guava/src/com/google/common/collect/MapMakerInternalMap.java
static <K>
      MapMakerInternalMap<K, Dummy, ? extends InternalEntry<K, Dummy, ?>, ?> createWithDummyValues(
          MapMaker builder) {
    if (builder.getKeyStrength() == Strength.STRONG
        && builder.getValueStrength() == Strength.STRONG) {
      return new MapMakerInternalMap<>(builder, StrongKeyDummyValueEntry.Helper.<K>instance());
    }
    if (builder.getKeyStrength() == Strength.WEAK
        && builder.getValueStrength() == Strength.STRONG) {
      return new MapMakerInternalMap<>(builder, WeakKeyDummyValueEntry.Helper.<K>instance());
    }
    if (builder.getValueStrength() == Strength.WEAK) {
      throw new IllegalArgumentException("Map cannot have both weak and dummy values");
    }
    throw new AssertionError();
  }
static <K>
      MapMakerInternalMap<K, Dummy, ? extends InternalEntry<K, Dummy, ?>, ?> createWithDummyValues(
          MapMaker builder) {
    if (builder.getKeyStrength() == Strength.STRONG
        && builder.getValueStrength() == Strength.STRONG) {
      return new MapMakerInternalMap<>(builder, StrongKeyDummyValueEntry.Helper.<K>instance());
    }
    if (builder.getKeyStrength() == Strength.WEAK
        && builder.getValueStrength() == Strength.STRONG) {
      return new MapMakerInternalMap<>(builder, WeakKeyDummyValueEntry.Helper.<K>instance());
    }
    if (builder.getValueStrength() == Strength.WEAK) {
      throw new IllegalArgumentException("Map cannot have both weak and dummy values");
    }
    throw new AssertionError();
  }
Returns a fresh {@link MapMakerInternalMap} with {@link MapMaker.Dummy} values but otherwise as
specified by the given {@code builder}. The returned {@link MapMakerInternalMap} will be
optimized to saved memory. Since {@link MapMaker.Dummy} is a singleton, we don't need to store
any values at all. Because of this optimization, {@code build.getValueStrength()} must be
{@link Strength#STRONG}.

<p>This method is intended to only be used by the internal implementation of {@link Interners},
since a map of dummy values is the exact use case there.