Using lambda expressions for summing up member variables?

Question

I have a class like followings.

public class Votes{
    String name;
    int likes;
    int dislikes;

    //constructors, getters and setters    
}

I am having a list like followings.

List<Votes> votesList;

Assume I am populating above list with some elements. I want to declare a method which performs grouping and summing operation in that list.

As an example, assume I am giving the following elements in a list as the input for that method.

votesList.add(new Votes("A", 10, 5));
votesList.add(new Votes("B", 15, 10));
votesList.add(new Votes("A", 20, 15));
votesList.add(new Votes("B", 10, 25));
votesList.add(new Votes("C", 10, 20));
votesList.add(new Votes("C", 0, 15));

That method should output a List<Votes> with the following elements.

("A", 30, 20),
("B", 25, 35),
("C", 10, 35)

Is there an easy way to do that using streams, lambda expressions in Java8? I know how it can be done using collectors if I only have one int memeber.

Can someone please explain me how can I address this situation?

Answer 1

One approach is to use groupingBy, combined with reducing:

Collection<Optional<Votes>> res =
     votesList.stream().collect(groupingBy(v -> v.name, reducing((v1, v2) -> new Votes(v1.name, v1.likes + v2.likes, v1.dislikes + v2.dislikes)))).values();

This will give you a Collection<Optional<Votes>>, you can get rid of it by combining the reducing collector and the finisher function Optional::get using the collectingAndThen collector but that will starts to look quite hard to read.

So another alternative could be to use toMap, and merge two votes whenever they have the same name:

Collection<Votes> res =
    votesList.stream().collect(toMap(v -> v.name,
                                     v -> v,
                                     (v1, v2) -> new Votes(v1.name, v1.likes + v2.likes, v1.dislikes + v2.dislikes))).values();

From there you can use the ArrayList copy-constructor (either by using collectingAndThen with m -> new ArrayList<>(m.values()), or by putting the previous expression in the parameter list of this copy constructor).

Answer 2

Finally, I came across this as the easiest way. :))

Map<String, List<Votes>> grouped = voteCountList.stream().collect(Collectors.groupingBy(r->r.getName()));

List<Votes> collectedList = new ArrayList<>();

grouped.forEach((groupName, votes) -> collectedList.add(new Votes(groupName,
                votes.stream().collect(Collectors.summingInt(r->r.getLikes())),
                votes.stream().collect(Collectors.summingInt(r->r.getDislikes())))));

Answer 3

This should get you started, it's not exactly what you want since it does not create the final list.

Map<String, Votes> collected = votesList.stream().collect(Collectors.groupingBy(Votes::getName,
        collectingAndThen(reducing(
                (originalVotes, newVotes) -> new Votes(originalVotes.getName(), originalVotes.getLikes() + newVotes.getLikes(), originalVotes.getDislikes() + newVotes.getDislikes()))
                , Optional::get)));

collected.forEach((key, value) -> {
    System.out.println(key + "," + value.getLikes() + "," + value.getDislikes());
});