@Override
    public int compare(WritableComparable w1, WritableComparable w2) {
      LinkKey k1 = (LinkKey) w1;
      LinkKey k2 = (LinkKey) w2;

      return k1.compareTo(k2);
    }
Comparator used to order linkKeys so that they are passed to a reducer in order. This is based
upon linkKey ChainId and Order.
hbase-it/src/test/java/org/apache/hadoop/hbase/mapreduce/IntegrationTestBulkLoad.java