code: public String getRecordId() {
          if (recordIdFieldStart == RECORD_ID_MISSING) {
              return null;
          }
          final int dataStart = baseAddress + recordIdFieldStart;
          final int dataLength = buffer.distanceTo(DATA_SEPARATORS, dataStart);
          return buffer.stringAt(dataStart, dataLength, charset);
      }
docs: Returns the contents of the record identifier field. The record identifier
  field has the tag <i>001</i>. It must be the first field in the record.
  <p>
  Defined in section 4.5.2 of the ISO 2709:2008 standard.
  
  @return a string which identifies the record or null if the record has
  no record identifier.