Use Java 8 Optional in existing Java 7 code

Question

I have an assignment in which I need to convert the following pre-Java 8 code to Java 8 code. Below is just one method which is giving me hard time to finish up:

Answer 1

I can't say this is very elegant, but it should satisfy your requirement. There are no explicit null checks, but it'll throw the exception if any input parameters are null, and it filters out vehicles with invalid names from the resulting list.

public static List loadMatching(Region region, String nameStartsWith, VehicleLoader loader) {
    return Optional.ofNullable(nameStartsWith)
            .flatMap(startWith -> Optional.ofNullable(loader)
                    .flatMap(vl -> Optional.ofNullable(region)
                            .map(Region::name)
                            .map(vl::getVehicleMakesByRegion))
                    .map(makes -> makes.stream()
                            .filter(make -> Optional.ofNullable(make.getName())
                                    .filter(name -> name.startsWith(startWith))
                                    .isPresent())
                            .collect(Collectors.toList())))
            .orElseThrow(() -> new IllegalArgumentException("The VehicleLoader and both region and nameStartsWith are required when loading VehicleMake matches"));