Working Effectively with Legacy Code

by Michael C. Feathers

To me, legacy code is simply code without tests.

To mitigate risk, we have to ask three questions: 1. What changes do we have to make? 2. How will we know that we’ve done them correctly? 3. How will we know that we haven’t broken anything?

The Legacy Code Change Algorithm When you have to make a change in a legacy code base, here is an algorithm you can use. 1. Identify change points. 2. Find test points. 3. Break dependencies. 4. Write tests. 5. Make changes and refactor.