code: public void warn(String format, Object arg) {
          if (logger.isEnabledFor(Level.WARN)) {
              FormattingTuple ft = MessageFormatter.format(format, arg);
              logger.log(FQCN, Level.WARN, ft.getMessage(), ft.getThrowable());
          }
      }
docs: Log a message at the WARN level according to the specified format and
  argument.
  
  <p>
  This form avoids superfluous object creation when the logger is disabled
  for the WARN level.
  </p>
  
  @param format
  the format string
  @param arg
  the argument