Question

Assume you divide up your systems in Value objects and Services objects (as suggested in “Growing Object-Oriented Software, Guided by Tests”. Misko Hevery calls these “newables” and “injectables”.

What happens when one of your value objects suddenly needs to access a service to implement it’s methods?

Let’s say you have a nice simple Value object. It’s immutable, holds a few bits of information and that’s about it. Let’s say we use it something like this:

CreditCard card = new CreditCard("4111-1111-1111-1111", "07/10");
if (card.isValid())
{
  // do stuff
} 
else
{
  // don't do stuff
}

So far so good. isValid() implements a check digit algorithm on the card number and returns true/false.

Now, let’s say I wish to enhance the system by validating the expiry date against the current time. How would you suggest this is done without breaking the Value object/Service object paradim? I should like this class to continue to be unit testable.

• CreditCard now has a dependency, but because of the way it is created it can not be injected, so dependency injection is out.
• The CreditCard class should not be calling out to Singletons (I am of the position that global access to a Singleton is bad practice)
• Putting the behaviour on CreditCardVerificationService.validateCard() means all the existing code has to be revisited. The implementation of isValid() is leaking out.

I know there are things that can be done to get around this, but what is the cleanest way?

Answer 1

I would argue that it isn’t a CreditCard object’s job to validate anything. A factory would validate the check digits to ensure that it is instantiating a conforming card, while a verification service would validate the card for expiration/$ limit.