Collect all values of a Set field

Question

I have a collection which has a field of type Set with some values. I need to create a new set collecting all these values.

I am wondering if this is possible using lambda expressions.

Below is the code line :

Set<String> teacherId = batches.stream()
                               .filter(b -> !CollectionUtils.isEmpty(b.getTeacherIds()))
                               .map(b -> b.getTeacherIds())
                               .collect(Collectors.toSet());

The problem is post map operation, it contains a collection of set of strings. So collect operation returns a Set<Set<String>> but i am looking to aggregate all the values to a single set.

Answer 1

You need to use flatMap instead of map:

Set<String> teacherIds = 
    batches.stream()
           .flatMap(b -> b.getTeacherIds().stream())
           .collect(Collectors.toSet());

Note that the filtering is redundant for empty collections - streaming an empty collection will just result in an empty stream, which won't affect the final result. If getTeacherIds() could return null, however, you'd still have to handle it. Using filter(Objects::nonNull) would suffice, and save you the dependency on Apache Commons.

Answer 2

You can use flatMap to obtain a flat Stream of all the values, and then collect to a Set<String>:

Set<String> teacherId = 
    batches.stream()
           .filter(b -> !CollectionUtils.isEmpty(b.getTeacherIds()))
           .flatMap(b -> b.getTeacherIds().stream())
           .collect(Collectors.toSet());

Answer 3

If you care that that getTeacherIds() is not null, use it explicitly via !=, that CollectionUtils.isEmpty just hides stuff. Especially since if getTeacherIds() returns an Empty collection - that is handled just fine by flatMap, so to me that is not needed at all.

  Set<String> teacherIds = batches
            .stream()
            .filter(x -> x.getTeacherIds() != null)
            .flatMap(x -> x.getTeacherIds().stream())
            .collect(Collectors.toSet());

Answer 4

I am wondering if this is possible using lambda expressions.

I capture the last fish, :).

Set<String> teacherIds = batches.stream()//v--- the class of `x`
                                .map(XClass::getTeacherIds)
                                .filter(Objects::nonNull)
                                .flatMap(Collection::stream)
                                .collect(Collectors.toSet());

Note: I'm sorry I'm forget to tell you if the getTeacherIds copy the internal IDs to a new set of IDs, the code above is appropriate for you. since it is read the IDs from XClass once.