checkArgument(forward.isEmpty());
    checkArgument(backward.isEmpty());
    checkArgument(forward != backward);
    return makeInverse(backward);
  }

  AbstractBiMap<V, K> makeInverse(Map<V, K> backward) {
    return new Inverse<>(backward, this);
  }

  void setInverse(AbstractBiMap<V, K> inverse) {
    this.inverse = inverse;
  }