Collect all objects from a Set of Sets with Java Stream

Question

I\'m trying to learn Java Streams and trying to get a HashSet from a HashSet>.

HashSet<         


        

Answer 1

You can flatMap each student into a stream formed by the student along with their teachers:

HashSet combined = 
    students.stream()
            .flatMap(student -> Stream.concat(Stream.of(student), student.getTeachers().stream()))
            .collect(Collectors.toCollection(HashSet::new));

concat is used to concatenate to the Stream of the teachers, a Stream formed by the student itself, obtained with of.