Partition a Stream by a discriminator function

Question

One of the missing features in the Streams API is the \"partition by\" transformation, for example as defined in Clojure. Say I want to reproduce Hibernate\'s fetch join

Answer 1

For those of you who just want to partition a stream, there are mappers and collectors for that.

class Person {

    String surname;
    String forename;

    public Person(String surname, String forename) {
        this.surname = surname;
        this.forename = forename;
    }

    @Override
    public String toString() {
        return forename;
    }

}

class Family {

    String surname;
    List members;

    public Family(String surname, List members) {
        this.surname = surname;
        this.members = members;
    }

    @Override
    public String toString() {
        return "Family{" + "surname=" + surname + ", members=" + members + '}';
    }

}

private void test() {
    String[][] data = {
        {"Kray", "Ronald"},
        {"Kray", "Reginald"},
        {"Dors", "Diana"},};
    // Their families.
    Stream families = Arrays.stream(data)
            // Build people
            .map(a -> new Person(a[0], a[1]))
            // Collect into a Map> as families
            .collect(Collectors.groupingBy(p -> p.surname))
            // Convert them to families.
            .entrySet().stream()
            .map(p -> new Family(p.getKey(), p.getValue()));
    families.forEach(f -> System.out.println(f));
}