- Mysterious Name
- Duplicated Code
- Long Function
- Long Parameter List
- Global Data
- Mutable Data
- Divergent Change
- Shotgun Surgery
- Feature Envy
- Data Clumps
- Primitive Obsession
- Repeated Switches
- Lazy Element
- Speculative Generality
- Temporary Field
- Message Chains
- Middle Man
- Insider Trading
- Large Class
- Alternative Classes with Different Interfaces
- Data Class
- Refused Bequest
Kent Beck helps Martin Fowler describe how to find bad smells in code and how to clean them up with refactorings.
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“If it stinks, change it.”
— Grandma Beck, discussing child-rearing philosophy
By now you have a good idea of how refactoring works. But just because you know how doesn’t mean you know when. Deciding when to start refactoring—and when to stop—is just as important to refactoring as knowing how to operate the mechanics of it.
Now comes the dilemma. It is easy to explain how to delete an instance variable or create a hierarchy. These are simple matters. Trying to explain when you should do these things is not so cut-and-dried. Instead of appealing to some vague notion of programming aesthetics (which, frankly, is what we consultants usually do), I wanted something a bit more solid.
When I was writing the first edition of this book, I was mulling over this issue as I visited Kent Beck in Zurich. Perhaps he was under the influence of the odors of his newborn daughter at the time, but he had come up with the notion of describing the “when” of refactoring in terms of smells.
“Smells,” you say, “and that is supposed to be better than vague aesthetics?” Well, yes. We have looked at lots of code, written for projects that span the gamut from wildly successful to nearly dead. In doing so, we have learned to look for certain structures in the code that suggest—sometimes, scream for—the possibility of refactoring. (We are switching over to “we” in this chapter to reflect the fact that Kent and I wrote this chapter jointly. You can tell the difference because the funny jokes are mine and the others are his.)
One thing we won’t try to give you is precise criteria for when a refactoring is overdue. In our experience, no set of metrics rivals informed human intuition. What we will do is give you indications that there is trouble that can be solved by a refactoring. You will have to develop your own sense of how many instance variables or how many lines of code in a method are too many.
Use this chapter and the table on the inside back cover as a way to give you inspiration when you’re not sure what refactorings to do. Read the chapter (or skim the table) and try to identify what it is you’re smelling, then go to the refactorings we suggest to see whether they will help you. You may not find the exact smell you can detect, but hopefully it should point you in the right direction.
Puzzling over some text to understand what’s going on is a great thing if you’re reading a detective novel, but not when you’re reading code. We may fantasize about being International Men of Mystery, but our code needs to be mundane and clear. One of the most important parts of clear code is good names, so we put a lot of thought into naming functions, modules, variables, classes, so they clearly communicate what they do and how to use them.
Sadly, however, naming is one of the two hard things [mf-2h] in programming. So, perhaps the most common refactorings we do are the renames: Change Function Declaration (124) (to rename a function), Rename Variable (137), and Rename Field (244). People are often afraid to rename things, thinking it’s not worth the trouble, but a good name can save hours of puzzled incomprehension in the future.
Renaming is not just an exercise in changing names. When you can’t think of a good name for something, it’s often a sign of a deeper design malaise. Puzzling over a tricky name has often led us to significant simplifications to our code.