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如何使用流獲取嵌套集合中的所有元素

Java

慕尼黑8549860 2022-12-28 10:48:58

我有一個包含不同嵌套集合的類，現在我想接收嵌套集合的所有元素，具體我想收集集合的所有 StrokePoints。我可以用“舊的”java 來解決它，但是如何用流來解決呢？ int strokesCounter = 0; List<StrokePoint> pointList = new ArrayList<>(); if (!strokesData.getListOfSessions().isEmpty()) { for (SessionStrokes session : strokesData.getListOfSessions()) { List<Strokes> strokes = session.getListOfStrokes(); for (Strokes stroke : strokes) { strokesCounter++; List<StrokePoint> points = stroke.getListOfStrokePoints(); pointList.addAll(stroke.getListOfStrokePoints()); } } }我正在尋找一種使用流功能填充 pointList 的方法。

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4 回答

波斯汪

TA貢獻1811條經驗獲得超4個贊

展平嵌套數據非常簡單：

List<StrokePoint> pointList = strokesData.getListOfSessions()
        .streams()
        .map(SessionStrokes::getListOfStrokes)
        .flatMap(List::stream)
        .map(Strokes::getListOfStrokePoints)
        .flatMap(List::stream)
        .collect(Collectors.toList());

沿途收集劃水次數比較棘手，而且有些爭議。你可以創建一個

AtomicInteger strokesCounter = new AtomicInteger();

并在第一個之后增加它flatMap：

.peek(strokesCounter::incrementAndGet)

反對回復 2022-12-28

白板的微信

TA貢獻1883條經驗獲得超3個贊

你可以只使用Stream.flatMap()兩次：

List<StrokePoint> pointList = strokesData.getListOfSessions().stream()
        .flatMap(session -> session.getListOfStrokes().stream())
        .flatMap(strokes -> strokes.getListOfStrokePoints().stream())
        .collect(Collectors.toList());

如果您需要計算筆畫列表，您可以將其分成兩部分并使用List.size()：

List<Strokes> strokesList = strokesData.getListOfSessions().stream()
        .flatMap(session -> session.getListOfStrokes().stream())
        .collect(Collectors.toList());int strokesCounter = strokesList.size();
List<StrokePoint> pointList = strokesList.stream()
        .flatMap(strokes -> strokes.getListOfStrokePoints().stream())
        .collect(Collectors.toList());

或者你可以增加一個AtomicIntegerin flatMap()：

final AtomicInteger strokesCounter = new AtomicInteger();
List<StrokePoint> pointList = strokesData.getListOfSessions().stream()
        .flatMap(session -> {
            List<Strokes> strokes = session.getListOfStrokes();
            strokesCounter.addAndGet(strokes.size());   
                     return strokes.stream();
        })
        .flatMap(strokes -> strokes.getListOfStrokePoints().stream())
        .collect(Collectors.toList());

或與peek()：

final AtomicInteger strokesCounter = new AtomicInteger();
List<StrokePoint> pointList = strokesData.getListOfSessions().stream()
        .flatMap(session -> session.getListOfStrokes().stream())
        .peek(i -> strokesCounter.incrementAndGet())
        .flatMap(strokes -> strokes.getListOfStrokePoints().stream())
        .collect(Collectors.toList());

反對回復 2022-12-28

慕后森

TA貢獻1802條經驗獲得超5個贊

由于主要目的是解決收集的問題List<StrokePoint>，您可以使用以下flatMap操作執行它：

List<StrokePoint> points = strokesData.getListOfSessions()

.stream()

.flatMap(ss -> ss.getListOfStrokes().stream()

.flatMap(s -> s.getListOfStrokePoints().stream()))

.collect(Collectors.toList());

此外，Strokes 的計數也可以通過對列表的大小求和來使用流來計算：

long strokeCount = strokesData.getListOfSessions()

.stream()

.mapToLong(ss -> ss.getListOfStrokes().size())

.sum();

要合并這些操作，您可以構造AbstractMap.SimpleEntrywhile 減少條目為：

AbstractMap.SimpleEntry<Integer, Stream<StrokePoint>> reduce = strokesData.getListOfSessions()

.stream()

.map(ss -> new AbstractMap.SimpleEntry<>(ss.getListOfStrokes().size(),

ss.getListOfStrokes()

.stream()

.flatMap(s -> s.getListOfStrokePoints().stream())))

.reduce(new AbstractMap.SimpleEntry<>(1, Stream.empty()),

(e1, e2) -> new AbstractMap.SimpleEntry<>(

Integer.sum(e1.getKey(), e2.getKey()),

Stream.concat(e1.getValue(), e2.getValue())));

使用此條目，您可以獲得 s 的計數和Strokes 的列表StrokePoint：

long strokeCount = reduce.getKey();

List<StrokePoint> strokePoints = reduce.getValue().collect(Collectors.toList());

反對回復 2022-12-28

江戶川亂折騰

TA貢獻1851條經驗獲得超5個贊

如果您擔心副作用，這樣做就可以了（但是在撰寫本文時其他兩個答案都更具可讀性）：

Entry<Integer, List<StrokePoint>> summary = //

strokesData.getListOfSessions()

.stream()

.flatMap(session -> session.getListOfStrokes().stream())

.map(strokes -> new SimpleEntry<>(1, strokes.getListOfStrokePoints()))

.reduce((l1, l2) -> {

int count = l1.getKey() + l2.getKey();

List<StrokePoint> list = l1.getValue();

list.addAll(l2.getValue());

return new SimpleEntry<>(count, list);

})

.orElse(new SimpleEntry<>(0, Collections.emptyList()));

strokesCounter = summary.getKey();

pointList = summary.getValue();

編輯以添加驗證：

public class Scratch {

public static void main(String[] args) {

int strokesCounter = 0;

List<StrokePoint> pointList = new ArrayList<>();

StrokesData strokesData = new StrokesData();

SessionStrokes sessionStrokes = new SessionStrokes();

strokesData.sessionStrokes.add(sessionStrokes);

Strokes s1 = new Strokes();

sessionStrokes.strokesList.add(s1);

s1.strokePoints.add(new StrokePoint());

Strokes s2 = new Strokes();

sessionStrokes.strokesList.add(s2);

s2.strokePoints.add(new StrokePoint());

Entry<Integer, List<StrokePoint>> summary = //

strokesData.getListOfSessions()

.stream()

.flatMap(session -> session.getListOfStrokes()

.stream())

.map(strokes -> new SimpleEntry<>(1, strokes.getListOfStrokePoints()))

.reduce((l1, l2) -> {

int count = l1.getKey() + l2.getKey();

List<StrokePoint> list = l1.getValue();

list.addAll(l2.getValue());

return new SimpleEntry<>(count, list);

})

.orElse(new SimpleEntry<>(0, Collections.emptyList()));

strokesCounter = summary.getKey();

pointList = summary.getValue();

System.out.println(strokesCounter);

System.out.println(pointList);

}

class StrokesData {

List<SessionStrokes> sessionStrokes = new ArrayList<>();

public List<SessionStrokes> getListOfSessions() {

return sessionStrokes;

}

class SessionStrokes {

List<Strokes> strokesList = new ArrayList<>();

public List<Strokes> getListOfStrokes() {

return strokesList;

}

class Strokes {

List<StrokePoint> strokePoints = new ArrayList<>();

public List<StrokePoint> getListOfStrokePoints() {

return strokePoints;

}

class StrokePoint {

}

反對回復 2022-12-28

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如何使用流獲取嵌套集合中的所有元素

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