private static class SerializedForm<K> implements Serializable {
      final ImmutableMap<K, ?> map;

      SerializedForm(ImmutableMap<K, ?> map) {
        this.map = map;
      }

      Object readResolve() {
        return map.keySet();
      }

      @J2ktIncompatible // serialization
      private static final long serialVersionUID = 0;
    }
  }

  @Override

    public int compareTo(Object o) {
      return toString().compareTo(o.toString());
    }

    @Override
    public String toString() {
      return "BY_TO_STRING";
    }

    private Object readResolve() {
      return INSTANCE;
    }
  }

  // Always equal is a valid total ordering. And it works for any Object.
  private static final class AlwaysEqual extends Ordering<@Nullable Object>

  @Override
  public <E extends Comparable<?>> E max(Iterable<E> iterable) {
    return NaturalOrdering.INSTANCE.min(iterable);
  }

  // preserving singleton-ness gives equals()/hashCode() for free
  private Object readResolve() {
    return INSTANCE;
  }

  @Override
  public String toString() {
    return "Ordering.natural().reverse()";
  }

  private ReverseNaturalOrdering() {}

  static class SerializedForm implements Serializable {
    final ImmutableCollection<?> collection;

    SerializedForm(ImmutableCollection<?> collection) {
      this.collection = collection;
    }

    Object readResolve() {
      return collection.asList();
    }

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible // serialization
  @J2ktIncompatible

  }

  @Override
  public int hashCode() {
    return 0x79a31aac;
  }

  @Override
  public String toString() {
    return "Optional.absent()";
  }

  private Object readResolve() {
    return INSTANCE;
  }

  private static final long serialVersionUID = 0;

    @Override
    public int hashCode() {
      return System.identityHashCode(this);
    }

    @Override
    public String toString() {
      return "-\u221e";
    }

    private Object readResolve() {
      return INSTANCE;
    }

    private static final long serialVersionUID = 0;
  }

  /*
   * The implementation neither produces nor consumes any non-null instance of

  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public <T extends B> T putInstance(Class<T> type, T value) {
    throw new UnsupportedOperationException();
  }

  Object readResolve() {
    return isEmpty() ? of() : this;
  }

  private static class EntrySetSerializedForm<K, V> implements Serializable {
    final ImmutableMap<K, V> map;

    EntrySetSerializedForm(ImmutableMap<K, V> map) {
      this.map = map;
    }

    Object readResolve() {
      return map.entrySet();
    }

    private static final long serialVersionUID = 0;
  }

  private static class SerializedForm<V> implements Serializable {
    final ImmutableMap<?, V> map;

    SerializedForm(ImmutableMap<?, V> map) {
      this.map = map;
    }

    Object readResolve() {
      return map.values();
    }

    private static final long serialVersionUID = 0;
  }

  @Override
  public <S extends Comparable<?>> Ordering<S> reverse() {
    return (Ordering<S>) ReverseNaturalOrdering.INSTANCE;
  }

  // preserving singleton-ness gives equals()/hashCode() for free
  private Object readResolve() {
    return INSTANCE;
  }

  @Override
  public String toString() {
    return "Ordering.natural()";
  }

  private NaturalOrdering() {}

  private static final long serialVersionUID = 0;