* @author Kevin Bourrillion
 */
@GwtCompatible
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {

  final transient K singleKey;
  final transient V singleValue;

  SingletonImmutableBiMap(K singleKey, V singleValue) {
    checkEntryNotNull(singleKey, singleValue);
    this.singleKey = singleKey;
    this.singleValue = singleValue;

import org.jspecify.annotations.Nullable;

/**
 * GWT emulation of {@link SingletonImmutableBiMap}.
 *
 * @author Hayward Chan
 */
final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {
  private final K singleKey;
  private final V singleValue;

  @Nullable transient SingletonImmutableBiMap<V, K> inverse;

  SingletonImmutableBiMap(K key, V value) {
    super(singletonMap(checkNotNull(key), checkNotNull(value)));

FIELD com.google.common.collect.JdkBackedImmutableBiMap.inverse
FIELD com.google.common.collect.Maps.FilteredMapValues.unfiltered
FIELD com.google.common.collect.Sets.SubSet.inputSet
FIELD com.google.common.collect.SingletonImmutableBiMap.inverse
FIELD com.google.common.collect.TreeTraverser.PostOrderNode.childIterator
FIELD com.google.common.collect.TreeTraverser.PreOrderIterator.stack

    return (ImmutableBiMap<K, V>) RegularImmutableBiMap.EMPTY;
  }

  public static <K, V> ImmutableBiMap<K, V> of(K k1, V v1) {
    checkEntryNotNull(k1, v1);
    return new SingletonImmutableBiMap<K, V>(k1, v1);
  }

  public static <K, V> ImmutableBiMap<K, V> of(K k1, V v1, K k2, V v2) {
    return new RegularImmutableBiMap<K, V>(ImmutableMap.of(k1, v1, k2, v2));
  }