How to groupBy object properties and map to another object using Java 8 Streams?

Question

Suppose I have a group of bumper cars, which have a size, a color and an identifier (\"car code\") on their sides.

class BumperCar {
    int size;
    String col         


        

Answer 1

If we assume that DistGroup has hashCode/equals based on size and color, you could do it like this:

bumperCars
    .stream()
    .map(x -> {
        List<String> list = new ArrayList<>();
        list.add(x.getCarCode());
        return new SimpleEntry<>(x, list);
    })
    .map(x -> new DistGroup(x.getKey().getSize(), x.getKey().getColor(), x.getValue()))
    .collect(Collectors.toMap(
        Function.identity(),
        Function.identity(),
        (left, right) -> {
            left.getCarCodes().addAll(right.getCarCodes());
            return left;
        }))
    .values(); // Collection<DistGroup>

Answer 2

Solution-1

Just merging the two steps into one:

List<DistGroup> distGroups = bumperCars.stream()
        .collect(Collectors.groupingBy(t -> new SizeColorCombination(t.getSize(), t.getColor())))
        .entrySet().stream()
        .map(t -> {
            DistGroup d = new DistGroup(t.getKey().getSize(), t.getKey().getColor());
            d.addCarCodes(t.getValue().stream().map(BumperCar::getCarCode).collect(Collectors.toList()));
            return d;
        })
        .collect(Collectors.toList());

Solution-2

Your intermediate variable would be much better if you could use groupingBy twice using both the attributes and map the values as List of codes, something like:

Map<Integer, Map<String, List<String>>> sizeGroupedData = bumperCars.stream()
        .collect(Collectors.groupingBy(BumperCar::getSize,
                Collectors.groupingBy(BumperCar::getColor,
                        Collectors.mapping(BumperCar::getCarCode, Collectors.toList()))));

and simply use forEach to add to the final list as:

List<DistGroup> distGroups = new ArrayList<>();
sizeGroupedData.forEach((size, colorGrouped) ->
        colorGrouped.forEach((color, carCodes) -> distGroups.add(new DistGroup(size, color, carCodes))));

Note: I've updated your constructor such that it accepts the card codes list.

DistGroup(int size, String color, List<String> carCodes) {
    this.size = size;
    this.color = color;
    addCarCodes(carCodes);
}

Further combining the second solution into one complete statement(though I would myself favor the forEach honestly):

List<DistGroup> distGroups = bumperCars.stream()
        .collect(Collectors.groupingBy(BumperCar::getSize,
                Collectors.groupingBy(BumperCar::getColor,
                        Collectors.mapping(BumperCar::getCarCode, Collectors.toList()))))
        .entrySet()
        .stream()
        .flatMap(a -> a.getValue().entrySet()
                .stream().map(b -> new DistGroup(a.getKey(), b.getKey(), b.getValue())))
        .collect(Collectors.toList());

Answer 3

Check out my library AbacusUtil:

StreamEx.of(bumperCars)
         .groupBy(c -> Tuple.of(c.getSize(), c.getColor()), BumperCar::getCarCode)
         .map(e -> new DistGroup(e.getKey()._1, e.getKey()._2, e.getValue())
         .toList();

Answer 4

You can collect by by using BiConsumer that take (HashMap<SizeColorCombination, DistGroup> res, BumperCar bc) as parameters

Collection<DistGroup> values = bumperCars.stream()
        .collect(HashMap::new, (HashMap<SizeColorCombination, DistGroup> res, BumperCar bc) -> {
                SizeColorCombination dg = new SizeColorCombination(bc.color, bc.size);
                DistGroup distGroup = res.get(dg);
                if(distGroup != null) {
                    distGroup.addCarCode(bc.carCode);
                }else {
                    List<String> codes = new ArrayList();
                    distGroup = new DistGroup(bc.size, bc.color, codes);
                    res.put(dg, distGroup);
                }
                },HashMap::putAll).values();