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

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

Collection values = bumperCars.stream()
        .collect(HashMap::new, (HashMap 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 codes = new ArrayList();
                    distGroup = new DistGroup(bc.size, bc.color, codes);
                    res.put(dg, distGroup);
                }
                },HashMap::putAll).values();