Java - order a collections by number of objects in each list?

Question

 Collection<List<Person>> personsByDepartment=
                persons.stream()
                        .collect(Collectors.groupingBy(Person::getDepartment))
                        .values();

I have the following Collection above that groups a list of People into lists based off their department. This is working as expected.

How can I ensure this list is sorted so that the list with most amount of people in it is first?

Answer 1

Provide a Comparator to compare the lists by their size. You could stream the values again to sort:

Collection<List<Person>> personsByDepartment = persons.stream()
            .collect(Collectors.groupingBy(Person::getDepartment))
            .values()
            .stream()
            .sorted((l1, l2) -> Integer.compare(l2.size(), l1.size()))
            .collect(Collectors.toList());

Edit: As Michael pointed out in comments the same comparator can be written as:

Comparator.<List<Person>>comparingInt(List::size).reversed()

It's more obvious, that the ordering is being reversed.