer ob);

      public void removeObserver(Observer ob);

      public void notifyObservers(Message msg);

}

public class WeatherStationBroadcasterSubject implements Subject {

     

      private Set<Observer> observerList = new HashSet<Observer>();

      @Override

      public void addObserver(Observer ob) {

            observerList.add(ob);

      }

      @Override

      public void removeObserver(Observer ob) {

            observerList.remove(ob);

      }

      @Override

      public void notifyObservers(Message msg) {

            Iterator<Observer> it = observerList.iterator();

            while(it.hasNext()) {

                  it.next().update(msg);

            }

      }

}

public interface Observer {

      public void update(Message msg);

}

public class MobileObserver implements Observer {

      @Override

      public void update(Message msg) {

            System.out.println("Mobile Phone Observer : Weather data changed :: "+msg.getMessage());

      }

}

public class SmartWatchObserver implements Observer {

      @Override

      public void update(Message msg) {

            System.out.println("Smart Watch Observer : Weather data changed :: "+msg.getMessage());

      }

}

public class Message {

     

      private String message;

      public Message(String message) {

            super();

            this.message = message;

      }

      public String getMessage() {

            return message;

      }

}

public class MainClass {

      public static void main(String[] args) {

            Subject subject = new WeatherStationBroadcasterSubject();

            Observer observer1 = new MobileObserver();

            subject.addObserver(observer1);

            Message msg = new Message("It's a hot day");

            subject.notifyObservers(msg);

            Observer observer2 = new SmartWatchObserver();

            subject.addObserver(observer2);

            msg = new Message("It's a cold day");

            subject.notifyObservers(msg);

            subject.removeObserver(observer1);

            msg = new Message("It's a cloudy day");

            subject.notifyObservers(msg);

      }

}

Result :

Mobile Phone Observer : Weather data changed :: It's a hot day

Mobile Phone Observer : Weather data changed :: It's a cold day

Smart Watch Observer : Weather data changed :: It's a cold day

Smart Watch Observer : Weather data changed :: It's a cloudy day


Code link : https://github.com/sksumit1/DesignPatterns