Duplication-Driven Development?

What is the primary force that drives you when doing test-driven development? Surprisingly, it might just be code duplication.

In the book “Test-Driven Development by Example“, Kent Beck uses an implementation of money as an example of TDD. At one point, a test case says “assert(someFunc(5) == 5 * 2)” and to satisfy the test case, someFunc is implemented simply by returning a hard-coded 10. “Now that is dirty” you say (and that’s what I said :). According to the book, the reason for hard-coding is that we want the test case to pass as soon as possible. After that, the refactoring phase can take over and we can clean up the code.

The Petronas Twin TowersHere comes the interesting part: hard-coding is a terrible habit, right? And we don’t want our bad habits to show up in the code. This sign of bad taste should be reason enough to fix it, right? However, there is a more fundamental reason for getting rid of the constant: code duplication. The constant 2 * 5 = 10 occurs both in the test case and in the code. To remove the duplication, change someFunc(param) to return “param * 2” (later, the book also gets rid of the 2 by replacing it with a member variable). To sum up, instead of acting on “taste”, we use the well proven design principle of (avoiding) code duplication.

A fascinating insight from the book is that the design of the code, too, can grow naturally from code duplication. For example, let’s say we have a class that encapsulates some algorithm. When another class with similar behavior is required, the book suggests we can accept some code duplication (even extensive copy-paste) to pass the tests. After getting the tests to pass, we refactor. Lets say the difference between the two classes is the fact that they use different algorithms to carry out their work. Apart from that, they are identical. To avoid code duplication, we combine the two classes. Quite naturally, this results in us isolating the algorithms behind an interface. By following the path of least resistance for removing code duplication, we have implemented the strategy pattern. To summarize, this suggests that code duplication helps us drive the high-level design of our code. If true, it is kind of a comforting feeling that refactoring and code duplication naturally leads us to good design.

All and all, “Test-Driven Development by Example” is well-written, easy to read and quite a good book, albeit maybe more suitable for the aspiring test-driven designer than the seasoned on.

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