SimpleDateFormat dt1 = new SimpleDateFormat("ddmmyyyyy"); List<MenuPrice> menuPrices = findAll(restaurant); menuPrices.parallelStream() .collect(Collectors.groupingBy(dt1.format(MenuPrice::getUpdateDate)));
Map<String,List<MenuPrice>> menuPrices.parallelStream() .collect(Collectors.groupingBy(m -> dt1.format(m.getUpdateDate())));