The Art of Agile Development

Test-Driven Development

Note

Programmers

We produce well-designed, well-tested, and well-factored code in small, verifiable steps.

“What programming languages really need is a ‘DWIM’ instruction,” the joke goes. “Do what I mean, not what I say.”

Programming is demanding. It requires perfection, consistently, for months and years of effort. At best, mistakes lead to code that won’t compile. At worst, they lead to bugs that lie in wait and pounce at the moment that does the most damage.

People aren’t so good at perfection. No wonder, then, that software is buggy.

Wouldn’t it be cool if there were a tool that alerted you to programming mistakes moments after you made them—a tool so powerful that it virtually eliminated the need for debugging?

There is such a tool, or rather, a technique. It’s test-driven development, and it actually delivers these results.

Test-driven development, or TDD, is a rapid cycle of testing, coding, and refactoring. When adding a feature, a pair may perform dozens of these cycles, implementing and refining the software in baby steps until there is nothing left to add and nothing left to take away. Research shows that TDD substantially reduces the incidence of defects [Janzen & Saiedian]. When used properly, it also helps improve your design, documents your public interfaces, and guards against future mistakes.

TDD isn’t perfect, of course. (Is anything?) TDD is difficult to use on legacy codebases. Even with greenfield systems, it takes a few months of steady use to ...