Java stream grouping by with custom value

Question

I have a List<Person>. With Java streams I can group the persons by last name using something like this:

Map<String, List<Person>> persons.stream()
    .collect(Collectors.groupingBy(Person::getLastName));

But what if I want to group the first name of the persons by the last names? In other words, grouped by last name "Smith" I might have a list of "Jane" and "John". Is there maybe a value extractor version of groupingBy() I can use? I'm thinking something like this:

Map<String, List<String>> persons.stream()
    .collect(Collectors.groupingBy(Person::getLastName, Person::getFirstName));

Answer 1

Try use Collectors.mapping() as a downstream collector within the Collectors.groupingBy() method:

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;


class Main {
    public static void main(String[] args) {
        List<Person> persons = List.of(
            new Person("Smith", "John"),
            new Person("Doe", "Jane"),
            new Person("Smith", "Alex"),
            new Person("Doe", "John"),
            new Person("Brown", "Charlie")
        );
        Map<String, List<String>> groupedByLastName = persons.stream()
            .collect(Collectors.groupingBy(
                Person::getLastName,
                Collectors.mapping(Person::getFirstName, Collectors.toList())
            ));
        System.out.println(groupedByLastName);
    }
}

class Person {
    private String lastName;
    private String firstName;

    public Person(String lastName, String firstName) {
        this.lastName = lastName;
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    @Override
    public String toString() {
        return "Person{lastName='" + lastName + "', firstName='" + firstName + "'}";
    }
}

Output:

{Brown=[Charlie], Smith=[John, Alex], Doe=[Jane, John]}