code: @Override
      public void debug(String format, Object arg) {
          if (logger.isDebugEnabled()) {
              FormattingTuple ft = MessageFormatter.format(format, arg);
              logger.debug(ft.getMessage(), ft.getThrowable());
          }
      }
docs: Delegates to the {@link Log#debug(Object)} method of the underlying
  {@link Log} instance.
  
  <p>
  However, this form avoids superfluous object creation when the logger is disabled
  for level DEBUG.
  </p>
  
  @param format
  the format string
  @param arg
  the argument