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

  /**
   * A dummy singleton value type used by {@link Interners}.
   *
   * <p>{@link MapMakerInternalMap} can optimize for memory usage in this case; see {@link

   */
  @GwtIncompatible // java.lang.ref.SoftReference
  @CanIgnoreReturnValue
  public CacheBuilder<K, V> softValues() {
    return setValueStrength(Strength.SOFT);
  }

  @CanIgnoreReturnValue
  CacheBuilder<K, V> setValueStrength(Strength strength) {
    checkState(valueStrength == null, "Value strength was already set to %s", valueStrength);
    valueStrength = checkNotNull(strength);

      builder.refreshAfterWrite(refresh.duration, refresh.unit);
    }
    if (keyStrength != null) {
      builder.setKeyStrength(keyStrength);
    }
    if (valueStrength != null) {
      builder.setValueStrength(valueStrength);
    }
    return builder;
  }

  static class DurationSpec {
    private final long duration;
    private final TimeUnit unit;

    MapMaker readMapMaker(ObjectInputStream in) throws IOException {
      int size = in.readInt();
      return new MapMaker()
          .initialCapacity(size)
          .setKeyStrength(keyStrength)
          .setValueStrength(valueStrength)
          .keyEquivalence(keyEquivalence)
          .concurrencyLevel(concurrencyLevel);
    }

    @SuppressWarnings("unchecked")
    @J2ktIncompatible // java.io.ObjectInputStream