Your class library works, but could it be better? Refactoring: Improving the Design of Existing Code shows
how refactoring can make object-oriented code simpler and easier to maintain. Today refactoring requires considerable
design know-how, but once tools become available, all programmers should be able to improve their code using refactoring
Besides an introduction to refactoring, this handbook provides a catalog of dozens of tips for improving code. The best
thing about Refactoring is its remarkably clear presentation, along with excellent nuts-and-bolts advice, from object
expert Martin Fowler. The author is also an authority on software patterns and UML, and this experience helps make this
a better book, one that should be immediately accessible to any intermediate or advanced object-oriented developer.
(Just like patterns, each refactoring tip is presented with a simple name, a "motivation," and examples using Java and
Early chapters stress the importance of testing in successful refactoring. (When you improve code, you have to test to
verify that it still works.) After the discussion on how to detect the "smell" of bad code, readers get to the heart of
the book, its catalog of over 70 "refactorings"--tips for better and simpler class design. Each tip is illustrated with
"before" and "after" code, along with an explanation. Later chapters provide a quick look at refactoring research.
Like software patterns, refactoring may be an idea whose time has come. This groundbreaking title will surely help
bring refactoring to the programming mainstream. With its clear advice on a hot new topic, Refactoring is sure to be
essential reading for anyone who writes or maintains object-oriented software. --Richard Dragan
Topics Covered: Refactoring, improving software code, redesign, design tips, patterns, unit testing, refactoring
research, and tools.
From the Inside Flap
Once upon a time, a consultant made a visit to a development project. The consultant looked at some of the code that
had been written; there was a class hierarchy at the center of the system. As he wandered through the hierarchy, the
consultant saw that it was rather messy. The higher-level classes made certain assumptions about how the classes would
work, assumptions that were embodied in inherited code. That code didn't suit all the subclasses, however, and was
overridden quite heavily. If the superclass had been modified a little, then much less overriding would have been
necessary. In other places some of the intention of the superclass had not been properly understood, and behavior
present in the superclass was duplicated. In yet other places several subclasses did the same thing with code that could
clearly be moved up the hierarchy.
The consultant recommended to the project management that the code be looked at and cleaned up, but the project
management didn't seem enthusiastic. The code seemed to work and there were considerable schedule pressures. The
managers said they would get around to it at some later point.
The consultant had also shown the programmers who had worked on the hierarchy what was going on. The programmers were
keen and saw the problem. They knew that it wasn't really their fault; sometimes a new pair of eyes are needed to spot
the problem. So the programmers spent a day or two cleaning up the hierarchy. When they were finished, the programmers
had removed half the code in the hierarchy without reducing its functionality. They were pleased with the result and
found that it became quicker and easier both to add new classes to the hierarchy and to use the classes in the rest of
The project management was not pleased. Schedules were tight and there was a lot of work to do. These two programmers
had spent two days doing work that had done nothing to add the many features the system had to deliver in a few months
time. The old code had worked just fine. So the design was a bit more "pure" a bit more "clean." The project had to ship
code that worked, not code that would please an academic. The consultant suggested that this cleaning up be done on
other central parts of the system. Such an activity might halt the project for a week or two. All this activity was
devoted to making the code look better, not to making it do anything that it didn't already do.
How do you feel about this story? Do you think the consultant was right to suggest further clean up? Or do you follow
that old engineering adage, "if it works, don't fix it"?
I must admit to some bias here. I was that consultant. Six months later the project failed, in large part because the
code was too complex to debug or to tune to acceptable performance.
The consultant Kent Beck was brought in to restart the project, an exercise that involved rewriting almost the whole
system from scratch. He did several things differently, but one of the most important was to insist on continuous
cleaning up of the code using refactoring. The success of this project, and role refactoring played in this success, is
what inspired me to write this book, so that I could pass on the knowledge that Kent and others have learned in using
refactoring to improve the quality of software. What Is Refactoring?
Refactoring 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. It is a disciplined way to clean up code that minimizes the chances of
introducing bugs. In essence when you refactor you are improving the design of the code after it has been written.
"Improving the design after it has been written." That's an odd turn of phrase. In our current understanding of
software development we believe that we design and then we code. A good design comes first, and the coding comes second.
Over time the code will be modified, and the integrity of the system, its structure according to that design, gradually
fades. The code slowly sinks from engineering to hacking.
Refactoring is the opposite of this practice. With refactoring you can take a bad design, chaos even, and rework it
into well-designed code. Each step is simple, even simplistic. You move a field from one class to another, pull some
code out of a method to make into its own method, and push some code up or down a hierarchy. Yet the cumulative effect
of these small changes can radically improve the design. It is the exact reverse of the normal notion of software decay.
