public void notifyObservers() {
        synchronized (this) {
            /* We don't want the Observer doing callbacks into
             * arbitrary code while holding its own Monitor.
             * The code where we extract each Observable from
             * the Vector and store the state of the Observer
             * needs synchronization, but notifying observers
             * does not (should not).  The worst result of any
             * potential race-condition here is that:
             * 1) a newly-added Observer will miss a
             *   notification in progress
             * 2) a recently unregistered Observer will be
             *   wrongly notified when it doesn't care
             */
            if (!changed) {
                return;
            }
            clearChanged();
        }
        for (Observer observer : obs) {
            observer.update(this);
        }
    }
@Test
    void testNotifyObservers() {
        observable.addObserver(observer);
        reset(observer);
        observable.notifyObservers();
        assertFalse(observable.hasChanged());
        verify(observer, never()).update(observable);
        observable.setChanged();
        assertTrue(observable.hasChanged());
        observable.notifyObservers();
        verify(observer).update(observable);
        assertFalse(observable.hasChanged());
    }