NotificationCenter class like ios

NotificationCenter.java

import java.util.HashMap;
import java.util.Observer;

public final class NotificationCenter {

    private static NotificationCenter instance;
    private final HashMap<String, NotificationObservable> observables;

    private NotificationCenter() {
        observables = new HashMap<>();
    }

    public static synchronized NotificationCenter getInstance() {
        if (instance == null) {
            instance = new NotificationCenter();
        }
        return instance;
    }

    public void addObserver(String notification, Observer observer) {
        NotificationObservable observable = observables.get(notification);
        if (observable == null) {
            observable = new NotificationObservable();
            observables.put(notification, observable);
        }
        observable.addObserver(observer);
    }

    public void removeObserver(String notification, Observer observer) {
        NotificationObservable observable = observables.get(notification);
        if (observable != null) {
            observable.deleteObserver(observer);
        }
    }

    public void postNotification(String notification, Object object) {
        NotificationObservable observable = observables.get(notification);
        if (observable != null) {
            observable.notifyObservers(object);
        }
    }

    public int getObserverCount(String notification) {
        NotificationObservable observable = observables.get(notification);
        if (observable != null) {
            return observable.countObservers();
        }
        return 0;
    }
}


/**
Post notification
NotificationCenter.getInstance().postNotification("your_key, object);

Notification Observer
NotificationCenter.getInstance().addObserver("your key", new Observer() {
        @Override
        public void update(Observable o, Object arg) {
        // if you want to remove when task complete then
        NotificationCenter.getInstance().removeObserver("your key", this);
    }
});
**/

NotificationObservable.java

import java.util.Observable;

class NotificationObservable extends Observable {

    public void notifyObservers(Object data) {
        setChanged();
        super.notifyObservers(data);
    }
}