With refactoring you find the balance of work changes. You find that design, rather than occurring all up front, occurs
continuously during development. You learn from building the system how to improve the design. The resulting interaction
leads to a program with a design that stays good as development continues. What's in This Book?
This book is a guide to refactoring; it is written for a professional programmer. My aim is to show you how to do
refactoring in a controlled and efficient manner. You will learn to refactor in such a way that you don't introduce bugs
into the code but instead methodically improve the structure.
It's traditional to start books with an introduction. Although I agree with that principle, I don't find it easy to
introduce refactoring with a generalized discussion or definitions. So I start with an example. Chapter 1 takes a small
program with some common design flaws and refactors it into a more acceptable object-oriented program. Along the way we
see both the process of refactoring and the application of several useful refactorings. This is the key chapter to read
if you want to understand what refactoring really is about.
In Chapter 2 I cover more of the general principles of refactoring, some definitions, and the reasons for doing
refactoring. I outline some of the problems with refactoring. In Chapter 3 Kent Beck helps me describe how to find bad
smells in code and how to clean them up with refactorings. Testing plays a very important role in refactoring, so
Chapter 4 describes how to build tests into code with a simple open-source Java testing framework.
The heart of the book, the catalog of refactorings, stretches from Chapter 5 through Chapter 12. This is by no means a
comprehensive catalog. It is the beginning of such a catalog. It includes the refactorings that I have written down so
far in my work in this field. When I want to do something, such as Replace Conditional with Polymorphism (255), the
catalog reminds me how to do it in a safe, step-by-step manner. I hope this is the section of the book you'll come back
In this book I describe the fruit of a lot of research done by others. The last chapters are guest chapters by some of
these people. Chapter 13 is by Bill Opdyke, who describes the issues he has come across in adopting refactoring in
commercial development. Chapter 14 is by Don Roberts and John Brant, who describe the true future of refactoring,
automated tools. I've left the final word, Chapter 15, to the master of the art, Kent Beck. Refactoring in Java
For all of this book I use examples in Java. Refactoring can, of course, be done with other languages, and I hope this
book will be useful to those working with other languages. However, I felt it would be best to focus this book on Java
because it is the language I know best. I have added occasional notes for refactoring in other languages, but I hope
other people will build on this foundation with books aimed at specific languages.
To help communicate the ideas best, I have not used particularly complex areas of the Java language. So I've shied away
from using inner classes, reflection, threads, and many other of Java's more powerful features. This is because I want
to focus on the core refactorings as clearly as I can.
I should emphasize that these refactorings are not done with concurrent or distributed programming in mind. Those
topics introduce additional concerns that are beyond the scope of this book. Who Should Read This Book?
This book is aimed at a professional programmer, someone who writes software for a living. The examples and discussion
include a lot of code to read and understand. The examples are all in Java. I chose Java because it is an increasingly
well-known language that can be easily understood by anyone with a background in C. It is also an object-oriented
language, and object-oriented mechanisms are a great help in refactoring.
Although it is focused on the code, refactoring has a large impact on the design of system. It is vital for senior
designers and architects to understand the principles of refactoring and to use them in their projects. Refactoring is
best introduced by a respected and experienced developer. Such a developer can best understand the principles behind
refactoring and adapt those principles to the specific workplace. This is particularly true when you are using a
language other than Java, because you have to adapt the examples I've given to other languages.
Here's how to get the most from this book without reading all of it.
If you want to understand what refactoring is, read Chapter 1; the example should make the process clear. If you want to
understand why you should refactor, read the first two chapters. They will tell you what refactoring is and why you
should do it. If you want to find where you should refactor, read Chapter 3. It tells you the signs that suggest the
need for refactoring. If you want to actually do refactoring, read the first four chapters completely. Then skip-read
the catalog. Read enough of the catalog to know roughly what is in there. You don't have to understand all the details.
When you actually need to carry out a refactoring, read the refactoring in detail and use it to help you. The catalog is
a reference section, so you probably won't want to read it in one go. You should also read the guest chapters,
especially Chapter 15.
Building on the Foundations Laid by Others
I need to say right now, at the beginning, that I owe a big debt with this book, a debt to those whose work over the
last decade has developed the field of refactoring. Ideally one of them should have written this book, but I ended up
being the one with the time and energy.
Two of the leading proponents of refactoring are Ward Cunningham and Kent Beck. They used it as a central part of their
development process in the early days and have adapted their development processes to take advantage of it. In
particular it was my collaboration with Kent that really showed me the importance of refactoring, an inspiration that
led directly to this book.
