ObserverPatternPractice

import java.util.ArrayList;

interface Subject {

    void register(Observer observer);
    void unregister(Observer observer);
    void notifyObservers();
}


interface Observer {
    void update(String notification);
}


class BarristerSumonFacebookPage implements Subject {

    // Barrister Sumon's page has to know his followers
    private ArrayList<Observer> observers = new ArrayList<Observer>();

    // His status
    private String status;


    // The way followers can 'Like' or register to his page
    @Override
    public void register(Observer observer) {
        observers.add(observer);
    }

    // The way followers can 'Unlike' or unsubscribe from his page
    @Override
    public void unregister(Observer observer) {
        observers.remove(observer);
    }

    // His page's way of notifying all this followers
    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update("Barrister Sumon has new notification!");
        }
    }

    // Barrister Sumon use's this method to update his status.
    // And when he does that, it should notify his followers
    public void updatePage(String status) {
        this.status = status;
        notifyObservers();
    }
}

class CrazyFan implements Observer {
    // Fan's name
    private String name;

    // Constructor to set Fan's name
    CrazyFan(String name) {
        this.name = name;
    }

    // This is the method, Barrister Sumon's FB page uses to notify the fan
    public void update(String notification) {
        // Just to prove that individual fan's get separate notifications
        System.out.print("This is " + this.name + "'s page! ");
        System.out.println(notification);
    }
}


class ObserverPatternTestDrive {
    public static void main(String[] args) {
        // Barrister Sumon creates his own page
        BarristerSumonFacebookPage fb_page = new BarristerSumonFacebookPage();

        // Let's create two crazy fans: Mofiz and Moznu
        CrazyFan crazyFan1 = new CrazyFan("Mofiz");
        CrazyFan crazyFan2 = new CrazyFan("Moznu");

        // Fans 'Like' his page, subscribes to it
        fb_page.register(crazyFan1);
        fb_page.register(crazyFan2);

        // Barrister Sumon updates his page with new video
        fb_page.updatePage("I'm now live!");
    }
}