In my previous editorial, I discussed the benefits and other effects on code bases of using small classes, which I defined using a limit of 50-60 lines. I received many letters on this article, most analogizing it to edicts of olden times to keep functions short enough to fit on one pane of glass.
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Much as I appreciate those letters, they miss a crucial point. I was not discussing a single function, but rather an entire class, which implies multiple functions in most cases. Coding classes as diminutive as 60 lines struck other correspondents as simply too much of a constraint and not worth the effort.
But it's precisely the discipline that this number of lines imposes that creates the very clarity that's so desirable in the resulting code. The belief expressed in other letters that this discipline could not be consistently maintained suggests that the standard techniques for keeping classes small are not as widely known as I would have expected. (Given that the original editorial was inspired by an extended effort to clean up a project that contains much of my own code, I say this with all due humility.)
Let's go over the principal techniques. I presume in this discussion that design has been done and it's now just a matter of writing the code. Or in the less attractive case, of maintaining code.
Diminish the workload. The first technique to apply is the single responsibility principle (SRP), which states that classes should do only one thing. How big that one thing is will determine in large part how big your classes are going to be. Reduce the work of each class; then, use other classes to marshal these smaller classes correctly.
Avoid primitive obsession. This obsession refers to the temptation to use collections in their raw form. This is definitely a code smell. If you have a linked list of objects, that linked list should be in its own class, with a descriptive name. Expose only the methods that the other classes need. This prevents other classes from performing operations without your knowledge on an object they don't own. The purpose of the list is also supremely clear and this encapsulation enables you to change easily to a different data structure if the need should arise later on.
Reduce the number of class and instance variables. A profusion of instance variables is a code smell. It strongly suggests that the class is doing more than one thing. It also makes it very difficult for subsequent developers to figure out what the class does. Very often, some subset of the variables form a natural grouping. Group them into a new class. And move the operations that manipulate them directly into that class.
Subclass special-case logic. If you have a class that includes rarely used logic, determine whether that logic can be moved to a subclass or even to another class entirely. The classic example of the benefits of object orientation is polymorphism. Use it to handle special variants.
Don't repeat yourself (DRY). This suggestion appears pointlessly obvious. However, even coders who are attentive to this rule will repeat code in two methods that differ only in a single detail. In addition, they can overlook the introduction of duplicate code during maintenance. More than the other guidelines here, which are all techniques, DRY is a discipline within a discipline.
Taken together, these tools get you most of the way to small classes. To see how they are implemented in real life, I suggest a book I've mentioned many times before, namely Martin Fowler's Refactoring, which is essentially a cookbook of techniques for cleaning up code, including the step-by-step process, and the illustrative code.
Returning back to my own experience, I am finding that as I insist on this particular discipline in my code rework, my brain is slowly developing a "muscle memory" and is beginning to think automatically about class size prior to class development and certainly during the cleanup of existing code. Cheers!