Question

I had an interview question that asked how I would design a system to show cars/trucks/vans at a kiosk so customers could look at specs on the vehicles. (Warning: I have not done much OO programming lately so bad vocabulary may follow)

I said I was start with a class, vehicle that had very basic properties like wheelbase, drive train, hp. Then I would subclass that into car, truck or van where I could have more specfic measurements like bed length for trucks or trunk capacity for cars.

Then they asked, how would I add options. I said that options could probably exist on any vehicle so I would say that a vehicle could have a list of options.

Finally they asked, what if there was an option that was only available on trucks and vans but not cars and I was a little stumped. Is there some way to make this nicely happen given the layout I described? Is there a better way to set up the class hierarchy to solve this? Or is this just a more complex problem that cant easily be solved without adding some extra logic?

Answer 1

There are a few options for that scenario.

First, the easy one:

Option #1 – Interface for Trucks/Vans

If only trucks and vans implement Options, then create an interface called IVehicleOptions and have trucks and vans implement it:

public interface VehicleOptions
{
  Options { get; }
}

The drawback to this is that now you have to treat cars differently than trucks and vans.

Option #2 – Null Design Pattern

However, you can use the Null Design Pattern. Have Car implement IVehicleOptions, and just return null:

public class Car : IVehicleOptions
{
  public VehicleOptions { get { return null; } }
}

Option #3 – Strategy Pattern

Create a base class, like Vehicle:

public abstract class Vehicle
{
  public Options Options { get; protected set; }
}

And have each concrete class set it:

public class Car : Vehicle
{
  public Car()
  {
    this.Options = NullOptions();  // This is the null design pattern used with this strategy pattern
  }
}

public class Truck : Vehicle
{
  public Truck
  {
    this.Options = SuperOptions();
  }
}

public class Van: Vehicle
{
  public Van
  {
    this.Options = ElegantOptions();
  }
}

Now all vehicles can be treated the same way (as a Vehicle).