* @see WeakReference
   */
  @CanIgnoreReturnValue
  @GwtIncompatible // java.lang.ref.WeakReference
  public MapMaker weakKeys() {
    return setKeyStrength(Strength.WEAK);
  }

  @CanIgnoreReturnValue
  MapMaker setKeyStrength(Strength strength) {
    checkState(keyStrength == null, "Key strength was already set to %s", keyStrength);
    keyStrength = checkNotNull(strength);