java8 stream grouping by merge objects or aggregate

Question

I have following situation, where two neighborhood objects and same street but different houses.

i am wondering using stream how can i group into one neighborhood object

in summary i have following data

Neighborhood: n11
streets: s11
houses: h1

Neighborhood: n11
streets: s11
houses: h2

and i want to merge it to display like as following

Neighborhood: n11
streets: s11
houses: h1,h2

code as follows

public class Neighborhood{
    public UUID id;
    public List<Street> streets;

    public Neighborhood(UUID id, List<Street> streets)
    {
        this.id=id;
        this.streets=streets;
    }
}

public class Streets
{
    public UUID id;
    public List<House> houses;

    public Streets(UUID id, List<House> houses)
    {
        this.id=id;
        this.houses=houses
    }
}

public class House
{
    public UUID id;

    public House(id)
    {
        this.id=id;
    }
}

House h1= new House("h11")
Street s1= new Street("s11", asList(h1))
Neighborhood n1 = new Neighborhood("n11", asList(s1));

House h2= new House("h12")
Street s2= new Street("s11", asList(h2))
Neighborhood n2 = new Neighborhood("n11", asList(s2));

I just tried writing this code and... it's insanely un-readable if I understood you correctly. I wish you would show us your attempts without streams — Eugene
– Eugene, Commented Feb 1, 2019 at 20:14
in essence, think about it: first there would be a Collectors.toMap( Neighborhood::getId, Function.identity(), (left, right) -> {merge two neighborhood}... but this merge is quite a beast. you would have to collect again to a Map so that you end up with a Map<String, List<String>> where Key is Street::id and Value is a List of House ids's and from that build a Neighborhood back... — Eugene
– Eugene, Commented Feb 1, 2019 at 20:19

Eugene · Accepted Answer · 2019-02-01 23:23:57Z

Well, you could build a Map<String, Map<String, List<String>>>, where key is Neighborhood::id and value is a Map that has key as Street::id and value a List of House::id. From here building it back to whatever you want is left an exercise to you...

Map<String, Map<String, List<String>>> map = new HashMap<>();

    neighborhoods.forEach(neighborhood -> {

        Map<String, List<String>> m = map.computeIfAbsent(neighborhood.getId(), (key) -> new HashMap<>());
        neighborhood.getStreets()
                    .forEach(street -> {
                        m.merge(street.getId(),
                                street.getHouses()
                                      .stream()
                                      .map(House::getId)
                                      .collect(Collectors.toCollection(ArrayList::new)),
                                (oldV, newV) -> {
                                    oldV.addAll(newV);
                                    return oldV;
                                }
                        );
                    });
    });

Koray Tugay · Accepted Answer · 2019-02-01 20:18:05Z

I had to change the code you provided a little:

import java.util.List;

public class Neighborhood {
    public String id;
    public List<Street> streets;

    public Neighborhood(String id, List<Street> streets) {
        this.id = id;
        this.streets = streets;
    }
}

class Street {

    public Street(String id, List<House> houses) {
        this.id = id;
        this.houses = houses;
    }

    public String id;
    public List<House> houses;
}

class House {
    public String id;

    public House(String id) {
        this.id = id;
    }
}

Here is my answer then:

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

    public static void main(String[] args) {
        House h1 = new House("h11");
        Street s1 = new Street("s11", Arrays.asList(h1));


        House h2 = new House("h12");
        Street s2 = new Street("s11", Arrays.asList(h2));

        Neighborhood n1 = new Neighborhood("n11", Arrays.asList(s1));
        Neighborhood n2 = new Neighborhood("n11", Arrays.asList(s2));

        Set<Street> collect = Stream.of(n1, n2).flatMap(neighborhood -> neighborhood.streets.stream())
                                          .collect(Collectors.toSet());

        System.out.println(collect);

        final Map<String, List<Street>> collect1 = collect.stream().collect(Collectors.groupingBy(street -> street.id));

        final List<Neighborhood> neighborhoods = new ArrayList<>();

        collect1.forEach((s, streets) -> {
            final List<House> collect2 = streets.stream().flatMap(street -> street.houses.stream())
                                                .collect(Collectors.toList());
            final Street street = new Street(s, collect2);
            neighborhoods.add(new Neighborhood(s, Arrays.asList(street)));
        });
    }
}

you at least assume that Street implements hashCod/equals, but there are other pieces of code that look weird. try to add this House h3 = new House("h13"); Street s3 = new Street("s13", Arrays.asList(h3));Neighborhood n3 = new Neighborhood("n11", Arrays.asList(s3));.. how many neighborhoods will you get?

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