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Chapter 1. Refactoring
Refactoring. What is it? Why do it?
In brief, refactoring is the gradual improvement of a code base by making small changes that don't modify a
program's behavior, usually with the help of some kind of automated tool. The goal of refactoring is to remove
the accumulated cruft of years of legacy code and produce cleaner code that is easier to maintain, easier to
debug, and easier to add new features to.
Technically, refactoring never actually fixes a bug or adds a feature. However, in practice, when refactoring I
almost always uncover bugs that need to be fixed and spot opportunities for new features. Often, refactoring
changes difficult problems into tractable and even easy ones. Reorganizing code is the first step in improving it.
If you have the sort of personality that makes you begin a new semester, project, or job by thoroughly cleaning
up your workspace, desk, or office, you'll get this immediately. Refactoring helps you keep the old from getting
in the way of the new. It doesn't let you start from a blank page. Instead, it leaves you with a clean, organized
workspace where you can find everything you need, and from which you can move forward.
The concept of refactoring originally came from the object-oriented programming community and dates back at
least as far as 1990 (William F. Opdyke and Ralph E. Johnson, "Refactoring: An Aid in Designing Application
Frameworks and Evolving Object-Oriented Systems," Proceedings of the Symposium on Object-Oriented
Programming Emphasizing Practical Applications [SOOPPA], September 1990, ACM), though likely it was in at
least limited use before then. However, the term was popularized by Martin Fowler in 1999 in his topic
Refactoring (Addison-Wesley, 1999). Since then, numerous IDEs and other tools such as Eclipse, IntelliJ IDEA,
and C# Refactory have implemented many of his catalogs of refactorings for languages such as Java and C#, as
well as inventing many new ones.
However, it's not just object-oriented code and object-oriented languages that develop cruft and need to be
refactored. In fact, it's not just programming languages at all. Almost any sufficiently complex system that is
developed and maintained over time can benefit from refactoring. The reason is twofold.
Increased knowledge of both the system and the problem domain often reveals details that weren't
apparent to the initial designers. No one ever gets everything right in the first release. You have to see a
system in production for a while before some of the problems become apparent.
Over time, functionality increases and new code is written to support this functionality. Even if the original
system solved its problem perfectly, the new code written to support new features doesn't mesh perfectly
with the old code. Eventually, you reach a point where the old code base simply cannot support the weight
of all the new features you want to add.
When you find yourself with a system that is no longer able to support further developments, you have two
choices: You can throw it out and build a new system from scratch, or you can shore up the foundations. In
practice, we rarely have the time or budget to create a completely new system just to replace something that
already works. It is much more cost-effective to add the struts and supports that the existing system needs
before further work. If we can slip these supports in gradually, one at a time, rather than as a big-bang
integration, so much the better.
Many sufficiently complex systems with large chunks of code are not object-oriented languages and perhaps are
not even programming languages at all. For instance, Scott Ambler and Pramod Sadalage demonstrated how to
refactor the SQL databases that support many large applications in Refactoring Databases (Addison-Wesley,
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