Refactoring Techniques

posted 15 Jun 2010, 13:48 by Sanjeev Kumar   [ updated 1 Dec 2011, 00:43 by Sanjeev Kumar ]
What is Refactoring anyways?
Refactoring is the process of changing a computer program's source code without modifying its external functional behavior in order to improve some of the non-functional attributes of the software. It is a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behaviour. Advantages include improved code readability and reduced complexity to improve the maintainability of the source code, as well as a more expressive internal architecture or object model to improve extensibility. 

Its heart is a series of small behavior preserving transformations. Each transformation (called a 'refactoring') does little, but a sequence of transformations can produce a significant restructuring. Since each refactoring is small, it's less likely to go wrong. The system is also kept fully working after each small refactoring, reducing the chances that a system can get seriously broken during the restructuring. By continuously improving the design of code, we make it easier and easier to work with. If you get into the hygienic habit of refactoring continuously, you'll find that it is easier to extend and maintain code.

Motivation for Refactoring

Refactoring is usually motivated by noticing a code smell. For example the method at hand may be very long, or it may be a near duplicate of another nearby method. Once recognized, such problems can be addressed by refactoring the source code, or transforming it into a new form that behaves the same as before but that no longer "smells". For a long routine, extract one or more smaller subroutines. Or for duplicate routines, remove the duplication and utilize one shared function in their place. Failure to perform refactoring can result in accumulating technical debt. There are two general categories of benefits to the activity of refactoring.
  • Maintainability. It is easier to fix bugs because the source code is easy to read and the intent of its author is easy to grasp. This might be achieved by reducing large monolithic routines into a set of individually concise, well-named, single-purpose methods. It might be achieved by moving a method to a more appropriate class, or by removing misleading comments.
  • Extensibility. It is easier to extend the capabilities of the application if it uses recognizable design patterns, and it provides some flexibility where none before may have existed.

Importance of Unit Tests

Before refactoring a section of code, a solid set of automatic unit tests is needed. The tests should demonstrate in a few seconds that the behaviour of the module is correct. The process is then an iterative cycle of making a small program transformation, testing it to ensure correctness, and making another small transformation. If at any point a test fails, you undo your last small change and try again in a different way. Through many small steps the program moves from where it was to where you want it to be. Proponents of extreme programming and other agile methodologies describe this activity as an integral part of the software development cycle.

List of refactoring techniques

Here is a very incomplete list of code refactorings. A longer list can be found in Fowler's Refactoring book and on Fowler's Refactoring Website.

Techniques that allow for more abstraction

  • Encapsulate Field – force code to access the field with getter and setter methods
  • Generalize Type – create more general types to allow for more code sharing
  • Replace type-checking code with State/Strategy
  • Replace conditional with polymorphism

Techniques for breaking code apart into more logical pieces

  • Extract Method, to turn part of a larger method into a new method. By breaking down code in smaller pieces, it is more easily understandable. This is also applicable to functions.
  • Extract Class moves part of the code from an existing class into a new class.

Techniques for improving names and location of code

  • Move Method or Move Field – move to a more appropriate Class or source file
  • Rename Method or Rename Field – changing the name into a new one that better reveals its purpose
  • Pull Up – in OOP, move to a superclass
  • Push Down – in OOP, move to a subclass
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