code: public static JobID forName(String str) throws IllegalArgumentException {
      if(str == null)
        return null;
      try {
        String[] parts = str.split("_");
        if(parts.length == 3) {
          if(parts[0].equals(JOB)) {
            return new org.apache.hadoop.mapred.JobID(parts[1], 
                                                      Integer.parseInt(parts[2]));
          }
        }
      }catch (Exception ex) {//fall below
      }
      throw new IllegalArgumentException("JobId string : " + str 
          + " is not properly formed");
    }
docs: Construct a JobId object from given string
  @return constructed JobId object or null if the given String is null
  @throws IllegalArgumentException if the given string is malformed