code: static String buildFormatString(FormatOptions options, FormatChar formatChar) {
      // The format char is guaranteed to be a lower-case ASCII character, so can be made upper case
      // by simply subtracting 0x20 (or clearing the 6th bit).
      char c = formatChar.getChar();
      c = options.shouldUpperCase() ? (char) (c & ~0x20) : c;
      return options.appendPrintfOptions(new StringBuilder("%")).append(c).toString();
    }
docs: Visible for testing.