assertTrue("map should not contain a zero", value.get() != 0);
    }
  }

  private static class MutateTask implements Callable<int[]> {
    private final ConcurrentHashMultiset<String> multiset;
    private final ImmutableList<String> keys;
    private final Random random = new Random();

    private MutateTask(ConcurrentHashMultiset<String> multiset, ImmutableList<String> keys) {
      this.multiset = multiset;