Converting nested optional list to optional list in Java?

Question

I have an optionalIdList ( Optional<Set<String>> optionalIdList ) that I want to iterate over, and then convert back to Optional<Set<String>> . My code thus far is like this:

optionalIdList
              .map(
                  idList ->
                      idList.stream()
                          .map(
                              id ->
                                  filterIds(
                                      partnerIds
                                  )))
              .flatMap(streamOfLists -> streamOfLists.map(item -> item.orElseGet(ImmutableSet::of)));

The filterIds list returns Optional<Set<String>>

However with my current solution I get the following error:

Required type: Optional<Set<String>>

Provided: Optional<Object>

Is there a nice way to do what I want to do?

Answer 1

EDIT: For some reason I assumed filterIds returns a single String , and not an Optional<Set<String>> as you wrote it does.

Here's the updated answer:

Optional<Set<String>> output =
    optionalIdList.map(idList -> idList.stream()
                                       .map(id -> filterIds(partnerIds))
                                       .filter (Optional::isPresent)
                                       .map (Optional::get)
                                       .flatMap (Set::stream)
                                       .collect(Collectors.toSet()));