Java 8 mapping to sub list entries of a collection using streams and collectors

Question

I have a collection of Person objects:.

public class Person {

  String name;

  ChildrenListHolder childrenListHolder;
}

public class ChildrenListHolder {
   List<Children> children;
}

public class Children {
   String childrensName;
}

(The entity structure is given by third party.)

Now, I need a Map<String,List<Person>> childrensName -> person-list

For example (simplified):

Person father: {name: "John", childrensListHolder -> {"Lisa", "Jimmy"}}
Person mother: {name: "Clara", childrensListHolder -> {"Lisa", "Paul"}}
Person george: {name: "George", childrensListHold -> "Paul"}}

The map, I need is

Map<String, List<Person>> map: {"Lisa"  -> {father, mother},
                                "Jimmy" -> {father},
                                "Paul"  -> {mother, george}}

I can do that with a bunch of for's and if's. But how can I do this using streams and collectors. I have tried many approaches, but I cannot get the expected result. TIA.

Answer 1

Given a List<Person> persons, you can have the following

Map<String,List<Person>> map =
    persons.stream()
           .flatMap(p -> p.childrenListHolder.children.stream().map(c -> new AbstractMap.SimpleEntry<>(c, p)))
           .collect(Collectors.groupingBy(
             e -> e.getKey().childrensName,
             Collectors.mapping(Map.Entry::getValue, Collectors.toList())
           ));

This is creating a Stream over the persons. Then each person is flat mapped by a tuple holding the child and the person for each child. Finally, we group by the child name and collect all the persons into a list.

Sample code assuming there are appropriate constructors:

public static void main(String[] args) {
    List<Person> persons = Arrays.asList(
        new Person("John", new ChildrenListHolder(Arrays.asList(new Children("Lisa"), new Children("Jimmy")))),
        new Person("Clara", new ChildrenListHolder(Arrays.asList(new Children("Lisa"), new Children("Paul")))),
        new Person("George", new ChildrenListHolder(Arrays.asList(new Children("Paul"))))
    );

    Map<String,List<Person>> map =
        persons.stream()
               .flatMap(p -> p.childrenListHolder.children.stream().map(c -> new AbstractMap.SimpleEntry<>(c, p)))
               .collect(Collectors.groupingBy(
                 e -> e.getKey().childrensName,
                 Collectors.mapping(Map.Entry::getValue, Collectors.toList())
               ));

    System.out.println(map);
}

Answer 2

I can do that with a bunch of for's and if's.

I know that you asked for a stream/collectors solution, but in any case a nested for loop using Map#computeIfAbsent works fine too:

Map<String, List<Person>> map = new HashMap<>();
for(Person p : persons) {
    for(Children c : p.childrenListHolder.children) {
        map.computeIfAbsent(c.childrensName, k -> new ArrayList<>()).add(p);
    }
}

and this written using the new forEach method introduced on collections:

Map<String, List<Person>> map = new HashMap<>();
persons.forEach(p -> p.childrenListHolder.children.forEach(c -> map.computeIfAbsent(c.childrensName, k -> new ArrayList<>()).add(p)));

Of course it's not a one-liner nor easy parallelizable like in Tunaki's solution (+1), but you don't need a "bunch" of if's to achieve that too (and you avoid also the creation of temporary map entries instances).