public final boolean retainAll(Collection<?> c) {
            if (c == null) throw new NullPointerException();
            boolean modified = false;
            for (Iterator<E> it = iterator(); it.hasNext();) {
                if (!c.contains(it.next())) {
                    it.remove();
                    modified = true;
                }
            }
            return modified;
        }
public final boolean retainAll(Collection<?> c)