Question

I have two parallel inheritance chains:

Vehicle <- Car         <- Truck <- etc.  VehicleXMLFormatter <- CarXMLFormatter                     <- TruckXMLFormatter <- etc. 

My experience has been that parallel inheritance hierarchies can become a maintenance headache as they grow.

i.e. NOT adding toXML(), toSoap(), toYAML() methods to my principal classes.

How do I avoid a parallel inheritance hierarchy without breaking the concept of separation of concerns?

Answer 1

I am thinking of using the Visitor pattern.

public class Car : Vehicle {    public void Accept( IVehicleFormatter v )    {        v.Visit (this);    } }  public class Truck : Vehicle {    public void Accept( IVehicleFormatter v )    {        v.Visit (this);    } }  public interface IVehicleFormatter {    public void Visit( Car c );    public void Visit( Truck t ); }  public class VehicleXmlFormatter : IVehicleFormatter { }  public class VehicleSoapFormatter : IVehicleFormatter { } 

With this, you avoid an extra inheritance tree, and keep the formatting logic separated from your Vehicle-classes. Offcourse, when you create a new vehicle, you’ll have to add another method to the Formatter interface (and implement this new method in all the implementations of the formatter interface).
But, I think that this is better then creating a new Vehicle class, and for every IVehicleFormatter you have, create a new class that can handle this new kind of vehicle.