Java reference

Collectors.flatMapping()

Turns each element into a stream and gathers all values with another collector. See also filtering().

Available since: Java 9

Syntax

static <T, U, A, R> Collector<T, ?, R> flatMapping(Function<? super T, ? extends Stream<? extends U>> mapper, Collector<? super U, A, R> downstream)

Parameters

Parameter	Type	Description
mapper	Function<? super T, ? extends Stream<? extends U>>	A function returning a stream of values for an element.
downstream	Collector<? super U, A, R>	The collector for all resulting values.

Return value

Collector<T, ?, R>A collector to use inside groupingBy and similar.

Examples

All tags of each author without duplicates:

record Post(String author, List<String> tags) {}
 
void main() {
    List<Post> posts = List.of(
        new Post("Anna", List.of("java", "streams")),
        new Post("Oleg", List.of("sql")),
        new Post("Anna", List.of("java", "records")));
 
    var tagsByAuthor = posts.stream()
        .collect(Collectors.groupingBy(Post::author, TreeMap::new,
            Collectors.flatMapping(p -> p.tags().stream(),
                Collectors.toCollection(TreeSet::new))));
    IO.println(tagsByAuthor);
}

Output:

{Anna=[java, records, streams], Oleg=[sql]}

filtering() selects elements inside a group. Unlike filter() before grouping, empty groups are kept:

record Person(String name, String city, int age) {}
 
void main() {
    List<Person> people = List.of(
        new Person("Anna", "Paris", 30),
        new Person("Oleg", "Berlin", 17),
        new Person("Mia", "Paris", 15));
 
    Map<String, Long> adults = people.stream()
        .collect(Collectors.groupingBy(Person::city, TreeMap::new,
            Collectors.filtering(p -> p.age() >= 18, Collectors.counting())));
    IO.println(adults);
}

Output:

{Berlin=0, Paris=1}

Notes

flatMapping() does inside a group what flatMap() does to a whole stream. Both collectors appeared in Java 9 and usually go as the second argument of groupingBy() or partitioningBy().

In the tutorial

Grouping — Grouping stream elements in Java — Collectors.groupingBy, counts and sums per group, mapping, TreeMap and partitioningBy.

Thanks for reading!