* @throws NullPointerException if any key or value in {@code map} is null
   */
  public static <K, V> ImmutableBiMap<K, V> copyOf(Map<? extends K, ? extends V> map) {
    if (map instanceof ImmutableBiMap) {
      @SuppressWarnings("unchecked") // safe since map is not writable
      ImmutableBiMap<K, V> bimap = (ImmutableBiMap<K, V>) map;
      // TODO(lowasser): if we need to make a copy of a BiMap because the

    }

    @Override
    ImmutableBiMap<K, V> buildJdkBacked() {
      return build();
    }
  }

  public static <K, V> ImmutableBiMap<K, V> copyOf(Map<? extends K, ? extends V> map) {
    if (map instanceof ImmutableBiMap) {
      @SuppressWarnings("unchecked") // safe since map is not writable
      ImmutableBiMap<K, V> bimap = (ImmutableBiMap<K, V>) map;
      return bimap;
    }

    super(singletonMap(checkNotNull(key), checkNotNull(value)));
    this.singleKey = key;
    this.singleValue = value;
    this.inverse = inverse;
  }

  @Override
  public ImmutableBiMap<V, K> inverse() {
    ImmutableBiMap<V, K> result = inverse;
    if (result == null) {
      return inverse = new SingletonImmutableBiMap<V, K>(singleValue, singleKey, this);
    } else {
      return result;
    }
  }

  @LazyInit @RetainedWith private transient @Nullable ImmutableBiMap<V, K> inverse;

  @Override
  public ImmutableBiMap<V, K> inverse() {
    if (isEmpty()) {
      return ImmutableBiMap.of();
    }
    ImmutableBiMap<V, K> result = inverse;
    return (result == null) ? inverse = new Inverse() : result;
  }

  private final class Inverse extends ImmutableBiMap<V, K> {

    @Override
    public int size() {

  public static class ImmutableBiMapGenerator extends TestStringBiMapGenerator {
    @Override
    protected BiMap<String, String> create(Entry<String, String>[] entries) {
      ImmutableBiMap.Builder<String, String> builder = ImmutableBiMap.builder();
      for (Entry<String, String> entry : entries) {
        checkNotNull(entry);
        builder.put(entry.getKey(), entry.getValue());
      }
      return builder.build();
    }

  }

  public void testOf() {
    assertMapEquals(ImmutableBiMap.of("one", 1), "one", 1);
    assertMapEquals(ImmutableBiMap.of("one", 1).inverse(), 1, "one");
    assertMapEquals(ImmutableBiMap.of("one", 1, "two", 2), "one", 1, "two", 2);
    assertMapEquals(ImmutableBiMap.of("one", 1, "two", 2).inverse(), 1, "one", 2, "two");
    assertMapEquals(
        ImmutableBiMap.of("one", 1, "two", 2, "three", 3), "one", 1, "two", 2, "three", 3);

  }

  private final transient @Nullable ImmutableBiMap<V, K> inverse;
  @LazyInit @RetainedWith private transient @Nullable ImmutableBiMap<V, K> lazyInverse;

  @Override
  public ImmutableBiMap<V, K> inverse() {
    if (inverse != null) {
      return inverse;
    } else {
      // racy single-check idiom
      ImmutableBiMap<V, K> result = lazyInverse;
      if (result == null) {

 *
 * @author Jared Levy
 * @author Hayward Chan
 */
@SuppressWarnings("serial")
final class RegularImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {
  static final RegularImmutableBiMap<Object, Object> EMPTY =
      new RegularImmutableBiMap<Object, Object>();

  private final ImmutableBiMap<V, K> inverse;

  @SuppressWarnings("unchecked") // used only for the empty map, which works for any types
  RegularImmutableBiMap() {

  public static class ImmutableBiMapGenerator extends TestStringBiMapGenerator {
    @Override
    protected BiMap<String, String> create(Entry<String, String>[] entries) {
      ImmutableBiMap.Builder<String, String> builder = ImmutableBiMap.builder();
      for (Entry<String, String> entry : entries) {
        checkNotNull(entry);
        builder.put(entry.getKey(), entry.getValue());
      }
      return builder.build();
    }

          .put(SortedMultiset.class, ImmutableSortedMultiset.of())
          .put(ImmutableSortedMultiset.class, ImmutableSortedMultiset.of())
          .put(BiMap.class, ImmutableBiMap.of())
          .put(ImmutableBiMap.class, ImmutableBiMap.of())
          .put(Table.class, ImmutableTable.of())
          .put(ImmutableTable.class, ImmutableTable.of())