Observer pattern

Here Observable is a Subject, Publisher, Event etc

Observer is listner, Subscriber  etc

Suppose there is a weather station which issues the weather warning . All the TV channels gets the update from this weather station. So weather station is a subject.

And TV channels are the Observer.

public interface Publisher {

      public void addSubscriber(Subscriber subscriber);

      public void removeSubscriber(Subscriber subscriber);

      public void notifySubscribers(String  string);

}

class WeatherStation implements Publisher {

      private String warning;

     

      void releaseWarning(String string) {

            this.warning = string;

            notifySubscribers(warning);

      }

     

      ArrayList<Subscriber> arrayList = new ArrayList<Subscriber>();

      @Override

      public void addSubscriber(Subscriber subscriber) {

            arrayList.add(subscriber);

      }

      @Override

      public void removeSubscriber(Subscriber subscriber) {

            arrayList.remove(subscriber);

      }

      @Override

      public void notifySubscribers(String  string) {

            for (Subscriber s : arrayList) {

                  s.getNotification4mPublisher(string);

            }

      }

}

Now we have a interface Subscriber

interface Subscriber {

      public void getNotification4mPublisher(String string);

}

class AaaTak implements Subscriber {

      @Override

      public void getNotification4mPublisher(String string) {

            System.out.println("AaaTak WEATHER WARNING " + string);

      }

}

class StarNews implements Subscriber {

      @Override

      public void getNotification4mPublisher(String string) {

            System.out.println("StarNews WEATHER WARNING " + string);

      }

}

class ZeeNews implements Subscriber {

      @Override

      public void getNotification4mPublisher(String string) {

            System.out.println("ZeeNews WEATHER WARNING " + string);

      }

}

public class TestObsrverPattern {

      public static void main(String[] args) {

            WeatherStation station = new WeatherStation();

            AaaTak aaaTak = new AaaTak();

            StarNews news = new StarNews();

            ZeeNews news2 = new ZeeNews();

            station.addSubscriber(aaaTak);

            station.addSubscriber(news);

            station.addSubscriber(news2);

            station.notifySubscribers("Heavy rains expected in NEW York");

      }

}

This can also be achieved by Observer class in java.util package

public class WeatherStation extends Observable{

     

      private String warning;

      void releaseWarning(String string) {

            this.warning = string;

         setChanged();    //this is mandatory

       //clearChanged();

            notifyObservers(warning);

      }

     

}

public class StarTV implements Observer {

      @Override

      public void update(Observable subject, Object object) {

            System.out.println(subject.hasChanged() + " --> " + object);

      }

}

class ZeeTv implements Observer {

      @Override

      public void update(Observable subject, Object object) {

            subject.notifyObservers();

            System.out.println(" --> " + object);

      }

}

      public static void main(String[] args) {

           

            WeatherStation weatherStation = new WeatherStation();

ZeeTv ztv = new ZeeTv();

StarTV starTV = new StarTV();

weatherStation.addObserver(ztv);

weatherStation.addObserver(starTV);

weatherStation.releaseWarning("Heavy rains expected in NEW York");

weatherStation.releaseWarning("Heavy rains expected in Minneapolis");

     

}