Test-driven development (TDD) is an approach to program development in which you interleave testing and code development. Essentially, you develop the code incrementally, along with a test for that increment. You don’t move on to the next increment until the code that you have developed passes its test. Test-driven development was introduced as part of agile methods such as Extreme Programming. However, it can also be used in plan-driven development processes.
The fundamental TDD process is shown in Figure. The steps in the process are as follows:
1. You start by identifying the increment of functionality that is required. This should normally be small and implementable in a few lines of code.
2. You write a test for this functionality and implement this as an automated test. This means that the test can be executed and will report whether or not it has passed or failed.
3. You then run the test, along with all other tests that have been implemented. Initially, you have not implemented the functionality so the new test will fail. This is deliberate as it shows that the test adds something to the test set.
4. You then implement the functionality and re-run the test. This may involve refactoring existing code to improve it and add new code to what’s already there.
5. Once all tests run successfully, you move on to implementing the next chunk of functionality.
