code: protected final void formatFileName(
      final Object obj, final StringBuffer buf) {
      for (int i = 0; i < patternConverters.length; i++) {
        int fieldStart = buf.length();
        patternConverters[i].format(obj, buf);
  
        if (patternFields[i] != null) {
          patternFields[i].format(fieldStart, buf);
        }
      }
    }
docs: Format file name.
  
  @param obj object to be evaluted in formatting, may not be null.
  @param buf string buffer to which formatted file name is appended, may not be null.