Ralph Johnson leads a group at the University of Illinois at Urbana-Champaign that is notable for its practical
contributions to object technology. Ralph has long been a champion of refactoring, and several of his students have
worked on the topic. Bill Opdyke developed the first detailed written work on refactoring in his doctoral thesis. John
Brant and Don Roberts have gone beyond writing words into writing a tool, the Refactoring Browser, for refactoring
Even with all that research to draw on, I still needed a lot of help to write this book. First and foremost, Kent Beck
was a huge help. The first seeds were planted in a bar in Detroit when Kent told me about a paper he was writing for the
Smalltalk Report Beck, hanoi. It not only provided many ideas for me to steal for Chapter 1 but also started me off in
taking notes of refactorings. Kent helped in other places too. He came up with the idea of code smells, encouraged me at
various sticky points, and generally worked with me to make this book work. I can't help thinking he could have written
this book much better himself, but I had the time and can only hope I did the subject justice.
As I've written this, I wanted to share much of this expertise directly with you, so I'm very grateful that many of
these people have spent some time adding some material to this book. Kent Beck, John Brant, William Opdyke, and Don
Roberts have all written or co-written chapters. In addition, Rich Garzaniti and Ron Jeffries have added useful
Any author will tell you that technical reviewers do a great deal to help in a book like this. As usual, Carter
Shanklin and his team at Addison-Wesley put together a great panel of hard-nosed reviewers. These were
Ken Auer, Rolemodel Software, Inc. Joshua Bloch, Javasoft John Brant, University of Illinois at Urbana-Champaign Scott
Corley, High Voltage Software, Inc. Ward Cunningham, Cunningham & Cunningham, Inc. Stephane Ducasse Erich Gamma, Object
Technology International, Inc. Ron Jeffries Ralph Johnson, University of Illinois Joshua Kerievsky, Industrial Logic,
Inc. Doug Lea, SUNY Oswego Sander Tichelaar
They all added a great deal to the readability and accuracy of this book, and removed at least some of the errors that
can lurk in any manuscript. I'd like to highlight a couple of very visible suggestions that made a difference to the
look of the book. Ward and Ron got me to do Chapter 1 in the side-by-side style. Joshua suggested the idea of the code
sketches in the catalog.
In addition to the official review panel there were many unofficial reviewers. These people looked at the manuscript or
the work in progress on my Web pages and made helpful comments. They include Leif Bennett, Michael Feathers, Michael
Finney, Neil Galarneau, Hisham Ghazouli, Tony Gould, John Isner, Brian Marick, Ralf Reissing, John Salt, Mark Swanson,
Dave Thomas, and Don Wells. I'm sure there are others who I've forgotton; I apologize and offer my thanks.
A particularly entertaining review group is the infamous reading group at the University of Illinois at
Urbana-Champaign. Because this book reflects so much of their work, I'm particularly grateful for their efforts captured
in real audio. This group includes Fredrico "Fred" Balaguer, John Brant, Ian Chai, Brian Foote, Alejandra Garrido,
Zhijiang "John" Han, Peter Hatch, Ralph Johnson, Songyu "Raymond" Lu, Dragos-Anton Manolescu, Hiroaki Nakamura, James
Overturf, Don Roberts, Chieko Shirai, Les Tyrell, and Joe Yoder.
Any good idea needs to be tested in a serious production system. I saw refactoring have a huge effect on the Chrysler
Comprehensive Compensation system (C3). I want to thank all the members of that team: Ann Anderson, Ed Anderi, Ralph
Beattie, Kent Beck, David Bryant, Bob Coe, Marie DeArment, Margaret Fronczak, Rich Garzaniti, Dennis Gore, Brian Hacker,
Chet Hendrickson, Ron Jeffries, Doug Joppie, David Kim, Paul Kowalsky, Debbie Mueller, Tom Murasky, Richard Nutter,
Adrian Pantea, Matt Saigeon, Don Thomas, and Don Wells. Working with them cemented the principles and benefits of
refactoring into me on a firsthand basis. Watching their progress as they use refactoring heavily helps me see what
refactoring can do when applied to a large project over many years.
Again I had the help of J. Carter Shanklin at Addison-Wesley and his team: Krysia Bebick, Susan Cestone, Chuck Dutton,
Kristin Erickson, John Fuller, Christopher Guzikowski, Simone Payment, and Genevieve Rajewski. Working with a good
publisher is a pleasure; they provided a lot of support and help.
Talking of support, the biggest sufferer from a book is always the closest to the author, in this case my (now) wife
Cindy. Thanks for loving me even when I was hidden in the study. As much time as I put into this book, I never stopped
being distracted by thinking of you.
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