}
  }

  @Override
  public boolean containsKey(@CheckForNull Object key) {
    return asMap().get(key) != null;
  }

  @Override
  public Collection<V> removeAll(@CheckForNull Object key) {
    return MoreObjects.firstNonNull(asMap().remove(key), unmodifiableEmptyCollection());
  }

  Collection<V> unmodifiableEmptyCollection() {