BiMap<K, V> bimap, @Nullable 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> {

      BiMap<K, V> bimap, @Nullable 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> {

  @J2ktIncompatible // Synchronized
  public void testSynchronizedBiMap() {
    BiMap<String, Integer> bimap = HashBiMap.create();
    bimap.put("one", 1);
    BiMap<String, Integer> sync = Maps.synchronizedBiMap(bimap);
    bimap.put("two", 2);
    sync.put("three", 3);
    assertEquals(ImmutableSet.of(1, 2, 3), bimap.inverse().keySet());
    assertEquals(ImmutableSet.of(1, 2, 3), sync.inverse().keySet());
  }

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

  @J2ktIncompatible // Synchronized
  public void testSynchronizedBiMap() {
    BiMap<String, Integer> bimap = HashBiMap.create();
    bimap.put("one", 1);
    BiMap<String, Integer> sync = Maps.synchronizedBiMap(bimap);
    bimap.put("two", 2);
    sync.put("three", 3);
    assertEquals(ImmutableSet.of(1, 2, 3), bimap.inverse().keySet());
    assertEquals(ImmutableSet.of(1, 2, 3), sync.inverse().keySet());
  }