collection.java

List<Person> people = new ArrayList<>();

String result = people.stream()
                      .filter(p -> p.getAge() > 20)
                      .map(Person::getLastname)
                      .collect(Collectors.joining(","));

List<String> list = people.stream()
                      .filter(p -> p.getAge() > 20)
                      .map(Person::getLastname)
                      .collect(Collectors.toList());
                      
Map<Integer, List<Person>> map = people.stream()
                                       .filter(p -> p.getAge() > 20)
                                       .collect(Collectors.groupingBy(Person::getAge));