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

Solution-1

Just merging the two steps into one:

List 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>> 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 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 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 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());