convert from Map<Object,Set<Object>> to Map<String,Set<String>>

Sorter :

I have a map Map<String, Set<String>>

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

I want to convert it to Map<String, Set<String>> . by grouping the values and swapping the places of key and value.

But this line gives me

Type mismatch: cannot convert from Map<Object,Set<Object>> to Map<String,Set<String>>

Andy Turner :

The problem that you've got here is the type of the map you are creating is:

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

not Map<String, Set<String>>.

As such, you need to expand the map's values first, for example:

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())));

Unless you really need the parallel processing, I think it would be much easier to use loops:

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()));
  }
}