如何使用Java的可选功能

Question

我有一个用例，想知道如何使用Java Optional最好地编写它，以便减少空检查？

// To get first residence place for family places or empty string if null
// What if familyPlaces is null?
public String getFirstResidencePlace(FamilyPlaces familyPlaces) {
    // What if getResidencePlaces() returns null in list?
    List<Place> places = familyPlaces.getResidencePlaces();
    for (Place place : places) {
        if (place.getResidencePlaces() != null &&
            !place.getResidencePlaces().isEmpty()) {
            return place.getResidencePlaces().toLowerCase();
        }
    }
    return "";
}

Answer 1

[这可以使用Optional来真正处理，确保第一个调用方法导致null值（familyPlaces.getResidencePlaces()）的情况：

Java 9 +

Optional.ofNullable(familyPlaces.getResidencePlaces())        // Optional<List<Place>>
        .stream()                                             // Stream<List<Place>>
        .flatMap(list -> list.stream()
                             .map(Place::getResidencePlaces)) // Stream<String>
        .filter(res -> res != null && !res.isEmpty())         // Stream<String>
        .map(String::toLowerCase)                             // Stream<String>
        .findFirst()                                          // the first String found
        .orElse("");                                          // default return value

注意，平面映射等于：

.flatMap(List::stream)                    // from Stream<List<Place>> to Stream<Place>
.map(Place::getResidencePlaces)           // from Stream<Place> to Stream<String>

Java 8：

Optional.ofNullable(familyPlaces.getResidencePlaces())        // Optional<List<Place>>
        .orElse(Collections.emptyList())                      // List<Place>
        .stream()                                             // Stream<Place>
        .map(Place::getResidencePlaces)                       // Stream<String>
        .filter(res -> res != null && !res.isEmpty())         // Stream<String>
        .map(String::toLowerCase)                             // Stream<String>
        .findFirst()                                          // the first String found
        .orElse("");                                          // default return value