code: public String formatExtendedISO(
          IsoDecimalStyle decimalStyle,
          ClockUnit precision
      ) {
  
          ClockInterval interval = this.toCanonical();
          StringBuilder buffer = new StringBuilder();
          ChronoPrinter<PlainTime> printer = Iso8601Format.ofExtendedTime(decimalStyle, precision);
          printer.print(interval.getStartAsClockTime(), buffer);
          buffer.append('/');
          printer.print(interval.getEndAsClockTime(), buffer);
          return buffer.toString();
  
      }
docs: /*[deutsch]
  <p>Formatiert die kanonische Form dieses Intervalls im angegebenen <i>extended</i> ISO-8601-Stil. </p>
  
  @param   decimalStyle    iso-compatible decimal style
  @param   precision       controls the precision of output format with constant length
  @return  String
  @throws  IllegalStateException if there is no canonical form (for example for [00:00/24:00])
  @see     #toCanonical()
  @since   4.18