Merge all list values together in a map [duplicate]

Question

This question already has an answer here:

• How to retainAll of List of Lists using stream reduce 2 answers

I would like to convert a Map like:

Map<String, List<String>> 

to

List<String> 

where the result list is the merge of all List values.

Answer 1

You can just have

List<String> result = map.values().stream().flatMap(List::stream).collect(Collectors.toList());

This retrieves the values of the map with values() then flat maps each list into a Stream formed by its elements and collects the result into a list.

Another alternative, without flat mapping each list, and thus may be more performant, is to collect directly the Stream<List<String>> (returned by values().stream()) by calling addAll on each accumulated result.

List<String> result = map.values().stream().collect(ArrayList::new, List::addAll, List::addAll);

Answer 2

Use flatMap on the Stream of map entries :

List<String> list = map.entrySet()
                       .stream()
                       .flatMap(e->e.getValue().stream())
                       .collect(Collectors.toList());