@Override
    public boolean add(E element) {
        return add(element, element.hashCode());
    }
@Test(expected = NullPointerException.class)
    public void testAddAllThrowsOnNullElement() {
        final OAHashSet<Integer> set = new OAHashSet<>(8);

        final Collection<Integer> elementsToAdd = new ArrayList<>(2);
        elementsToAdd.add(1);
        elementsToAdd.add(null);

        set.addAll(elementsToAdd);
    }