Code duplication is evil

As you’re programming, have you ever had the sense that a great evil was lurking in your code base? Well, most likely there is, and it’s name is: code duplication! This monster will sneak it’s way into your code with the promise of “fast” implementation and a “simple” solution. But make no mistake, once it has a foothold in your code base, what you thought was your friend will turn on you with a vengeance and destroy you. The code will rot in it’s place, and you’ll be cursed with recurring bugs that come back because you only fixed them in one place. Then your friends will laugh at your plight and make up nick names for you like “Mr. Duplication” or “Copy and paste Man.” In the end you’ll be left homeless and penniless. Then you will rue the day that you ever gave in to the subtle and poisonous promises of code duplication.

I’m not sure everyone sees it this way, though. I’ve said for a while now that the best way I know of to get to a well factored system is to have an acute aversion to code duplication. Both the obvious and subtle forms of it. However, despite the fact that the DRY principle is well known, I find that many developers (perhaps most) not only often repeat themselves in their code, but they also seem to be unconcerned when they find duplicated code and have to modify it. Personally I think the latter is the biggest issue.

We all write bad code some times, and we do stupid things like giving in to the temptation to duplicate. Many strange and terrible things can be done in the heat of the moment while programming and trying to implement a feature (even if you are pairing). To ere is human and it’s perfectly understandable. However, to come across blatant duplication and not only do nothing about it, but to be unmoved by it, that my friends is inexcusable.

Of all the code smells there are, code duplication is one of the most telling. So much can be learned from it, if you’re paying attention. Some times it can tell you that your design is not quite right. Or that you’re missing an abstraction. Or that you’re thinking about the problem in the wrong way. Often a lot of code duplication can be removed by solving the coding problem from a different angle.

There’s more that can be learned than just the few things I mentioned, but you will see none of these things if, when you encounter code duplication, you merely make the required changes in multiple places. You have to learn to hate it! You should be outraged by it. Don’t stand for it! When you see blatant code duplication, leap out of your chair, grab your keyboard and start slamming in on the table while yelling at your monitor, “NO! NO! NO! NO!” Sure, everyone will think you’re crazy, and I suppose you might even get fired, but after an out burst like that you’ll be determined to do something about the duplication. And, who knows, after a few of them, maybe others will be more careful about it, being frightened by what you might do next time.

When you see the evil of code duplication, don’t let yourself be unmoved by it. Be outraged if it helps, but do something about it. How long will we allow this great evil of our time to endure? As the saying goes: all that is necessary for the triumph of evil is that good programmers do nothing.