You didn't catch the end where they explicitly say > Secondly, nothing is free. My code traded the ability to change requirements for reduced duplication, and

You didn't catch the end where they explicitly say > Secondly, nothing is free. My code traded the ability to change requirements for reduced duplication, and it was not a good trade. For example, we later needed many special cases and behaviors for different handles on different shapes. My abstraction would have to become several times more convoluted to afford that, whereas with the original “messy” version such changes stayed easy as cake. Clean code is easy to understand and to change. Ideally, any given file or method only deals with one concept. Messy code usually results from someone merging concepts to avoid duplication. After that happens, differences in the concepts cause the code to become much harder to reason about. I've seen this particular problem happen a million times. The worst code I ever stepped into was completely DRY. When, where, and why something happened wasn't something the original authors could answer. That's because everytime they found code duplication, they'd abstract. And everytime that didn't work, that ended up adding little "if (this instanceof Foo) special case;" to get over the most recent bug.