Having had discussions with co-workers on the utility of private methods and recently on the scala mailing list, there’s been the notion that private methods (or even protected methods) are code smells – indicators that there is something wrong with the design of your code. I believe in many cases they can be, but that private (and protected) methods still have their uses, especially in maintaining a clean design under constraints of getting things done. First, we need to know what we are talking about.
While the technical meaning of access modifiers varies with the language, the intent these concepts communicate is what we’re talking about here. I assert that the intent of public/protected/private is (and/or should be) as follows:
- public - this method is part of the published API and will not change within major versions of the class
- protected - this method is a hook for modifying the behavior of this class using subclasses. It, too, will not change within major versions of the class (of course, it also might exist for code-reuse internal to the class hierarchy).
- private - this method was refactored out of a well tested public or protected method for reasons of clarity or internal re-use. This method may absolutely change, even in patch releases, and should not be relied upon to even exist
- package private (bonus for Java) - this method was written by someone lazy or ignorant, or by someone who acknowledges that this code should be pulled out into its own class, but hasn’t done so, yet still wants to test it separately, thus breaking encapsulation.
Why would private methods be code smells?
The most concise argument is that private methods could indicate that the class that contains them is doing too many things. Consider this code from shorty, my URL shortener:
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Is it totally clear what this method does? If not, it basically checks that the directory configured for our database exists and is a directory, giving us a specific error message if not. It’s a bit long and full of error checking, so the meat where it creates our URI hasher and gives it to the servlet is somewhat obscured. Here’s a cleaned up version:
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We’ve added lines of code, but our public method is a lot clearer: we get the dir for our DB; if we get a “right”, we have a usable dir, and if we get a “left” (the Scala convention for an error), we have the error message to use for our exception.
getDBDir now a code smell? This is a private method and, this means that
contextInitialized was tested and we extracted the private method as step three of the TDD “Test/Fix/Refactor” cycle.
getDBDir has nothing to do with the
ServletContextListener and is really all about checking for a valid path. So, we should extract it to a new class, right?
Well, if we did do that, we’ve now actually added a public API to our codebase. Unless we make the new class completely private to the enclosing class, we’ve now introduced a new public class that must be tested and managed as part of our system’s public API.
I don’t see the benefit to doing that. I don’t particularly want this method to be in the public API (at least not now). So, what about making it a private inner class? That doesn’t seem to be much different than what we have now, though it might save a few minutes on a future extraction.
When IS this a code smell?
This was a simple example. Consider a class like
SignedUrl in my Gliffy Ruby client. This class has a fair number of private methods and, overall, is pretty large. The private methods are vague things like
get_signing_key. The private methods aren’t really the problem, however; They are a symptom of the fact that this class is just doing too many things. A better design would be to split this class up into something like a
UrlAssembler (just off the top of my head). The result would be more smaller classes, each with fewer public and private methods.
What about protected methods?
In Ruby, you don’t see a lot of protected methods. In well-designed Java frameworks (such as Spring), you’ll see them as I’ve described above: hooks into a helper class that allow you to customize how that class behaves via subclassing. In non-library code, protected methods tend to get used for code-reuse in narrowly-focused class hierarchies. For example, the project I’m working on has a
BaseController that provides common methods to the actual web controllers. Having used protected methods for both of these purposes, I think that, honestly, both can be code smells.
Protected methods as hooks
In the case of providing “hooks”, I think it’s clear that by providing them at all, you are acknowledging that your class might be poorly designed (or that your language lacks the expressiveness to better design it). Consider the venerable (and now deprecated)
SimpleFormController, a Spring MVC class that provides many protected-method hooks, one of which is
protected Map referenceData(HttpServletRequest request). This is called during the lifecycle of the controller to get any data needed by the form that should be available to the view (for example, a list of U.S. states from the database). Your subclass of
SimpleFormController overrides this to provide the data as a map.
Why not create an interface called
ServletRequestToReferenceData that contains this method, and inject an instance into
SimpleFormController? This would be very Spring-like. My guess as to why this isn’t the case is that there is simply too much overhead in making yet another one-method interface and the designer of
SimpleFormController just felt this was a reasonable tradeoff.
In Scala, however, this could simply take a function:
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In Ruby, the overhead of creating a new class isn’t nearly as onerous, and we could still inject a block as we do in Scala.
Ultimately, I would say that you have to make a tradeoff here, and it has to take the language and frameworks into account; a class with many, many hooks might be a code smell, but using it as a means to avoid language ceremony in the name of OO purity is probably a reasonable design tradeoff.
Protected methods as code sharing
This use of protected methods is harder to justify, but incredibly handy. Still, this could be another case of Java (e.g.) not providing the necessary language features to make class extraction more straightforward. As mentioned, in my current application, I have a
BaseController. It has a helper method as follows:
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Because of Spring MVC, we have specialized exceptions for HTTP errors, such as “NOT_FOUND”. Here we contain the logic to identify the id of the logged-in person as well as the check for existence. Having this available to all controllers in the entire system is very handy.
But, is this a code smell?
We could make a class that does this, turning this code:
creating a one method class that has many lines of injected dependencies and other boilerplate with a few actual lines of code.
I think this is yet another case of pragmatically dealing with language ceremony. In Scala or Ruby, the overhead of creating re-usable bits of code like this is far lower than for Java; In Scala one could envision a simple function, possibly with some implicit parameters. Ultimately, “too much” of this sort of thing should be a code smell, but small bits of this, in the name of simplicity, clarity, and simply keeping the codebase smaller isn’t an automatic red flag.
So, are non-public methods code smells? If we take the wikipedia definition, which is clear that a code smell indicates merely the possibility of deeper problems in your code, then, yes, private and protected methods are code smells.
But, should they be avoided at all costs? Absolutely not. There’s many valid reasons to use them, and they do not deserve the blame for an ill-conceived class. Coding in the real world is about tradeoffs; we have to do the best thing we can with the time, resources, and tools at our disposal. These tradeoffs may not result in an According-to-Hoyle OO (or functional) design, but we’re not writing code to provide examples of programming paradigms or design patterns; we’re writing it to accomplish something and provide value. And, as professionals, we need to do it at a predictable rate that doesn’t incur too much technical debt.