BiMap<String, Integer> bimap = create();
    BiMap<Integer, String> inverse = bimap.inverse();
    assertSame(bimap, inverse.inverse());
    assertTrue(inverse instanceof SynchronizedBiMap);
    assertSame(mutex, ((SynchronizedBiMap<?, ?>) inverse).mutex);
  }

  @Override
  public void testValues() {
    BiMap<String, Integer> map = create();
    Set<Integer> values = map.values();

    BiMap<String, Integer> bimap = create();
    BiMap<Integer, String> inverse = bimap.inverse();
    assertSame(bimap, inverse.inverse());
    assertTrue(inverse instanceof SynchronizedBiMap);
    assertSame(mutex, ((SynchronizedBiMap<?, ?>) inverse).mutex);
  }

  @Override
  public void testValues() {
    BiMap<String, Integer> map = create();
    Set<Integer> values = map.values();

      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 {

  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());
  }

  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:
   *
   * <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!