when(backingMap.get(KEY)).thenReturn(null);
    when(backingMap.putIfAbsent(eq(KEY), isA(AtomicInteger.class))).thenReturn(null);

    assertEquals(0, multiset.add(KEY, COUNT));
  }

  public void testAdd_laterFewWithSuccess() {
    int INITIAL_COUNT = 32;
    int COUNT_TO_ADD = 400;

    AtomicInteger initial = new AtomicInteger(INITIAL_COUNT);
    when(backingMap.get(KEY)).thenReturn(initial);