OOP design problem

Question

What is good design in this simple case:

Let's say I have a base class Car with a method FillTank(Fuel fuel) where fuel is also a base class which have several leaf classes, diesel, ethanol etc.

On my leaf car class DieselCar.FillTank(Fuel fuel) only a certain type of fuel is allowed (no surprises there:)). Now here is my concern, according to my interface every car can be tanked with any fuel, but that seems wrong to me, in every FillTank() implementation check the input fuel for the correct type and if not throw error or something.

How can I redesign such case to a more accurate one, is it even possible? How to design a base method which takes a base-class for input without getting these "strange results"?

Answer 1

Use a generic base class (if your language supports it (the below is C#)):

public abstract class Car<TFuel>  where TFuel : Fuel {     public abstract void FillTank(TFuel fuel); } 

Basically this enforces any class that inherits from car to specify which type of fuel it uses. Furthermore, the Car class imposes a restriction that TFuel must be some subtype of the abstract Fuel class.

Lets say we have some class Diesel which is simple:

public class Diesel : Fuel {     ... } 

And a car which only runs on diesel:

public DieselCar : Car<Diesel> {      public override void FillTank(Diesel fuel)      {           //perform diesel fuel logic here.      } }