Martin Fowler 1999 defines refactoring in two ways---in the noun form, and the verb form:
- Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing its observable behaviour.
- Refactoring (verb): to restructure software by applying a series of refactorings without changing its observable behaviour.
Both of these definitions carry a common line: "without changing its observable behaviour". Refactoring is the process of restructuring code to make it easier to use, cleaner to work with, and removing Code smells. An alternative definition is "Extending a program to make it more extensible".
Refactoring is a big deal. It overturns the older culture of If it ain't broke don't fix it by allowing developers to change code to meet their own needs, rather than for fixing bugs or adding features. It is an integral part of Agile methods, expecially Test Driven Development.
Refactoring is much more disciplined than just editing code. It is an anti-hacking practice that reduces code changes to simple, atomic transactions. Martin Fowler 1999 says "the cumulative effect of these small changes can radically improve the design".
Refactoring is heavily dependent on Unit testing. Before any code is refactored, a self-checking test suite should be developed. It is used to check that the refactoring didn't break anything. If you are performing a series of small refactorings, run the tests between each one. This makes it easy to identify & fix bugs immediately. Fowler states that, when refactoring, one needs to know what can go wrong, and figure out how to refactor the program safely, with Martin Fowler 1999 providing safe steps for the various types of refactoring.
The culture of refactoring says that, if you are looking at some code and you see a way to make it better, you should. You should also refactor if you can't understand what it does. Fowler argues that this Refactoring and design approach makes a big difference to design culture.
Fowler also discusses Refactoring and performance.
Moving Features Between Objects
Simplifying Conditional Expressions
Making Method Calls Simpler
Dealing with Generalisation
- If you change the external behaviour of code, you aren't refactoring, you're just coding.
- If you don't have automated tests in place, you don't know whether you changed the behaviour, so you aren't refactoring.
- Refactoring is essential to ideas like Do the simplest thing that could possibly work and You ain't gonna need it.
- Refactoring helps us escape Big design up front.
- Most IDEs now have some level of support for refactoring.
- Ward's wiki pages:
- Refactoring humour:
- Martin Fowler 1999
- Code smells