/**
 * GWT emulation of {@link com.google.common.collect.ImmutableBiMap}.
 *
 * @author Hayward Chan
 */
@ElementTypesAreNonnullByDefault
public abstract class ImmutableBiMap<K, V> extends ForwardingImmutableMap<K, V>
    implements BiMap<K, V> {
  public static <T extends @Nullable Object, K, V>
      Collector<T, ?, ImmutableBiMap<K, V>> toImmutableBiMap(
          Function<? super T, ? extends K> keyFunction,