Stream - Collect by property and max

Question

Problem Statement

Given the following class (simplified for the question):

public static class Match {

  private final String type;
  private fina         


        

Answer 1

I have two methods for you.

First Method: Collectors.toMap()

A possible implementation is to use Collectors.toMap() to answers your problem.

stream.collect(Collectors.toMap(Match::getType, Match::getScore, Math::max));

And if you prefer to get a List(), you can remap the result

stream
    .collect(Collectors.toMap(Match::getType, Match::getScore, Math::max))
    .entrySet()
    .stream()
    .map(e -> new Match(e.getKey(), e.getValue()))
    .collect(Collectors.toList());

---

Second Method: Custom Collector

Like you say in your question, it is possible to extract this logic in a custom collector. You can do it like this:

public class MaxMatch implements Collector, List> {
    @Override
    public Supplier> supplier() {
        return HashMap::new;
    }

    @Override
    public BiConsumer, Match> accumulator() {
        return (map, match) -> {
            Integer score = match.getScore();
            if(map.containsKey(match.getType())) {
                score = Math.max(score, map.get(match.getType()));
            }
            map.put(match.getType(), score);
        };
    }

    @Override
    public BinaryOperator> combiner() {
        return (mapA, mapB) -> {
            mapA.forEach((k, v) -> {
                if(mapB.containsKey(k)) { mapB.put(k, Math.max(v, mapB.get(k))); }
                else { mapB.put(k, v); }
            });
            return mapB;
        };
    }

    @Override
    public Function, List> finisher() {
        return (map) -> map.entrySet().stream().map(e -> new Match(e.getKey(), e.getValue())).collect(Collectors.toList());
    }

    @Override
    public Set characteristics() {
        return Collections.emptySet();
    }
}

and use it like this:

stream.collect(new MaxMatch());

Hope this help you :)