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

It can be done by collapse with StreamEx

StreamEx.of(queryBuilder.createQuery(
    "SELECT f.name, m.name"
    + " FROM Family f JOIN Member m on m.family_id = f.id"
    + " ORDER BY f.name"))
        .collapse((a, b) -> a.string(0).equals(b.string(0)), Collectors.toList())
        .map(l -> new Family(l.get(0).string(0), StreamEx.of(l).map(r -> r.string(1)).toList())) 
        .forEach(System.out::println);