Jake Goulding

Test Double Terminology

While listening to the Code Quality panel discussion at CodeMash, I tweeted about “the state of mocking”:

The state of “mocking” is that the term “mocking” is misused by everyone. You often mean “test double” or “stub”.

This garnered some retweets and some great replies, so I wanted to have the opportunity to respond in a little longer form. I’ve written this post between sessions and in the halls, so give me a little leeway and feel free to send me a tweet with corrections, arguments, whatever.

Test frameworks encourage misuse

[I]t doesn’t help that most mocking frameworks are used for stubs

@F1nglas mentions that moq does this, and I know that Mockito does as well. RSpec does better in this area. From the rspec-mock documentation:

Use the double method to create [a test double.] You can also use the mock and stub methods to create test doubles, however these methods are there for backward compatibility only

Remember that most of these tools we use are open source: we can submit patches to improve them. In the meantime, we can add local alias methods that better express our intent.

Saying “mock” is just more fluid

[P]eople find it easier to say “I’ll mock that out” than “I’ll test-double that out”.

@jitterted’s point is spot on; it is easier to say “mock”. However, I don’t usually want to use just a test double. I always want to do/use something particular: stub, mock, dummy, etc.

I’d love it if we as a community could come up with a better verb to use to mean the generic “use a test double”.

Doesn’t language evolve?

[A] term takes on the meaning that “everyone” gives it


The meaning of a word is defined by common understanding


[M]isused by everyone is an evolving definition


Let me get this out of the way first: I love the evolution of language. Dropping irrelevant words, new words being created, and expanding the meaning of existing words are all good things.

The root issue here is not that language is evolving. The root issue is that our understanding of the tools and techniques we are referring to by that language hasn’t gotten to the point where we can substitute one for the other and know conclusively what we really mean.

If I said to you, the programmer: “Use a list to store the names”. You go off and write whatever you need, then come back for code review and I now say: “Why can’t I look up a specific name efficiently?”. You would be outraged; why did I tell you to use a list when I really meant a hashtable instead? My response? “Oh, you know that language evolves”.

If I’d originally said to use a “collection”, a more abstract concept than either list or hashtable, that would force you to think about what you need to do and how you were going to do it. I’m not conflating a specific idea with a general concept.

As community, we have heaps of experience with various types of collections; we don’t have that same strong background with test doubles yet. When we use “mock” when we should use “dummy” or “stub” or “test double”, we avoid thinking about what we really need to do and why we need to do it.

By using “mock” inappropriately, we confuse two distinct concepts. We then complain that “mocks” suck and don’t do what we need them to do. Mocks, stubs, fakes and dummies all have a place and a purpose. Just as using a list when you should use a hashtable reflects poorly on the programmer, so does using a mock when you should use a stub.

Mocks are not stubs. Don’t believe me. Believe Martin Fowler. Believe Gerard Meszaros. Watch Why You Don’t Get Mock Objects by Gregory Moeck. Whatever you do, make sure you understand what a mock object truly is before you start conflating terms.

Note: Don’t take from this post that I always do or say the right thing. I have other posts on this same blog that incorrectly refer to test doubles as “mocks”. Most of those posts were written before I learned the difference myself. I should edit those posts to clarify my real intent. The important thing is that as we go forward we should be careful and precise about what we mean.

Discuss on reddit or Hacker News.