code: @Override
    public int compareTo(ID o) {
      TaskAttemptID that = (TaskAttemptID)o;
      int tipComp = this.taskId.compareTo(that.taskId);
      if(tipComp == 0) {
        return this.id - that.id;
      }
      else return tipComp;
    }
docs: Compare TaskIds by first tipIds, then by task numbers.