Best way to get list of id's from hashMap values

Question

I have a HashMap having Integer as keys and List<Employee> as values. I want to get all Ids from all Values of HashMap

List<Long> ids = new ArrayList<>();
    hm.forEach((key, value) -> {
        List<Integer> c = value.stream()
                .map(Employee::getId).collect(Collectors.toList());
        ids.addAll(c);
     }

Here is how I am trying to do so far. Is there a way to directly stream from values of HashMap and get all distinct values?

Answer 1

hm.values()
      .stream()
      .flatMap(List::stream)
      .map(Employee::getId)
      .collect(Collectors.toSet());

Since you are interested in Ids only, stream over values of HashMap , and since each of those are List (s), you would use flatMap , the rest is probably obvious. Also since these are distinct as you say, a return type of Set makes a lot more sense.

If you still require a List just use:

.... .map(Employee::getId)
     .distinct()
     .collect(Collectors.toList())

Answer 2

List<Long> ids = hm.values().stream().map(Employee::getId).collect(Collectors.toList());

Here what we have done is get values as stream and map employee id and collect as list.

Its not needed re add to list.