@Override
    public boolean isEmpty() {
        return items.isEmpty();
    }
@Test
    public void testIsEmpty() throws Exception {
        //test empty condition
        assertTrue("The set should be initialized empty.", set.isEmpty());
        fillSet(5, this.set);
        assertFalse("The set should no longer be empty.", set.isEmpty());
        set.clear();
        assertTrue("The set should have been cleared.", set.isEmpty());
    }