I’ve espoused, on many occasions, making your methods short. The pattern Composed Method, defined by Kent Beck in Smalltalk Best Practice Patterns, says that methods should do things only at one level of abstraction. Methods should be short; a good size for Smalltalk methods is one to a half-dozen lines. That translates to about one to twelve lines (not counting braces!) for Java methods.
I recently engaged in a discussion at JavaRanch where someone thought “20 to 25 lines” was too short. After being taken aback, my response was that there are many reasons that your average method size should be considerably smaller. Here are nine benefits to short methods:
Maintenance costs. The longer a method, the more it will take you to figure out what it does, and where your modifications need to go. With shorter methods, you can quickly pinpoint where a change needs to go (particularly if you are coding using test-driven development).
Code readability. After the initial learning curve, smaller methods make it far easier to understand what a class does. They can also make it easier to follow code by eliminating the need for scrolling.
Reuse potential. If you break down methods into smaller components, you can start to recognize common abstractions in your code. You can minimize the overall amount of code dramatically by reusing these common methods.
Subclassing potential. The longer a method is, the more difficult it will be to create effective subclasses that use the method.
Naming. It’s easier to come up with appropriate names for smaller methods that do one thing.
Performance profiling. If you have performance issues, a system with composed methods makes it easier to spot the performance bottlenecks.
Flexibility. Smaller methods make it easier to refactor (and to recognize design flaws, such as feature envy).
Coding quality. It’s easier to spot dumb mistakes if you break larger methods into smaller ones.
Comment minimization. While comments can be valuable, most are unnecessary and can be eliminated by prudent renaming and restructuring. Comments that restate what the code says are unnecessary.
You should get the idea that most of the benefits are about improving the design of your system. Breaking methods up into smaller ones obviously leads to lots of smaller methods. You will find that not all of those methods should remain in the same class in which the original method was coded. The small methods will almost always point out to you that you are violating the basic rule of class design: the Single Responsibility Principle (SRP).
The SRP states that a class should have only one reason to change. Put another way, a class should do one thing and one thing only. A class to present a user interface (UI) should do just that. It shouldn’t act as a controller; it shouldn’t retrieve data; it shouldn’t open files; it shouldn’t contain calculations or business logic. A UI class should interact with other small classes that do each of those things separately.
Break a class into small methods, and you will usually find all sorts of violations of the SRP.
Initially, you may find it more difficult to work with code with lots of methods. There is certainly a learning curve associated with doing so. You will find that the smart navigational features of your IDE (for example, Eclipse) can go a long way toward helping you understand how all the little methods fit together. In short time, you will find that well-composed code imparts much greater clarity to your system.
One oft-repeated resistance to short methods is that it can degrade performance in your system. Indeed, method calls are usually fairly expensive operations. However, you rarely create performance problems with more methods. Poor performance can usually be attributed to other factors, such as IO operations, network latency, poor choice of algorithm, and other inefficient uses of resources.
Even if additional method calls do noticeably impact performance (performance is not an issue until someone recognizes it as one), it’s very easy to inline methods. The rule of performance is always: make it run, make it right (e.g. small methods), make it fast (optimize).
While it’s difficult to do, you can go too far and create too many methods. Make sure each method has a valid reason for existing; otherwise inline it. A method is useless if the name of the method and the body of the method say exactly the same thing.
There are exceptions to every rule. There will always be the need for the rare method larger than 20 lines. One way to look at things, however, is to look at smaller methods as a goal to strive for, not a hard number. Instead of debating whether 25 lines, 12 lines or 200 lines is acceptable, see what you can do to reduce method length in code that you would’ve otherwise not touched. You’ll be surprised at how much code you can eliminate.
Few people promote pushing in the other direction. Increasing the average method size represents sheer carelessness or laziness. Take the time and care to craft your code into a better design. Small methods!