正在将foreach转换为筛选器[duplicate]

Question

我想转换

List<FlightPositionRoute> flightPositionRoutes = new ArrayList<>();

            dtos.stream()
                    .map(a-> positions.getPositions(a.getId()
                            .getValue()))
                    .forEach(flightPositionRoute -> 
                               flightPositionRoute.ifPresent(flightPositionRoutes::add));

对此：

List<FlightPositionRoute> flightPositionRoutes = dtos.stream()
                    .map(a-> positions.getPositions(a.getId()
                            .getValue()))
                    .filter(Optional::isPresent)
                    .collect(Collectors.toList());

不幸的是，我

Required type: List <FlightPositionRoute>
Provided: List <Optional<FlightPositionRoute>>

我该怎么修理呢？

Answer 1

您只需解压缩可选:

List<FlightPositionRoute> flightPositionRoutes = dtos.stream()
    .map(a-> positions.getPositions(a.getId().getValue()))
    .filter(Optional::isPresent)
    .map(Optional::get)
    .collect(Collectors.toList());