* @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;
  }