@Override
    public boolean contains(Object o) {
        if (!(o instanceof Integer)) {
            throw new ClassCastException("PartitionIdSet can be only used with Integers");
        }
        return bitSet.get((Integer) o);
    }
@Test(expected = ClassCastException.class)
    public void test_contains_whenNotInteger() {
        partitionIdSet.contains(new Object());
    }