The Art of Agile Development, 2nd Edition

Test-Driven Development

AUDIENCE

Programmers

We produce high-quality 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.

Wouldn’t it be wonderful if there were a way to make computers do you what you mean? A technique so powerful, it virtually eliminates the need for debugging?

There is such a technique. It’s test-driven development, and it really works.

Test-driven development, or TDD, is a rapid cycle of testing, coding, and refactoring. When adding a feature, you’ll perform dozens of these cycles, implementing and refining the software in tiny steps until there is nothing left to add and nothing left to take away. Done well, TDD ensures that the code does exactly what you mean, not just what you say.

When used properly, TDD also helps you improve your design, documents your code for future programmers, enables refactoring, and guards against future mistakes. Better yet, it’s fun. You’re always in control and you get this constant reinforcement that you’re on the right track.

TDD isn’t perfect, of course. TDD helps programmers code what they intended to code, but it doesn’t stop programmers from misunderstanding what they need to do. It helps ...