Streams: groupingBy with mapping into a sorted Set

record Employee(String dept, String name) {}
var staff = List.of(new Employee("eng", "Ava"), new Employee("eng", "Ben"),
                    new Employee("eng", "Ava"), new Employee("ops", "Cara"));

Map<String, Set<String>> namesByDept = staff.stream().collect(
        Collectors.groupingBy(Employee::dept, TreeMap::new,
                Collectors.mapping(Employee::name, Collectors.toCollection(TreeSet::new))));

System.out.println(namesByDept);

{eng=[Ava, Ben], ops=[Cara]}