Java 8 grouping using custom collector?

Question

I have the following class.

class Person {

    String name;
    LocalDate birthday;
    Sex gender;
    String emailAddress;

    public int getAge() {
             


        

Answer 1

You can also use Collectors.toMap and provide mapping for key, value and merge function(if any).

Map<Integer, String> ageNameMap = 
    members.stream()
            .collect(Collectors.toMap(
              person -> person.getAge(), 
              person -> person.getName(), (pName1, pName2) -> pName1+"|"+pName2)
    );

Answer 2

You can use a mapping Collector to map the list of Person to a list of person names :

Map<Integer, List<String>> collect = 
    members.stream()
           .collect(Collectors.groupingBy(Person::getAge,
                                          Collectors.mapping(Person::getName, Collectors.toList())));

Answer 3

When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns the name of the Person, and we collect that into a List.

Map<Integer, List<String>> collect = 
    members.stream()
           .collect(Collectors.groupingBy(
               Person::getAge,
               Collectors.mapping(Person::getName, Collectors.toList()))
           );