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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.

Tiago Gomes
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    What do you mean by " I heard that using instance of is not a good practice so I have no idea how to do this.". You could add a property called Model in your base class. Then you would extract objects from your list using this property. I would personnaly do something like myobjects.OfType() ... –  Oct 20 '17 at 01:16
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    [You could implement the visitor pattern](https://stackoverflow.com/questions/29458676/how-to-avoid-instanceof-when-implementing-factory-design-pattern/29459571#29459571) – Vince Oct 20 '17 at 01:18
  • @Seb my teachers are not fans of us using instance of. I thought about adding a property but I would like to use polymorphism to solve this problem. – Tiago Gomes Oct 20 '17 at 01:19
  • https://stackoverflow.com/questions/20589590/why-not-use-instanceof-operator-in-oop-design – alejandrogiron Oct 20 '17 at 01:21
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    Putting a protected model field on the base class that the subclasses set and then using that field to sort the objects _is_ using polymorphism to solve the problem. – astidham2003 Oct 20 '17 at 01:34

2 Answers2

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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.

pirs
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s.pasynkov
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Java 8 Collectors grouping by can be useful in above case

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

Code_Eat_Sleep
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