Design Patterns: Introduction

A design pattern is defined as “general, reusable solution to a commonly occurring problem within a given context in software design.” The idea is that when you encounter a problem, it provides a ready to use template for helping to solve that problem.

If you look at the Wikipedia article linked above, we then drop into a bunch of things which are then called “design patterns.” And while they are useful–I have never really cared for the way we use the definition “design pattern” in practice because many of them are so “small,” in a sense. That is, a lot of design patterns seem applicable to user interface design–which is good, I suppose–but many of them aren’t more than just a way to rearrange objects in an object-oriented system. (And as the criticism in the article notes, many of these “design patterns” are more a reflection of missing features in a programming language than do they represent true “patterns.”)

For me, the more interesting design patterns I have encountered in my life are far more complex than this. But I see them often enough in my coding that I resort to these “algorithmic” design patterns to solve far more complex problems than “how to unspagetify my code.”

I think it’s also worth noting the difference between a “pattern” and an “algorithm.”

To me, an algorithm is something that can be built as a self-contained unit. For example, sorting an array can be written in most modern languages using things like Java Generics: define the object, specify the comparator which compares objects, toss to a sorting object with the generic specified as the object you’re sorting. Things like red/black trees and the like can similarly be coded in self-contained packages in such a way so that you simply specify the object you want stored–and you’re good to go.

The implementation details at that point, in a well-written library, becomes almost academic: you have no need to know how Merge sort or Red-Black trees work; just pass to Arrays.sort() or create a TreeSet and call it a day.

But some things we need to do are as much “design pattern” as they are “algorithm”; that is, they’re techniques which cannot exactly be packaged, but are larger than a pattern like the observer pattern.

So that is the point of this series of essays: to describe design patterns that are much bigger and more interesting than design patterns that are arguably missing features in a programming language. Though I may touch upon more interesting variations of existing design patterns as I encounter them. And I may discuss things that are not quite ‘algorithmic design patterns’ as much as they are ‘ways to solve problems’ that are beyond simple design patterns.

Mostly when I encounter a thing that’s interesting to me, I plan to describe it here.