Working Effectively with Legacy Code

Chapter 23: How Do I Know That I’m Not Breaking Anything?

Code is a strange sort of building material. Most materials that you can make things from, such as metal, wood, and plastic, fatigue. They break when you use them over time. Code is different. If you leave it alone, it never breaks. Short of the stray cosmic ray flipping a bit on your storage media, the only way it gets a fault is for someone to edit it. Run a machine made of metal over and over again, and it will eventually break. Run the same code over and over again, and, well, it will just run over and over again.

This puts a large burden on us as developers. Not only are we the primary agents that introduce faults in software, but it’s also pretty easy to do so. How easy is it to ...