int priceSum= list.stream().mapToInt(Order::getPrice).sum(); double priceSum= list.stream().mapToDouble(Order::getPrice).sum(); long priceSum= list.stream().mapToLong(Order::getPrice).sum();
BigDecimal priceSum =list.stream().map(Order::getPrice).reduce(BigDecimal.ZERO,BigDecimal::add);
int prices = list.stream().mapToInt(x-> x.getprice * x.getTotal).sum();
BigDecimal prices = list.stream().map(x-> x.getPrice().multiply(new BigDecimal(x.getTotal()))).reduce(BigDecimal.ZERO, BigDecimal::add);
List<Long> userIds= userList.stream().map(User::getUserId).collect(Collectors.toList());