java8 stream grouping and sorting on aggregate sum

Question

Given a java class Something

class Something {
  private int parentKey;
  private String parentName;
  private int childKey;
  private int noThings;

  public Something(int parentKey, String parentName, int childKey, 
    int noThings) {
    this.parentKey = parentKey;
    this.parentName = parentName;
    this.childKey = childKey;
    this.noThings = noThings;
  }

  public int getParentKey() {
    return this.parentKey;
  }

  public int getNoThings() {
    return this.noThings;
  }
}

I have a list of something objects

List<Something> somethings = newArrayList(
            new Something(425, "Lemon", 44, 23),
            new Something(123, "Orange", 125, 66),
            new Something(425, "Lemon", 11, 62),
            new Something(123, "Orange", 126, 32),
            new Something(323, "Lime", 25, 101),
            new Something(123, "Orange", 124, 88)
);

I want to be able to sort them so that the they are sorted by the cumulative sum of the noThings per parent object and then by the noThings.

So that I end up with

List<Something> sortedSomethings = newArrayList(
            new Something(123, "Orange", 124, 88),
            new Something(123, "Orange", 125, 66),
            new Something(123, "Orange", 126, 32),
            new Something(323, "Lime", 25, 101),
            new Something(425, "Lemon", 11, 62),
            new Something(425, "Lemon", 44, 23)
);

I know that to map it by parentKey and sum of noThings is

Map<Integer, Integer> totalNoThings = colns
            .stream()
            .collect(
                    Collectors.groupingBy(
                            Something::getParentKey,
            Collectors.summingInt(ClientCollectionsReceived::getNoThings)));

I thought that maybe wrapping my Something class and having the total per Parent Key might work in someway.

class SomethingWrapper {
  private int totalNoThingsPerClient;
  private Something something;
}

But it seems like a lot of work and not very elegant.

Any observations/ ideas would be gratefully appreciated.

Answer 1

Well, you already did the main work by collecting the aggregate information

Map<Integer, Integer> totalNoThings = somethings.stream()
    .collect(Collectors.groupingBy(Something::getParentKey,
        Collectors.summingInt(Something::getNoThings)));

then all you need to do is utilizing these information in a sort operation:

List<Something> sorted=somethings.stream().sorted(
    Comparator.comparing((Something x)->totalNoThings.get(x.getParentKey()))
          .thenComparing(Something::getNoThings).reversed())
    .collect(Collectors.toList());

Answer 2

Actually had to make one small tweak, rather than totalNoThings.get, it was totalNothings.indexOf

So final soln. was

List<Integer> totalNoThings
    = somethings.stream()
    .collect(Collectors.groupingBy(Something::getParentKey,
                    Collectors.summingInt(Something::getNoThings)))
    .entrySet().stream()
    .sorted(Map.Entry.comparingByValue())
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());


List<Something> sorted 
    = somethings.stream().sorted(
            Comparator.comparing(
            (Something obj)->totalNoThings.indexOf(
            obj.getParentKey()))
    .thenComparing(Something::getNoThings).reversed())
            .collect(Collectors.toList());