Java 8, Lambda: Sorting within grouped Lists and merging all groups to a list

Question

Based on the following answer: https://stackoverflow.com/a/30202075/8760211

How to sort each group by stud_id and then return a List with all Students as result of the g

Answer 1

Since the result is supposed to be a list, you‘re not grouping but simply sorting (in the sense of changing the order according to a defined rule). The main obstacle is that you want the locations to be ordered after their first encounter in the original list.

The straight-forward approach is to fix this location order first, followed by a single sort operation:

Map locationOrder = studlist.stream()
    .collect(HashMap::new,
             (m,s)->m.putIfAbsent(s.stud_location, m.size()),
             (m1,m2)->m2.keySet().forEach(l->m1.putIfAbsent(l, m1.size())));

studlist.sort(Comparator.comparingInt((Student s) -> locationOrder.get(s.stud_location))
                        .thenComparing(s -> s.stud_id));

If you can not or do not want to modify the original list, you can simply use a copy:

List result = new ArrayList<>(studlist);
result.sort(Comparator.comparingInt((Student s) -> locationOrder.get(s.stud_location))
                      .thenComparing(s -> s.stud_id));

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It’s also possible to solve this with a grouping operation, but that’s not easier:

List result = studlist.stream()
    .collect(Collectors.collectingAndThen(
                Collectors.groupingBy(s -> s.stud_location,
                                      LinkedHashMap::new, Collectors.toList()),
                m -> m.values().stream()
                      .flatMap(l -> l.stream().sorted(Comparator.comparing(s->s.stud_id)))
                      .collect(Collectors.toList())));

Note that you have to collect into a LinkedHashMap to ensure that the order of the groups is retained.