Java - Is it possible to extend all the subclasses of a class with a single class?

Question

Java - Is it possible to extend all the subclasses of a class with a single class?

Let's explain it with an example, the actual code is quite more complex. I have an Animal class with its own class hierarchy. Let's say that it has two subclasses: Testarrosa and Viper.

public class Car {

    public abstract String getManufacturer();
}

public class Testarossa extends Car{

    public String getManufacturer(){
        return "Ferrari";
    }
}

public class Viper extends Car{

    public String getManufacturer(){
        return "Dodge";
    }
}

I want to extend all the Car subclasses with a RegisteredCar subclass.

public class RegisteredCar extends Car {

   private String plateNumber;

   public RegisteredCar (String plateNumber){
       this.plateNumber=plateNumber;
   }

   public String getPlateNumber() {
       return plateNumber;
   }
}

At some point, I should be able to create a new RegisteredCar of a specific subclass. Something like

RegisteredCar c = new RegisteredCar<Viper>("B-3956-AC");

And call the c.getManufacturer() to obtain "Dodge" and c.getPlateNumber() to obtain B-3956-AC. Obviously, I should still be able to create a Car c = new Viper();

That is an example. Having an attribute in Car with null value if not registered is not enough for what I need.

Answer 1

In short, no that is not possible. You have to unfortunately modify your object model.

For example, what about having a Registration class this way:

public interface Registration<C extends Car> {
    C getCar();

    String getPlateNumber();
}

This way you can extract the information relating to registration in a single class, while maintaining your Car models.

You can then do helper methods like:

Registration<Viper> registeredViper = createRegistration(new Viper(), "B-3956-AC");