code: public static ValueIndex valueIndex(@NonNull ValueIndexItem... items) {
          if (items == null) {
              throw new IllegalArgumentException("items cannot be null.");
          }
          return new ValueIndex(items);
      }
docs: Create a value index with the given index items. The index items are a list of
  the properties or expressions to be indexed.
  
  @param items The index items
  @return The value index