How to get a List from a HashMap>

Question

I want to extract a List from a Map> (E is a random Class) using stream().

I

Answer 1

map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:

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

Answer 2

You can use Collection.stream with flatMap as:

Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
                  .stream()
                  .flatMap(Collection::stream)
                  .collect(Collectors.toList());

or use a non-stream version as:

List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)

Answer 3

Here's an alternate way to do it with Java-9 and above:

List<E> result = map.values()
                    .stream()
                    .collect(Collectors.flatMapping(List::stream, Collectors.toList()));

Answer 4

Simply use :-

map.values().stream().flatMap(List::stream).collect(Collectors.toList());

Answer 5

In addition to other answers:

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

This could also do the trick.

Answer 6

Or use forEach

 map.forEach((k,v)->list.addAll(v));

or as Aomine commented use this

map.values().forEach(list::addAll);