@Override
    public int size() {
        return items.size();
    }
@Test
    public void testSize() throws Exception {
        //Checks size values throughout addition and removal
        for (int i = 0; i < 10; i++) {
            map.put(i, i);
            assertEquals("The map size is wrong.", i + 1, map.size());
        }
        for (int i = 0; i < 10; i++) {
            map.remove(i);
            assertEquals("The map size is wrong.", 9 - i, map.size());
        }
    }