I am a huge advocate of Test Driven Development (TDD). I’ve even written an introduction to TDD. In his book Test Driven Development by Example [TDDbyExample] Kent Beck defines TDD as a process where you must write tests for code before writing the code itself. Therefore if you’re doing TDD you have to write the tests first, right? Wrong!
I really feel like unburdening, so here’s another admissions. I’m addicted to the green bar and high code coverage percentages. For those that are unfamiliar with the green bar, it’s a feature of the JUnit (and other testing framework) GUI. If all your tests pass you get a green bar. If any of your tests fail, you get a red bar. I’m addicted to the green bar, I can’t sleep without it. There are tools that allow you to measure how much of your code is exercised by your tests as a percentage. Usually anything over 80% coverage is considered good. I always strive for 100%, but usually achieve high 90s. I can’t sleep without high code coverage either. It’s very important that you bear these two vices in mind as you read on as they give me a discipline that not all software developers have. This isn’t because I’m some super software developer. It’s an affliction, believe me. If you don’t have this discipline, write your tests first.
There are many advantages to TDD. The two that stand out for me are loosely coupled code and code that’s easy to test (obviously). Actually by making your code easy to test it becomes loosely coupled and easy to change by default and that’s the point I’m making. If you want to get the green bar and have high code coverage then you need to make your code testable. Writing the tests first forces you to make your code testable, but it’s not the only way. If your tests are automated and measuring code coverage is automated or better still your continuous integration system runs your tests and measures the code coverage you’re forced to make your code testable. The key, when writing your code is to think about how you’re going to test it and write the tests soon after. This is a skill that has to be learnt and the best way to learn it is to start test first. Once you have it nailed though, you too will become addicted to the green bar and high code coverage. Of course you still have to write the tests, before you’ve written too much code, and refactor to remove duplicate code. It’s important to keep running the tests and to make sure they pass every time you make a change to the code. Many developers are lazy and don't bother. My affliction means that if I'm lazy, I don't sleep. It's a curse.