The term 'Mock Objects' has become a popular one to describe special case objects that mimic real objects for testing. Most language environments now have frameworks that make it easy to create mock objects. What's often not realized, however, is that mock objects are but one form of special case test object, one that enables a different style of testing. In this article I'll explain how mock objects work, how they encourage testing based on behavior verification, and how the community around them uses them to develop a different style of testing.
Final Thoughts
As interest in unit testing, the xunit frameworks and Test Driven Development has grown, more and more people are running into mock objects. A lot of the time people learn a bit about the mock object frameworks, without fully understanding the mockist/classical divide that underpins them. Whichever side of that divide you lean on, I think it's useful to understand this difference in views. While you don't have to be a mockist to find the mock frameworks handy, it is useful to understand the thinking that guides many of the design decisions of the software.
The purpose of this article was, and is, to point out these differences and to lay out the trade-offs between them. There is more to mockist thinking than I've had time to go into, particularly its consequences on design style. I hope that in the next few years we'll see more written on this and that will deepen our understanding of the fascinating consequences of writing tests before the code.
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