[oalae5niMiel7qu]

I saw this in my codebase firsthand. At first, we had to communicate with one device. Then two. For that second device, I subclassed the class that handled the communication with the first thing, and changed 3 methods in the subclass. Now it was possible to substitute that second device for the first, allowing a different product configuration. After a few years, we had to support a third device type, and now they wanted it to be possible to have more than one of them, and to mix and match them.

Supporting the third device was handed off to a junior dev. I pointed him at my subclass and said to just do that, we'd figure out the mixing and matching later. But he looked at my subclass like it was written in Greek. He ended up writing his own class that re-imagined the functionality of the superclass and supported the new device (but not the old ones). Integrating this new class into the rest of the codebase would've been nigh impossible, since he also re-implemented some message-handling code, but with only a subset of the original functionality, and what was there was incorrect.

His work came back to me, and I refactored that entire section of the code, and this is when the generalization occurred: Instead of a superclass, I took the stuff that had to be inherited and made that its own thing, having the same interface as before. The device communication part would be modeled as drivers, with a few simple functions that would perform the essential functions of the devices, implemented once per device type. I kept the junior dev's communication code for the new device, but deleted his attempt to re-imagine that superclass. Doing it this way also made it easy to mix and match the devices.