public MutableNullValueVector() {
    _nullBitmap = new ThreadSafeMutableRoaringBitmap();
  }
@Test
  public void testMutableNullValueVector() {
    int[] docIds = new int[NUM_DOCS];
    for (int i = 0; i < NUM_DOCS; i++) {
      int docId = RANDOM.nextInt(MAX_DOC_ID);
      _nullValueVector.setNull(docId);
      docIds[i] = docId;
    }
    for (int i = 0; i < NUM_DOCS; i++) {
      Assert.assertTrue(_nullValueVector.isNull(docIds[i]));
    }
  }