Add feature on legacy code – code cast
Blog post series
This is a code cast in Java. We have a new feature request and we need to add it to a legacy code base.
We start by understanding what we need to change. After that we run the existing system tests, that we wrote during the session From Nothing to System Tests. We use a test coverage tool to understand how much of the code is covered with tests. After this step we start writing new system tests until all the code
that will be changed is covered by tests.
When all of the system is covered with tests, we refactor the code to make room for a new feature. We extract a method, then a class and then an interface. The interface is implemented in a new way so that we add the new functionality. Adding the new functionality is done with tests written up-front.
At the end we write one system test to verify that the whole system can work well with the new feature. This is a kind of partial acceptance test that helps us understand if all the system works well together. And of course, manual testing and refactoring is essential before we can say we are done.
See more about the technique in the blog post Add Features on Legacy Code.
Many thanks to Thomas Sundberg for proofreading this post.