converting foreach to filter [duplicate]

Question

I would like to convert

List<FlightPositionRoute> flightPositionRoutes = new ArrayList<>(); dtos.stream() .map(a-> positions.getPositions(a.getId() .getValue())) .forEach(flightPositionRoute -> flightPositionRoute.ifPresent(flightPositionRoutes::add));

To this:

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

I mean I would like to use filter instead of foreach, because it's prettier ;)

Unfortunately, I'm getting

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

How can I repair it?

flatMap by Optional:stream should be enough: dtos.stream().map(a-> positions.getPositions(a.getId().getValue())).flatMap(Optional::stream).collect(Collectors.toList()); will provide List<FlightPositionRoute> — Nowhere Man
– Nowhere Man, Commented Oct 7, 2020 at 19:25

Polygnome · Accepted Answer · 2020-10-07 18:19:01Z

You just need to unpack the Optional:

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

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converting foreach to filter [duplicate]

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