从映射转换为映射

Question

Map<String, Set<String> result = map.entrySet().parallelStream().collect(
            Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toSet())));

Answer 1

这里的问题是您正在创建的地图的类型是：

Map<Set<String>, Set<String>>

不是映射 > 。

因此，您需要首先扩展映射的值，例如：

Map<String, Set<String>> collect = map.entrySet()
    .parallelStream()
    // Expand (k, {v1, v2, v3}) to [(v1, k), (v2, k), (v3, k)]
    .flatMap(e -> e.getValue().stream().map(ee -> new SimpleEntry<>(ee, e.getKey())))
    .collect(
        Collectors.groupingBy(
            Map.Entry::getKey,
            Collectors.mapping(Map.Entry::getValue, Collectors.toSet())));

Map<String, Set<String>> collect = new HashSet<>();
for (Map.Entry<String, Set<String>> entry : map.entrySet()) {
  for (String v : entry.values()) {
      collect.computeIfAbsent(v -> new HashSet<>())
              .add(entry.getKey()));
  }
}