code: PartitionStrategy getSubpartitionStrategy(int startIndex) {
      if (startIndex == 0) {
        return this;
      }
      if (startIndex >= fieldPartitioners.size()) {
        return null;
      }
      return new PartitionStrategy(fieldPartitioners.subList(startIndex,
          fieldPartitioners.size()));
    }
docs: Return a {@link PartitionStrategy} for subpartitions starting at the given
  index.