Ho to convert an Arraylist<Person> to Map<String, List<Long>> with Stream?

Question

I have a Person ArrayList

List<Person> personList = getPersonList();

And I want to create a map of Map<String, List<Long>>

key based on Person::getDepartment and value based on a List<Long> of timestamps Person::getTimestamp

There are multiple Person records in the List<Person> personList , so I need to remove duplicates on Person::getDepartment , but since a Map doesn't let key duplicates, that should not be a problem, i think.

I can do that perfect with a forach loop but my question is can I do that with only one stream?

public class Person {

    private String department;
    private long timestamp;

    //getters and setters
}

Answer 1

In the collect, use groupingBy() to group Person by department and mapping() to map the Person to its timestamp.

Map<String, List<Long>> map = 
personList.stream()
          .collect(groupingBy(
                         Person::getDepartment,
                         mapping(Person::getTimestamp, toList())
                        )
                  );

And add these imports :

import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toList;

or just :

import static java.util.stream.Collectors.*;