code: @Override
    public int compareTo(JobId o) {
      if (o == null) {
        // nulls sort last
        return -1;
      }
  
      return new CompareToBuilder()
          .append(this.jobEpoch, o.getJobEpoch())
          .append(this.jobSequence, o.getJobSequence())
          .toComparison();
    }
docs: Compares two JobId objects on the basis of their
  jobEpoch (jobtracker start time from the job ID)
  and
  jobSequence( jobtracker assigned sequence number for the job,)
  
  @param other
  @return 0 if this jobEpoch and jobSequence are equal to
  other jobEpoch and jobSequence,
  1 if this jobEpoch and jobSequence are greater than
  other jobEpoch and jobSequence,
  -1 if this jobEpoch and jobSequence less than
  other jobEpoch and jobSequence