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May 2014 12 www.drdobbs.com The Relationship Between Testability and Good Design Problems writing good unit tests reveal shortcomings in design and coding. R ecently, the profile of unit testing has risen dramatically. In the 1990s and early 2000s, it was a practice that seemed to languish. As is often the case when people adopt a new technology — back then, organizations were moving from structured design toward object-oriented design — they place all their focus on getting the essentials right and discard practices that don't seem to fit neatly into the new picture. And so they neglected unit testing. But if we've learned anything over the past 10 years, it is that unit testing is an essential discipline. Tests help us better reason about our code, and they form a regression bedrock that makes refac- toring and feature addition much easier. There is, however, a very subtle effect of unit testing that few people discuss. There seems to be an eerily consistent connection between testability at the unit level and good design. Almost uniformly, code that is hard to test has design problems. When you fix the design problems, it be- comes easy to test. Let's take a look at an example. In Figure 1, we have a class that seems to have decent structure. There is one public method, evaluate(), which is the user's point of contact with the class. It delegates its work to a series of private methods. Overtly, there is nothing wrong with this — class RuleEvaluator does the job we've intended it to. Now, how should we write unit tests for this class? We should be able to in- stantiate it, call evaluate() with various arguments, and check the results. But suppose we wanted to test getNextToken()? By Michael Feathers [ GOOD DESIGN ] Table of Contents @mfeathers Previous Next Previous Next Download Download Register Register Subscribe Subscribe Previous Next Previous Next

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