过滤收集问题，使用 Stream API

Question

I try to filter collection of objects that have collection field, using Stream API:

@Override
public void method (List<Flight> flightList) 
{
List<Flight> trueFlights = flightList.forEach(flight -> flight.getSegments().stream().filter(segment -> segment.getArrivalDate().isBefore(segment.getDepartureDate())).collect(Collectors.toList()));
return trueFlights;
}

But it does not compile:

Error:(32, 27) java: incompatible types: void cannot be converted to java.util.List

Segment:

class Segment {
    private final LocalDateTime departureDate;

    private final LocalDateTime arrivalDate; 
    //..getter setter  

Flight:

class Flight {
    private final List<Segment> segments;
   //.. getter setter 

What am I doing wrong?

Answer 1

You could be looking for filter operation over a Stream . That would further require you to include a predicate, which is possible with the use of terminal operations anyMatch , allMatch over the inner stream.

For example, if you want to include input flights to trueFlights if any of its segment satisfies the condition that the arrival date is before departure date, you could perform anyMatch :

List<Flight> trueFlights = flightList.stream()
        .filter(flight -> flight.getSegments().stream()
                .anyMatch(segment -> segment.getArrivalDate().isBefore(segment.getDepartureDate())))
        .collect(Collectors.toList());

What you did incorrectly on the other hand was to make use of forEach which has a void return type and then trying to collect elements further from it.