code: public void addSort(WritableComparable<?> writable, int sort, int priority) {
          key.addHadoopValue(String.format(SORT_LABEL, priority), writable, sort, priority);
      }
docs: Add sort key
  @param writable Hadoop sort key
  @param sort sort type SORT_NON or SORT_LOWER or SORT_UPPER
  @param priority sort order