Refactoring

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Revision as of 00:18, 27 July 2009

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.

Contents 1 Discussion 2 Refactoring Techniques 2.1 Composing Methods 2.2 Moving Features Between Objects 2.3 Organising Data 2.4 Simplifying Conditional Expressions 2.5 Making Method Calls Simpler 2.6 Dealing with Genralisation 2.7 Big Refactorings 3 Notes 4 See also

Discussion

Refactoring is 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.

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 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.

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.

Refactoring Techniques

Composing Methods

Extract Method

Inline Method

Inline Temp

Replace Temp with Query

Introduce Explaining Variable

Split Temporary Variable

Remove assignments to Parameters

Replace Method with Method Object

Substitute Algorithm

Moving Features Between Objects

Move Method

Move Field

Extract Class

Inline Class

Hide Delegate

Remove Middle Man

Introduce Foreign Method

Introduce Local Extension

Organising Data

Self Encapsulate Field

Replace Data Value with Object

Change Value to Reference

Change Reference to Value

Replace Array with Object

Duplicate Observed Data

Change Unidirectional Association to Bidirectional

Change Bidirectional Association to Unidirectional

Replace Magic Number with Symbolic Constant

Encapsulate Field

Encapsulate Collection

Replace Type Code with Class

Replace Type Code with Subclass

Replace Type Code with State/Strategy

Simplifying Conditional Expressions

Decompose Conditional

Consolidate Conditional Expression

Consolidate Duplicate Conditional Fragments

Remove Control Flag

Replace Nested Conditional with Guard Clauses

Replace Conditional with Polymorphism

Introduce Null Object

Introduce Assertion

Making Method Calls Simpler

Rename Method

Add Parameter

Remove Parameter

Separate Query from Modifier

Parameterise Method

Replace Parameter with Explicit Methods

Preserve Whole Object

Replace Parameter with Method

Introduce Parameter Object

Replace Parameter Object

Remove Setting Method

Hide Method

Replace Constructor with Factory Method

Encapsulate Downcast

Replace Error Code with Exception

Replace Exception with test

Dealing with Genralisation

Pull Up Field

Pull Up Method

Pull Up Constructor Body

Push Down Field

Push Down Method

Extract Subclass

Extract Superclass

Extract Interface

Collapse Hierarchy

Form Template Method

Replace Inheritance with Delegation

Big Refactorings

Tease Apart Inheritance

Convert Procedural Design to Objects

Separate Domain from Presentation

Extract Hierarchy

Thats just the ones from Martin Fowler 1999 There are more some of them are up here (refactoring.com)

Notes

• 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.

See also

• Ward's wiki pages:
  • [WhatIsRefactoring]
  • [RefactorMercilessly]
  • [WikiPagesAboutRefactoring]
  • [Refactoring.com] Maintained by Martin Fowler
• Refactoring humour:
  • [Refuctoring & Code Smiles]
  • [Writing unmaintainable code]
• Martin Fowler 1999
• Code smells