Collectors.groupby 用于地图<String,List<String>

2022-01-22 00:00:00 java-8 java java-stream collectors

如果解决方案非常明显，请原谅我，但我似乎无法弄清楚如何做到这一点

Forgive me if the solution is very obvious but I can't seem to figure out how to do this

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("b1", "a1");
    map.put("b2", "a2");
    map.put("b3", "a1");
    Map<String, List<String>> mm = map.values().stream().collect(Collectors.groupingBy(m -> m));
    System.out.println(mm);
}

我想根据 hashmap 中的值进行分组.我希望输出为 {a1=[b1, b3], a2=[b2]} 但它目前以 {a1=[a1, a1], a2=[a2]}

I want to group by based on values in hashmap. I want the output to be {a1=[b1, b3], a2=[b2]} but it is currently coming as {a1=[a1, a1], a2=[a2]}

推荐答案

目前，您正在流式传输地图值(我认为这是一个错字)，根据您需要的输出，您应该流式传输地图 entrySet 并使用基于映射值的 groupingBy 和基于映射键的 mapping 作为下游收集器:

Currently, you're streaming over the map values (which I assume is a typo), based on your required output you should stream over the map entrySet and use groupingBy based on the map value's and mapping as a downstream collector based on the map key's:

 Map<String, List<String>> result = map.entrySet()
            .stream()
            .collect(Collectors.groupingBy(Map.Entry::getValue,
                          Collectors.mapping(Map.Entry::getKey, 
                                        Collectors.toList())));

你也可以通过 forEach + computeIfAbsent 执行这个没有流的逻辑:

You could also perform this logic without a stream via forEach + computeIfAbsent:

Map<String, List<String>> result = new HashMap<>();
map.forEach((k, v) -> result.computeIfAbsent(v, x -> new ArrayList<>()).add(k));

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