如何使用流获取嵌套集合中的所有元素 [英] How to get all elements in nested collection with streams

问题描述

我有一个包含不同嵌套集合的类，现在我想接收嵌套集合的所有元素，具体来说，我想收集集合的所有StrokePoints.我可以使用旧的" java解决它，但如何使用流来解决呢?

I have a class which contains different nested collections, now I want to receive all the elements of the nested collections, concrete I want to collect all the StrokePoints of the collections. I can solve it with "old" java but how to do it with streams?

    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());        
            }
        }
    }

我正在寻找一种使用流功能填充pointList的方法.

I am looking for a way to fill the pointList with the stream functionality.

推荐答案

您可以只使用

You can just use Stream.flatMap() twice:

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

如果需要计算笔画列表，可以将其分为两部分并使用List.size():

If you need to count the strokes list you can split this into two parts and use 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，您可以在flatMap()中增加AtomicInteger:

Alternatively you can increment an AtomicInteger in 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());

或使用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());

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