The implementation currently has: -

private List<WeakReference<Observer>> observer = new ArrayList<WeakReference<Observer>>();

However there is nothing to remove the WeakReference as it looses it's reference.

Secondly notifyOberservers has: -

for (WeakReference<Observer> w : observer)

{ Observer tmp = w.get(); tmp.update(this, arg); }

As w is a WeakReference the value from get should be checked to ensure it is not null.