How to get list from objects' list field with java 8?

Question

I get a Manager model which have List of Companies. I need to get all companies from few managers. Now i have this code:

List<Company> companies = managers.stream().map(Manager::getCompanies).collect(Collectors.toList());

What i need to do to get it correct?

Do you want to filter managers by some criteria? – NikMashei Sep 16 '20 at 12:52 — NikMashei, Sep 16 '20 at 12:52

score 4 · Answer 1 · answered Sep 16 '20 at 12:52

4

FlatMap is your friend:

List<Company> companies = managers.stream().flatMap(m -> m.getCompanies().stream()).collect(Collectors.toList());

Flat map will put all companies in one list instead having multiple lists with companies.

answered Sep 16 '20 at 12:52

jmizv

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score 1 · Answer 2 · answered Sep 16 '20 at 13:02

It sounds like you have a List<Manager> and each one has a List<Company>, and you want to get a new list of companies that fall under some subset of those managers. If that is what you are trying to do, and you want to do it with streams, then you need something like this:

List<Company> companies = managers.stream().
                                  .filter(/* limit to managers you want */)
                                  .map(Manager::getCompanies) // get each manager's list
                                  .flatMap(Collection::stream) // combine the lists
                                  .collect(Collectors.tolist());

If you have to worry about nulls you can add .filter(Objects::nonNull) after the .map() line.

How to get list from objects' list field with java 8?

2 Answers2