Collector to create a List of List based on a property of the object

Question

I have the following object:

List persons

And I want to create the following structure

List> based on the family name.

I am able to group them in a Map by using groupingBy(p -> p.getFamilyName()). But I would like not to have a map, but a List of Lists.

You can do it in two steps by using the method values() of the map. But I would like to know if you can do it with a collector.

The best solution is what you are doing, If you want to make it with your idea, It be more complicated than your's take a look at this https://stackoverflow.com/questions/45817262/java-8-how-to-turn-a-list-into-a-list-of-lists-using-lambda — Youcef LAIDANI, Apr 26 '18 at 08:01

score 3 · Answer 1 · answered Apr 26 '18 at 08:39

You can use collectingAndThen:

ArrayList<List<Person>> collect = stream.collect(
    Collectors.collectingAndThen(Collectors.groupingBy(Person::getFamilyName),
                                 m -> new ArrayList<>(m.values())));

This first applies the groupingBy, "and then" creates an ArrayList for each grouped values.

OldCurmudgeon · Accepted Answer · 2018-04-26T08:37:22.313

You could collect using groupingBy and then re-stream the entrySet into your list.

    List<Map.Entry<String, List<Person>>> families = people.stream()
            .collect(Collectors.groupingBy(p -> p.familyName))
            .entrySet()
            .stream()
            .collect(Collectors.toList());
    System.out.println(families);

If you don't need the family name as key then just add .map(e -> e.getValue()) before the final collect:

    List<List<Person>> families = people.stream()
            .collect(Collectors.groupingBy(p -> p.familyName))
            .entrySet()
            .stream()
            .map(e -> e.getValue())
            .collect(Collectors.toList());

or just stream the values:

    List<List<Person>> families = people.stream()
            .collect(Collectors.groupingBy(p -> p.familyName))
            .values()
            .stream()
            .collect(Collectors.toList());

This is not exactly what I want, I want a List>, not Map.Entry — Manuelarte, Apr 26 '18 at 08:31
@Manuelarte - Then just add a `.map(e -> e.getValue())` before the final collect. — OldCurmudgeon, Apr 26 '18 at 08:35

Collector to create a List of List based on a property of the object

2 Answers2