how to differentiate subclasses from the same abstract class

Question

Imagine I have a class named Car and a bunch of subclasses that extend car like BMW,FORD,etc. So I have this ArrayList of cars and i am trying to separate each object in this ArrayList to diferent ArrayLists, one for each brand. I heard that using instance of is not a good practice so I have no idea how to do this.

Answer 1

I don't really know how to solve this polymorphism use, but i propose to not use instanceof and use a Map instead, with the class of the car and the list of the cars as arguments.

In this case, the code should look something like :

private static Collection<List<Car>> separateCars(List<Car> cars) {
    Map<Class, List<Car>> result = new HashMap<>();     // creating the empty map with results
    for (Car car : cars) {                              // iterating over all cars in the given list
        if (result.containsKey(car.getClass())) {       // if we have such car type in our results map
            result.get(car.getClass()).add(car);        // just getting the corresponding list and adding that car in it
        } else {                                        // if we faced with the class of the car that we don't have in the map yet
            List<Car> newList = new ArrayList<>();      // creating a new list for such cars
            newList.add(car);                           // adding this car to such list
            result.put(car.getClass(), newList);        // creating an entry in results map with car's class and the list we just created
        }
    }

    return result.values();     // returning only the lists we created as we don't need car's classes
}

Hope that will help you.

Answer 2

Java 8 Collectors grouping by can be useful in above case

https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/