BiMap<K, V> bimap, @CheckForNull Object mutex) {
    if (bimap instanceof SynchronizedBiMap || bimap instanceof ImmutableBiMap) {
      return bimap;
    }
    return new SynchronizedBiMap<>(bimap, mutex, null);
  }

  static final class SynchronizedBiMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMap<K, V> implements BiMap<K, V>, Serializable {

      BiMap<K, V> bimap, @CheckForNull Object mutex) {
    if (bimap instanceof SynchronizedBiMap || bimap instanceof ImmutableBiMap) {
      return bimap;
    }
    return new SynchronizedBiMap<>(bimap, mutex, null);
  }

  static final class SynchronizedBiMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMap<K, V> implements BiMap<K, V>, Serializable {

   *
   * <p>It is imperative that the user manually synchronize on the returned map when accessing any
   * of its collection views:
   *
   * <pre>{@code
   * BiMap<Long, String> map = Maps.synchronizedBiMap(
   *     HashBiMap.<Long, String>create());
   * ...
   * Set<Long> set = map.keySet();  // Needn't be in synchronized block
   * ...
   * synchronized (map) {  // Synchronizing on map, not set!

   *
   * <p>It is imperative that the user manually synchronize on the returned map when accessing any
   * of its collection views:
   *
   * <pre>{@code
   * BiMap<Long, String> map = Maps.synchronizedBiMap(
   *     HashBiMap.<Long, String>create());
   * ...
   * Set<Long> set = map.keySet();  // Needn't be in synchronized block
   * ...
   * synchronized (map) {  // Synchronizing on map, not set!