package java.util;
public class Observable {
    private boolean changed = false;
    private final ArrayList<Observer> observers;
public Observable() {
        observers = new ArrayList<>();
}
public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new 
 
NullPointerException();
        if (!observers.contains(o)) {
            observers.add(o);
}
    }
public synchronized void deleteObserver(Observer o) {
        observers.remove(o);
}
public void notifyObservers() {
        notifyObservers(null);
}
public void notifyObservers(Object arg) {Observer[] arrLocal;
 
        synchronized (this) {if (!hasChanged())
                return;arrLocal = observers.toArray(new Observer[observers.size()]);
clearChanged();
}
        for (int i = arrLocal.length-1; i>=0; i--)
            arrLocal[i].update(this, arg);
}
public synchronized void deleteObservers() {
        observers
 
.clear();
}
protected synchronized void setChanged() {
        changed = true;
}
protected synchronized void clearChanged() {
        changed = false;
}
public synchronized boolean hasChanged() {
        return changed;
}
public synchronized int countObservers() {
        return observers.size();
}
}