code: public final void renderMessageString() {
          Object[] coreFields = event.getFieldsToLog();
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < coreFields.length + xFields.length; ++i) {
              if (i > 0) {
                  sb.append(fieldSeperator);
              }
              Object field = i < coreFields.length ? coreFields[i] : xFields[i - coreFields.length];
              renderObject(sb, field);
  
          }
          messageString = sb.toString();
  
      }
docs: /* (non-Javadoc)
  @see com.betfair.cougar.logging.records.LoggableEvent#renderMessageString()