Merge all list values together in a map [duplicate]

Answer 1

Use flatMap on the Stream of map entries :

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

Answer 2

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);