Java 8转换Map< K，List< V>>映射< V，列表< K>> [英] Java 8 convert Map<K, List<V>> to Map<V, List<K>>

问题描述

我需要将地图< K，列表< V>> 转换为地图< V，列表< K>> 。
我一直在努力解决这个问题。

I need to convert Map<K, List<V>> to Map<V, List<K>>. I've been struggling with this issue for some time.

很明显如何进行转换 Map< K，V> 到地图< V，列表< K>> ：

It's obvious how to do conversion Map<K, V> to Map<V, List<K>>:

.collect(Collectors.groupingBy(
     Map.Entry::getKey, 
     Collectors.mapping(Map.Entry::getValue, toList())
)

但我找不到解决初始问题。是否有一些易于准备的java-8方式这样做？

But I can't find solve an initial issue. Is there some easy-to-ready-java-8 way to do it?

推荐答案

我认为你很接近，你需要 flatMap 那些 Stream 的条目并从那里收集。我已经使用了现在的 SimpleEntry ，但是你可以使用某种 Pair 。

I think you were close, you would need to flatMap those entries to a Stream and collect from there. I've used the already present SimpleEntry, but you can use a Pair of some kind too.

initialMap.entrySet()
          .stream()
          .flatMap(entry -> entry.getValue().stream().map(v -> new SimpleEntry<>(entry.getKey(), v)))
          .collect(Collectors.groupingBy(
               Entry::getValue,
               Collectors.mapping(Entry::getKey, Collectors.toList())
         ));

好吧，如果你不想创造那些的额外开销SimpleEntry 实例，你可以做的有点不同：

Well, if you don't want to create the extra overhead of those SimpleEntry instances, you could do it a bit different:

    Map<Integer, List<String>> result = new HashMap<>();

    initialMap.forEach((key, values) -> {
        values.forEach(value -> result.computeIfAbsent(value, x -> new ArrayList<>()).add(key));
    });

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