How to convert List of list to Hashset in Java using Lambda

Question

I have a class that contains the user data.

public class UserData {
    @JsonProperty("uid")
    private String userId;

    @JsonProperty("cities")
    private List<Integer> cities;

}

From the List<UserData>, I want to extract all the unique cities. Changes, I am doing for this,

Set<Integer> citiesList = new HashSet<>();

for (UserData userData : userDataList)
       citiesList.addAll(userData.getCities());

What could be the lambda expressions of these changes?

Any help would be much appreciated.

I'm not sure how a lambda expression would fit in there. Unless you mean you want to do this using streams? — Federico klez Culloca, Jun 14 '21 at 06:53

Karol Dowbecki · Accepted Answer · 2021-06-14T06:58:11.167

8

It feels like you should use .flatMap() with List.stream() operation, not a specific lambda expression:

Set<Integer> citiesList = userDataList.stream()
  .map(UserData::getCities)
  .flatMap(List::stream)
  .collect(Collectors.toSet());

edited Jun 14 '21 at 06:58

answered Jun 14 '21 at 06:55

Karol Dowbecki

score 1 · Answer 2 · answered Jun 14 '21 at 07:09

If you want to be sure that the returned Set is a HashSet, you must use Collectors.toCollection(HashSet::new), like this:

userDataList.stream().map(d -> d.getCities())
      .flatMap(d -> d.stream())
      .collect(Collectors.toCollection(HashSet::new));

2 Answers2