Wsrtjtyk

Lets suppose we have 10 types of Car:Bmw, Renault etc. And we have Repo. So there are two ways for developing API of Repo:

The first way:

class Repo{

  public <T extends Car> T getCarByType(Class<T> clazz){..}

}

The second way:

class Repo{

   public Bmw getBmw(){..}

   public Reno getRenault(){..}

   ...

}

Firstly I thought that I should follow the first way as it lets write less code and -> it is better for supporting. And besides, I thought that if I have 20 types of car the first way is obvious advantage (less code).

However, as the number of car is growing - you start to forget what car you have. When you follow the second way - you have clear API of the repo and the types.

So,could anyone explain when to use which method?

The first way will produce less duplicate code. But if you want to model the types of cars, you could use an enum

enum Cars {

  BMW(Bmw.class),

  RENO(Reno.class)

  ;



  Class<? extends Car> type;



  Cars(Class<? extends Car> type){

    this.type = type;

  }



  Class<? extends Car> getType() {

    return type;

  }



}

And the access the car using this enum

public <T extends Car> T getCarByType(Cars car){

  Class<T> type = car.getType();

  ...

}

First if your cars aren't so different you may use only one class Car that will have a type property which will allow you to make difference between cars type, however if you wan't to continue with your solution I think you may use both of the ways, you can create a generic Repo which will have all code used to get Data then you can make on top of it a specific layer that will expose methods to retrieve each car by it's type

Generic Repo

class Repo{

  public <T extends Car> T getCarByType(Class<T> clazz){..}

}

RepoFacade

class RepoFacade{

   public Bmw getBmw(){

       Repo<Bmw> = new Repo<>();

       return repo.getCarByType(BMW.class);

   }

   public Reno getReno(){..}

}

I would go with something simpler. No polymorphism based on brand, because it is just a label, it does not add any behavior:

public class Car {

    public enum Make { BMW, ... };



    public Make make;

    public Color color;

    public int year;

    // Cars have other properties, I suppose

}



public class Repo {

    private List<Car> cars = new ArrayList<Car>;



    // This could also return a list of all cars of that model, 

    // because there is no reason to have exactly one of each make.

    @Nullable

    public Car findByMake(Car.Make make) {

        for (Car car : cars) {

            if (car.make == make) {

                return car;

            }

        }

        return null;

    }

}