Inheritance and interfaces

Question

This is somewhat of a follow-up question to this question.

Suppose I have an inheritance tree as follows:

Car -> Ford -> Mustang -> MustangGT

Is there a benefit to defining interfaces for each of these classes? Example:

ICar -> IFord -> IMustang -> IMustangGT

I can see that maybe other classes (like Chevy) would want to implement Icar or IFord and maybe even IMustang, but probably not IMustangGT because it is so specific. Are the interfaces superfluous in this case?

Also, I would think that any class that would want to implement IFord would definitely want to use its one inheritance by inheriting from Ford so as not to duplicate code. If that is a given, what is the benefit of also implementing IFord?

Answer 1

In my view interfaces are a tool to enforce a requirement that a class implement a certain signature, or (as I like to think of it) a certain "Behavior" To me I think if the Capital I at the beginning of my onterface names as a personal pronoun, and I try to name my interfaces so they can be read that way... ICanFly, IKnowHowToPersistMyself IAmDisplayable, etc... So in your example, I would not create an interface to Mirror the complete public signature of any specific class. I would analyze the public signature (the behavior) and then separate the members into smaller logical groups (the smaller the better) like (using your example) IMove, IUseFuel, ICarryPassengers, ISteerable, IAccelerate, IDepreciate, etc... And then apply those interfaces to whatever other classes in my system need them

Answer 2

Interfaces are intended to be a generic public API, and users will be restricted to using this public API. Unless you intend users to be using the type-specific methods of IMustangGT, you may want to limit the interface hierarchy to ICar and IExpensiveCar.

Answer 3

Only inherit from Interfaces and abstract classes.

If you have a couple of classes wich are almost the same, and you need to implement the majority of methods, use and Interface in combination with buying the other object.
If the Mustang classes are so different then not only create an interface ICar, but also IMustang.
So class Ford and Mustang can inherit from ICar, and Mustang and MustangGT from ICar and IMustang.

If you implement class Ford and a method is the same as Mustang, buy from Mustang:

class Ford{
  public function Foo(){
    ...
    Mustang mustang  = new Mustang();
    return mustang.Foo();
}

Answer 4

Don't build stuff you don't need. If it turns out you need the interfaces, it's a small effort to go back and build them.

Also, on the pedantic side, I hope you're not actually building something that looks like this hierarchy. This is not what inheritance should be used for.

Answer 5

In my experience, interfaces are best used when you have several classes which each need to respond to the same method or methods so that they can be used interchangeably by other code which will be written against those classes' common interface. The best use of an interface is when the protocol is important but the underlying logic may be different for each class. If you would otherwise be duplicating logic, consider abstract classes or standard class inheritance instead.

And in response to the first part of your question, I would recommend against creating an interface for each of your classes. This would unnecessarily clutter your class structure. If you find you need an interface you can always add it later. Hope this helps!

Adam

Answer 6

Create it only once that level of functionality becomes necessary.

Re-factoring Code is always on on-going process.

There are tools available that will allow you to extract to interface if necessary. E.G. http://geekswithblogs.net/JaySmith/archive/2008/02/27/refactor-visual-studio-extract-interface.aspx