Java 8流收集项目列表的地图 [英] Java 8 stream to collect a Map of List of items

问题描述

我有一个地图列表，其中存储了角色和人的名字.例如:

I have a List of Maps which stores the roles and the names of people. For ex:

List<Map<String, String>> listOfData

1) Role:  Batsman
   Name:  Player1

2)Role:  Batsman
   Name:  Player2

3)Role:  Bowler
   Name:  Player3

角色和名称是地图的键. 我想将其转换为Map<String, List<String>> result，这将为我提供每个角色的名称列表，即

Role and Name are the Keys of the map. I want to convert this into a Map<String, List<String>> result, which will give me a list of names for each role, i.e

k1: Batsman  v1: [Player1, Player2]
k2: Bowler   v2: [Player3]


listOfData
    .stream()
    .map(entry -> new AbstractMap.SimpleEntry<>(entry.get("Role"), entry.get("Name"))
    .collect(Collectors.toList());

这样做不会给我角色的名称列表，而只会给我一个名称.我如何继续收集列表中的元素，然后将其添加到键中?

Doing this way will not give me a list of names for the role, it will give me a single name. How do i keep collecting the elements of the list and then add it to a key ?

用于创建基本结构的Java代码:

Java code to create base structure:

Map<String, String> x1 = ImmutableMap.of("Role", "Batsman", "Name", "Player1");

        Map<String, String> y1 = ImmutableMap.of("Role", "Batsman", "Name", "Player2");

        Map<String, String> z1 = ImmutableMap.of("Role", "Bowler", "Name", "Player3");


        List<Map<String, String>> list = ImmutableList.of(x1, y1, z1);
        Map<String, List<String>> z = list.stream()
                    .flatMap(e -> e.entrySet().stream())
                    .collect(Collectors.groupingBy(Map.Entry::getKey,
                            Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

推荐答案

基于Aomine给出的想法:

Based on the idea given by Aomine:

list.stream()
    .map(e -> new AbstractMap.SimpleEntry<>(e.get("Role"), e.get("Name")))
    .collect(Collectors.groupingBy(Map.Entry::getKey,
                    Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

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