import java.util.ArrayList;

import java.util.List;

class Topic implements Subject {

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

private String message;

private boolean changed;

private final Object MUTEX = new Object();

@Override

public void register(Observer observer) {

if (observer == null) {

throw new NullPointerException("Null Observer");

}

synchronized (MUTEX) {

if (!observers.contains(observer)) {

observers.add(observer);

}

}

}

@Override

public void unregister(Observer observer) {

synchronized (MUTEX) {

observers.remove(observer);

}

}

@Override

public void notifyObserver() {

List<Observer> localObservers = null;

synchronized (MUTEX) {

if (!changed) {

return;

}

localObservers = new ArrayList<>(this.observers);

this.changed = false;

for (Observer observer : localObservers) {

observer.update();

}

}

}

@Override

public Object getUpdate(Observer observer) {

return this.message;

}

public void postMessage(String msg) {

System.out.println("Message Posted To Topic : " + msg);

this.message = msg;

this.changed = true;

notifyObserver();

}

}