downstream.java

Map<Integer, Long> map = people.stream()
                               .filter(p -> p.getAge() > 20)
                               .collect(
                                 Collectors.groupingBy(Person::getAge),
                                 Collectors.counting() // the downstream collector
                                );
                                
Map<Integer, Set<String> map = people.stream()
                               .filter(p -> p.getAge() > 20)
                               .collect(
                                 Collectors.groupingBy(
                                   Person::getAge,
                                   Collectors.mapping(
                                      Person::getLastname,
                                      Collectors.toCollection(TreeSet::new)
                                    )
                                  ),
                                );