The Four Elements of Simple Design

I realize that I've never really written this down before, and I say it so frequently in my work as a trainer and mentor, that I think it bears repeating.

I have reduced everything I've ever learned about effective object-oriented design to the four elements of simple design that I first learned from Kent Beck's work. Maybe you can, too.
I define simple design this way. A design is simple to the extent that it:
* Passes its tests
* Minimizes duplication
* Maximizes clarity
* Has fewer elements
Note that I put these properties in priority order. I'm willing to copy-and-paste to get a test passing, but once the test passes, I can usually remove the duplication quickly. I'm willing to extract code into a method and call it foo() in order to get rid of duplicate code, although that name foo() rarely survives more than 15 minutes. Finally, I will gladly introduce interfaces, classes, methods and variables to clarify the intent of a piece of code, although generally speaking once I make things more clear, I can find things to cut.

Some people put “minimize duplication” and “maximize clarity” in a tie for second place. I don’t. My experience has led me to conclude that removing duplication helps more than fixing bad names does. Moreover, removing duplication tends to allow a suitable structure to emerge, whereas bad names highlight an inappropriate distribution of responsibilities. I use this observation as a key element of my demonstration, “Architecture Without Trying”.
Now I should point out that, as a test-driven development (and now also a behavior-driven development) practitioner, I write tests as I draw breath, so I don't really need to emphasize that part.
* Passes its tests
* Minimizes duplication
* Maximizes clarity
* Has fewer elements
I should also point out that I’ve yet to see a codebase with low duplication and high clarity that, nonetheless, had considerably more design elements than it needed, so I don't really need to emphasize that part, either.
* Passes its tests
* Minimizes duplication
* Maximizes clarity
* Has fewer elements
That leaves me with two key elements of simple design: remove duplication and fix bad names. When I remove duplication, I tend to see an appropriate structure emerge, and when I fix bad names, I tend to see responsibilities slide into appropriate parts of the design.
I claim these to be axioms of modular design, with a “parallel postulate” of whether you use objects or not. If you use objects, you get object-oriented design, and if you don’t, you get structured design. (I don’t know how functional design fits into this yet, because I haven’t done enough of it.)
I claim that developing strong skills of detecting duplication, removing duplication, identifying naming problems, and fixing naming problems equates to learning everything ever written about object-oriented design.
Put more simply, if you master removing duplication and fixing bad names, then I claim you master object-oriented design.
Now I wouldn't bother burning your old OOD/OOP books, but I will tell you that if you have an interested buyer, then feel free to sell them. Sphere: Related Content