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".
According to Martin Fowler 1999, p. xvii, Kent Beck is the foremost "master of the art" of refactoring. Beck has opinions about Why refactoring works.
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.
These techniques are from Martin Fowler 1999. There are more techniques here (refactoring.com).
Remove assignments to Parameters
Replace Method with Method Object
Moving Features Between Objects
Replace Data Value with Object
Change Unidirectional Association to Bidirectional
Change Bidirectional Association to Unidirectional
Replace Magic Number with Symbolic Constant
Replace Type Code with Subclass
Replace Type Code with State/Strategy
Simplifying Conditional Expressions
Consolidate Conditional Expression
Consolidate Duplicate Conditional Fragments
Replace Nested Conditional with Guard Clauses
Replace Conditional with Polymorphism
Making Method Calls Simpler
Replace Parameter with Explicit Methods
Replace Constructor with Factory Method
Replace Error Code with Exception
Dealing with Generalisation
Replace Inheritance with Delegation
Convert Procedural Design to Objects
Separate Domain from Presentation
- 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