* @throws IllegalArgumentException if {@code countMap} is not empty
   * @since 20.0
   */
  public static <E> ConcurrentHashMultiset<E> create(ConcurrentMap<E, AtomicInteger> countMap) {
    checkArgument(countMap.isEmpty(), "the backing map (%s) must be empty", countMap);
    return new ConcurrentHashMultiset<>(countMap);
  }

  private ConcurrentHashMultiset(ConcurrentMap<E, AtomicInteger> countMap) {
    this.countMap = countMap;
  }

   * @throws IllegalArgumentException if {@code countMap} is not empty
   * @since 20.0
   */
  public static <E> ConcurrentHashMultiset<E> create(ConcurrentMap<E, AtomicInteger> countMap) {
    checkArgument(countMap.isEmpty(), "the backing map (%s) must be empty", countMap);
    return new ConcurrentHashMultiset<>(countMap);
  }

  private ConcurrentHashMultiset(ConcurrentMap<E, AtomicInteger> countMap) {
    this.countMap = countMap;
  }

      return new OldConcurrentHashMultiset<E>(new ConcurrentHashMap<E, Integer>());
    }

    @VisibleForTesting
    OldConcurrentHashMultiset(ConcurrentMap<E, Integer> countMap) {
      checkArgument(countMap.isEmpty());
      this.countMap = countMap;
    }

    // Query Operations

    /**
     * Returns the number of occurrences of {@code element} in this multiset.
     *