Java 8 Stream API-用Java 8重写的Java 9 Collectors.flatMapping [英] Java 8 Stream API - Java 9 Collectors.flatMapping rewritten in Java 8

问题描述

由于 java-9 称为此处的示例):

List<List<Integer>> list = Arrays.asList(
    Arrays.asList(1, 2, 3, 4, 5, 6), 
    Arrays.asList(7, 8, 9, 10));

Map<Integer, List<Integer>> map =list.stream()
    .collect(Collectors.groupingBy(
         Collection::size,
         Collectors.flatMapping(
             l -> l.stream().filter(i -> i % 2 == 0),
             Collectors.toList())));

{4 = [8，10]，6 = [2，4，6]}

{4=[8, 10], 6=[2, 4, 6]}

仅使用 3 个收集器，这是一种相当优雅的方法.我需要在java-8 尚不支持.我尝试使用 6 收集器，该收集器使用范围很广，我无法找出减少使用它们的方法:

This is a fairly elegant way using just 3 collectors. I need to rewrite the collector in java-8 where is not yet supported. My attempt use 6 Collectors that is quite an extensive usage and I am not able to figure out a way using less of them:

Map<Integer, List<Integer>> map = list.stream()
    .collect(Collectors.groupingBy(
        Collection::size,
        Collectors.collectingAndThen(
            Collectors.mapping(
                l -> l.stream().filter(i -> i % 2 == 0).collect(Collectors.toList()),
                Collectors.toList()),
            i -> i.stream().flatMap(j -> j.stream()).collect(Collectors.toList()))));

是否仅使用

Is there a shorter better way using solely java-8?

推荐答案

对于这种特殊情况，我想这将是一个更简单的版本:

For just this particular case, I guess this would be a simpler version:

Map<Integer, List<Integer>> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toList())
            ));

如果要合并(两个具有相同大小的集合)，我将添加一个非常简单的merge函数:

If there would be merging involved (two collections that would have the same size), I would add a merge function that is pretty trivial:

 Map<Integer, List<Integer>> map =
        list.stream()
            .collect(Collectors.toMap(
                Collection::size,
                x -> x.stream().filter(y -> y % 2 == 0).collect(Collectors.toCollection(ArrayList::new)),
                (left, right) -> {
                    left.addAll(right);
                    return left;
                }
            ));

否则，我在此评论中同意Michael的观点，将其移植回java-8并不难.

Otherwise, I agree with Michael in this comment, this is not that hard to back-port to java-8.

这篇关于Java 8 Stream API-用Java 8重写的Java 9 Collectors.flatMapping的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！