Observer.java

class Subject {
    protected ArrayList<Observer> observers = new ArrayList<>();
    public void addObserver(Observer o) {
        this.observers.add(o);
    }
    public void removeObserver(Observer o) {
        this.observers.remove(o);
    }
    public void notify(Object arg) {
        for (Observer o : this.observers)
            o.update(this, arg);
    }
}