Java 8 显示此错误。在封闭作用域中定义的局部变量 itemList 必须是 final 或有效 final

Question

我正在使用 java 8 编写代码，但我迭代了一个 List然后找到RestaurantOrderBook使用类别类型。然后把那个 List成Map .它显示此错误:

Local variable itemList defined in an enclosing scope must be final or effectively final

Query query = new Query();
String categ = category;
query.addCriteria(Criteria.where("restaurantId").is(restaurantId));

List<RestaurantOrderBook> itemList = new ArrayList<RestaurantOrderBook>();
itemList = mongoTemplate.find(query, RestaurantOrderBook.class);
System.out.println("size : " + itemList.size());
Map<String , List<RestaurantOrderBook>> map = new HashMap<String , List<RestaurantOrderBook>>();
        Arrays.asList("TakeAway", "Dining").forEach(e ->{

//Following line throws error:              
List<RestaurantOrderBook> list  = itemList.stream().filter(a -> !a.getOrderType().isEmpty() && a.getOrderType().equals(e)).collect(Collectors.toList());
                map.put(e, list);
        });

我还有另一种情况:

@Override
    public EventReportRewardsPoints eventReportRewardsPoints(String organizerId) {
        try{
            List<Event> listOfEvents = eventRepo.findByOrganizerId(organizerId);
            EventReportRewardsPoints eventTransReport = new EventReportRewardsPoints();
            Integer soldTics = 0;
            Double totalRevenue = 0d;
            Integer soldToday  = 0;

            Integer findTotalAvailableTics = 0;
            Integer findTotalQtytics = 0;

            for (Event event : listOfEvents) {
                List<EventTicket> eventTicket = eventTicketRepo.findByEventId(event.getId());

                Integer sumOfAvailabletics = eventTicket.stream()
                        .mapToInt(EventTicket::getRemainingTickets).sum();
                findTotalAvailableTics = findTotalAvailableTics + sumOfAvailabletics;

                Integer sumOfQtytics = eventTicket.stream().mapToInt(EventTicket::getQty).sum();
                findTotalQtytics  = findTotalQtytics + sumOfQtytics;

                List<EventTicketBook> listOfEventsTic = eventTicketBookRepository.findByEventId(event.getId());
                for (EventTicketBook eventTicketBook : listOfEventsTic) {
                    Double sumOfSales = eventTicketBook.getTickets().stream().mapToDouble(EventTicket::getPrice).sum();
                    totalRevenue = totalRevenue + sumOfSales;
                    Date now = new Date();
                    System.out.println("create date : " + eventTicketBook.getCreateOn());
                    /*if(now.compareTo(eventTicketBook.getCreateOn()) == 0){
                        Integer sumOfSoldToday = eventTicketBook.getTickets().stream().mapToInt(EventTicket::getQty).sum();
                        soldToday = soldToday + sumOfSoldToday;
                    }*/
                }
            }
            System.out.println("findTotalQtytics : " + findTotalQtytics);
            System.out.println("findTotalAvailableTics : " + findTotalAvailableTics);
            soldTics = findTotalQtytics - findTotalAvailableTics;
            eventTransReport.setTotalRevenue(totalRevenue);
            eventTransReport.setSoldToday(soldToday);
            eventTransReport.setTicketsSold(soldTics);
            return eventTransReport;
        }catch(Exception e ){
            e.printStackTrace();
        }
        return null;
    }

我如何使用 lamda 表达式来实现这一点。？？

Answer 1

您正在使用 itemList 在 lambda 表达式中。因此它必须是最终的。

Java 8 引入了 的新概念最终生效 ，这意味着编译器会检查使用的变量是否在使用中是最终的，并且不会强制开发人员将其显式声明为最终的。

因此，如果您将代码更改为

final List<RestaurantOrderBook> itemList = new ArrayList<RestaurantOrderBook>();

你会看到，编译器给你一个错误:

itemList = mongoTemplate.find(query, RestaurantOrderBook.class);

因为你要重新签约 itemList .这就是为什么 itemList 也不是有效的最终结果。如果你挤压这两行

List<RestaurantOrderBook> itemList = mongoTemplate.find(query, RestaurantOrderBook.class);

它应该工作。