public String toString() {
    return Maps.toStringImpl(this);
  }

  /**
   * Serialized type for all ImmutableMap instances. It captures the logical contents and they are
   * reconstructed using public factory methods. This ensures that the implementation types remain
   * as implementation details.
   */
  @J2ktIncompatible // serialization
  static class SerializedForm<K, V> implements Serializable {