@Override
    public boolean isEmpty() {
        return map.isEmpty();
    }
@Test
    public void testIsEmpty() {
        ExtendedSet<TestValue> nonemptyset = new ExtendedSet<>(Maps.newConcurrentMap());
        TestValue val = new TestValue("foo", 1);
        assertTrue(nonemptyset.add(val));
        assertTrue(nonemptyset.contains(val));
        assertFalse(nonemptyset.isEmpty());

        ExtendedSet<TestValue> emptyset = new ExtendedSet<>(Maps.newConcurrentMap());
        assertTrue(emptyset.isEmpty());

    }