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Improving the Design of Existing Code

Martin Fowler, Kent Beck, John Brant, William Opdyke, Don Roberts

Jul 1999, Hardback, 464 pages
ISBN13: 9780201485677
ISBN10: 0201485672
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Refactoring is about improving the design of existing code. It is the process of changing a software system in such a way that it does not alter the external behavior of the code, yet improves its internal structure. With refactoring you can even take a bad design and rework it into a good one. This book offers a thorough discussion of the principles of refactoring, including where to spot opportunities for refactoring, and how to set up the required tests. There is also a catalog of more than 40 proven refactorings with details as to when and why to use the refactoring, step by step instructions for implementing it, and an example illustrating how it works The book is written using Java as its principle language, but the ideas are applicable to any OO language.

As the application of object technology--particularly the Java programming language--has become commonplace, a new problem has emerged to confront the software development community. Significant numbers of poorly designed programs have been created by less-experienced developers, resulting in applications that are inefficient and hard to maintain and extend. Increasingly, software system professionals are discovering just how difficult it is to work with these inherited, "non-optimal" applications. For several years, expert-level object programmers have employed a growing collection of techniques to improve the structural integrity and performance of such existing software programs. Referred to as "refactoring," these practices have remained in the domain of experts because no attempt has been made to transcribe the lore into a form that all developers could use. . .until now. In Refactoring: Improving the Design of Existing Code, renowned object technology mentor Martin Fowler breaks new ground, demystifying these master practices and demonstrating how software practitioners can realize the significant benefits of this new process.

With proper training a skilled system designer can take a bad design and rework it into well-designed, robust code. In this book, Martin Fowler shows you where opportunities for refactoring typically can be found, and how to go about reworking a bad design into a good one. Each refactoring step is simple--seemingly too simple to be worth doing. Refactoring may involve moving a field from one class to another, or pulling some code out of a method to turn it into its own method, or even pushing some code up or down a hierarchy. While these individual steps may seem elementary, the cumulative effect of such small changes can radically improve the design. Refactoring is a proven way to prevent software decay.

In addition to discussing the various techniques of refactoring, the author provides a detailed catalog of more than seventy proven refactorings with helpful pointers that teach you when to apply them; step-by-step instructions for applying each refactoring; and an example illustrating how the refactoring works. The illustrative examples are written in Java, but the ideas are applicable to any object-oriented programming language.

1. Refactoring, a First Example.
The Starting Point.The First Step in Refactoring.Decomposing and Redistributing the Statement Method.Replacing the Conditional Logic on Price Code with Polymorphism.Final Thoughts.

2. Principles in Refactoring.
Defining Refactoring.Why Should You Refactor?When Should You Refactor?What Do I Tell My Manager?Problems with Refactoring.Refactoring and Design.Refactoring and Performance.Where Did Refactoring Come From?

3. Bad Smells in Code.
Duplicated Code.Long Method.Large Class.Long Parameter List.Divergent Change.Shotgun Surgery.Feature Envy.Data Clumps.Primitive Obsession.Switch Statements.Parallel Inheritance Hierarchies.Lazy Class.Speculative Generality.Temporary Field.Message Chains.Middle Man.Inappropriate Intimacy.Alternative Classes with Different Interfaces.Incomplete Library Class.Data Class.Refused Bequest.Comments.

4. Building Tests.
The Value of Self-testing Code.The JUnit Testing Framework.Adding More Tests.

5. Toward a Catalog of Refactorings.
Format of the Refactorings.Finding References.How Mature Are These Refactorings?

6. 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.

7. Moving Features Between Objects.
Move Method.Move Field.Extract Class.Inline Class.Hide Delegate.Remove Middle Man.Introduce Foreign Method.Introduce Local Extension.

8. Organizing 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 Record with Data Class.Replace Type Code with Class.Replace Type Code with Subclasses.Replace Type Code with State/Strategy.Replace Subclass with Fields.

9. 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.

10. Making Method Calls Simpler.
Rename Method.Add Parameter.Remove Parameter.Separate Query from Modifier.Parameterize Method.Replace Parameter with Explicit Methods.Preserve Whole Object.Replace Parameter with Method.Introduce Parameter Object.Remove Setting Method.Hide Method.Replace Constructor with Factory Method.Encapsulate Downcast.Replace Error Code with Exception.Replace Exception with Test.

11. Dealing with Generalization.
Pull Up Field.Pull Up Method.Pull Up Constructor Body.Push Down Method.Push Down Field.Extract Subclass.Extract Superclass.Extract Interface.Collapse Hierarchy.Form Template Method.Replace Inheritance with Delegation.Replace Delegation with Inheritance.

12. Big Refactorings.
Tease Apart Inheritance.Convert Procedural Design to Objects.Separate Domain from Presentation.Extract Hierarchy.

13. Refactoring, Reuse, and Reality.
A Reality Check.Why Are Developers Reluctant to Refactor Their Programs?A Reality Check (Revisited).Resources and References for Refactoring.Implications Regarding Software Reuse and Technology Transfer.A Final Note.References.

14. Refactoring Tools.
Refactoring with a Tool.Technical Criteria for a Refactoring Tool.Practical Criteria for a Refactoring Tool.Wrap Up.

15. Putting It All Together.


List of Soundbites.

List of Refactorings.

Index. 0201485672T04062001

Martin Fowler is the Chief Scientist of ThoughtWorks, an enterprise-application development and delivery company. He's been applying object-oriented techniques to enterprise software development for over a decade. He is notorious for his work on patterns, the UML, refactoring, and agile methods. Martin lives in Melrose, Massachusetts, with his wife, Cindy, and a very strange cat. His homepage is

Kent Beck consistently challenges software engineering dogma, promoting ideas like patterns, test-driven development, and Extreme Programming. Currently affiliated with Three Rivers Institute and Agitar Software, he is the author of many Addison-Wesley titles.

John Brant and Don Roberts are the authors of the Refactoring Browser for Smalltalk, which is found at They are also consultants who have studied both the practical and theoretical aspects of refactoring for six years.

William Opdyke's doctoral research on refactoring object-oriented frameworks at the University of Illinois led to the first major publication on this topic. He is currently a Distinguished Member of Technical Staff at Lucent Technologies/Bell Laboratories.

John Brant and Don Roberts are the authors of the Refactoring Browser for Smalltalk, which is found at They are also consultants who have studied both the practical and theoretical aspects of refactoring for six years.

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