Prefer Fakes over Mocks

> if I write it the way that I understand you're suggesting, I would have to consider the behavior of all of the ICartDiscountStrategy implementations.

No, you would need to consider only the business requirements.

> Some time later, I implement a new AcmeCorpDiscountStrategy which results in a different discount being returned.

Why did you implement that? Is it a business requirement change, that "existing customers should get this different discount"? If yes, then your tests break - but for a good reason. If it's not about existing customers, but new sort of customers, then your tests should be unaffected and you just need to write new tests that target these new sorts of customers and validate that the new discount strategy is applied.

> But that is an implementation detail. Only the BulkPurchaseDiscountStrategy knows whether it's 9 or 10 that's the trigger.

Language is poor, but I think there is consensus on what "implementation detail" means, and I don't think that's it. Whether it's 9 or 10 is customer facing behaviour. It's behaviour that might come from your marketing team or product owner or whatever, but it is outward facing. It is something that someone could write in a word document and call part of your specification.

The implementation detail would be whether that logic lives in BulkPurchaseDiscountStrategy or as one case in a big ShoppingCart class. A user pressing buttons on your UI can see the result of the price break, but they can't see the difference in code organisation.

You can say "price breaks should occur at 9 items" without ever saying "BulkPurchaseDiscountStrategy". In one ideal, all tests would test user facing requirements and not know anything about your code. You could refactor your code to your heart's content and know you the result was still what you wanted. There are downsides to that though because those end to end or system tests are fragile to write and expensive to run. It's much easier to cover all possible code paths with more focused testing. Hence in this case you could test the top interface of ShoppingCart, with all it's assembled dependencies. Not the same as pushing button in a Web Browser, but a lot more convenient to write and run.

I prefer to express it as the purpose of tests is to ensure something never changes. I don't care how future maintainer touches the code so long as given this set of inputs this is the output.

My hierarchy goes: None > Real > Fake > Mock

The best solution doesn't need anything, e.g. if we have calls to our persistence layer mixed in with calculations, the latter shouldn't be tested with fakes/mocks; instead we should refactor the code so the calculations don't depend on the persistence layer.

If the behaviour of some code depends inextricably on some external system, then our tests should use that system. This avoids unnecessary code/work/duplication, allows tests to exercise more paths through our codebase, exposes problems in our understanding of that system, etc.

If calling an external system in our tests is dangerous, unacceptably slow, costly (if it's some Web service), etc. then we should make a fake implementation to test against. If our code only uses a small part of some large, complicated dependency, we might want to define an interface for the subset that we need, refactor our code to use that, and only have our fake implement that part.

If a fake implementation isn't practical, or would require so much overriding per-test as to be useless on its own, then I might consider using mocks. I would seriously consider whether that code could be refactored to avoid such coupling. I would also never make assertions about irrelevant details, like whether a particular method was called, or what order things are run in.

But the price break is a requirement that I expect to change. Today it is 10. Tomorrow someone looks at the production process the machines give groups of 11, so the bulk discount should be multiples of 11 (if you order 12 you get 11 at the bulk rate and 1 and the regular price). Then next week management says small orders are too hard to serve, and so the minimum bulk order is 100, but bigger discounts if the total order is a multiple off 11. Then the week after...

Point is that your implementation of the discounts is expected to change. I should not have to change my shopping cart tests at all just because the discount requirement changed - it makes the costs of those tests too high.

Of course this is domain specific. I expect discounts to change all the time. Likewise I expect taxes to change often. In both cases they need to be mocked/faked just to keep my test code sane. there are many other domain specific things that won't change. I'm always going to store the shipping mass in KG, and for some customers convert it to lbs. It is reasonable to hardcode the weight in lbs as that won't change. (future me who has to ship to mars will disagree, but I don't know what the requirements are)