Java – format date in function

I tried to format the date in the function interface, but I don't know if it is possible

SimpleDateFormat dt1 = new SimpleDateFormat("ddmmyyyyy");

List<MenuPrice> menuPrices = findAll(restaurant);

menuPrices.parallelStream()
          .collect(Collectors.groupingBy(dt1.format(MenuPrice::getUpdateDate)));

Solution

This is possible, but not a method reference:

Map<String,List<MenuPrice>>
    menuPrices.parallelStream()
              .collect(Collectors.groupingBy(m -> dt1.format(m.getUpdateDate())));