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.

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Refactoring refers to taking existing, working software, and changing it about to improve its design, so that future modifications and enhancements are easier to add. _Refactoring_ is primarily a catalog of 70 or so different kinds of improvements you can make to object-oriented software.

Each entry in the catalog describes an implementation problem, the solution, motivation for applying the solution, the mechanics of the refactoring, and examples. The book's examples are all in Java, but C++ programmers should be able to approach the refactorings with ease. Often, Fowler diagrams the refactorings in UML, so a little Unified Modeling Language experience will help, too.

While the catalog is nice, the kinds of refactorings are obvious is most cases. Even moderately experienced programmers won't need the step-by-step mechanics described. The real benefit, though, is that the mechanics of each refactoring help guarantee that you can pull off the refactoring without introducing new bugs or side effects. They encourage you to take smaller, verifiable steps, than the more gross refactorings that most developers would naturally take. You actually save time doing so.

How do you know your refactorings are safe? Unit testing is the answer that Fowler et al. provide. Java developers will find the introduction to the Junit Testing Framework the most valuable part of the book, more so than the catalog of refactorings itself.

There's more to the book than the catalog and Junit, of course. There's discussion of the history of refactoring, how to evaluate refactoring tools, and how to convince management that what appears to be an overhead activity is actually useful in the long run.

Unfortunately, these sections are all too brief. And there is no discussion of how refactoring fits in with various software development processes. For example, programmers using Extreme Programming (XP) would probably feel right at home with Fowler's recommendations of refactoring in duets and unit testing, but developers stuck with a Software Engineering Institute process like PSP categorize testing as failure time and something to be minimized if not avoided. Cleanroom developers are taught that unit testing inteferes with metrics for quality, and that verifications are what should be done. Should such developers redo verifications after each refactoring? There's no answer in this book.

An unusual chapter, called "Bad Smells in Code," gives overall motivation for the refactorings. These vague notions, such as "long methods" or "lazy classes" humorously provide a foundation for starting your own refactorings. I say "humorously" because (mostly) Beck's and Fowler's odd analogies (classes becoming too intimate and delving in each others' private parts) provoke a chuckle (as if a chapter about "bad smells" in code weren't enough).

Overall, I've enjoyed reading this book and referring to the catalog while putting my own unit tests and refactorings into practice. Fowler's writing style is smooth and fluid, and it's easy to digest the catalog in no time. The book's typesetting is crisp, the figures quite clean, and both the refactoring index and "smell" index are enormously useful.

101 of 110 found the following review helpful:

Refactoring: Improving the Design of Existing CodeNov 12, 2002
By Christopher Taylor "aspmatrix"
A little while back I was introduced to a word I had never heard before, Refactoring. I was told to
get Martin Fowler's book and read it so I could gain a better understanding of what Refactoring
was. Well folks, I would classify this book as a 'Hidden Treasure'.

Although it is not a flashy or well known title, I believe its impact can be much deeper and long
lasting than many of the mainstream, more popular technology books. The underlying theories
that it teaches can be applied for years, even when languages change.

There are only a couple of things I would change about this book, which I will mention below.

Preface
The Preface it brief enough, and gives the definition for the word Refactoring. This is a good thing
because right form the start you get the true definition of Refactoring. In short, refactoring is the
process of changing code to improve the internal structure, but not changing the external
behavior.

Chapter 1: Refactoring, a First Example

In this chapter Mr. Fowler tries to start by showing a simple Refactoring example. The problem is
that the chapter then goes on for 50+ pages. Mr. Fowler explains his reasons for doing this, but I
think that a simple example should have been much simpler. Especially when it is in the first
chapter of the book. It's not that this isn't a good chapter. I feel it's just too soon in the book. I
would have put it at the end.

Chapter 2: Principles of Refactoring
This is an excellent chapter. The definition of Refactoring is discussed as well as the following
questions: Why should you refactor? When should you refactor? What do I tell my manager? This
last question may seem funny, but when you read this chapter you will understand why it is in
there. This chapter also discusses common problems that occur during Refactoring, and
Refactoring and performance.

Chapter 3: Bad Smells in Code
In this chapter things that cause code to 'smell' are discussed. When code 'smells' it could be an
indicator that refactoring is needed. 22 different 'smells' are discussed. My favorites were
Duplicated Code, Large Class, and Lazy Class. This is a chapter full of awesome hints.

Chapter 4: Building Tests
Building tests is an important part refactoring. Refactoring is done in small steps, and after every
step you should test. In this chapter the discussion covers the processes and methodology of
applying tests during refactoring.

Chapter 5: Toward a Catalog of Refactorings
This chapter is a quick setup for chapters 6 to 12. Mr. Fowler explains his method for cataloging
the individual refactorings. What is pretty amazing is that he has taken a lot of time naming and
detailing each refactoring.

Chapter 6: Composing Methods
One of my favorite chapters. Mr. Fowler opens by saying, "A large part of my refactoring is
composing methods to package code properly." This chapter is all about that. 9 total refactorings
are explained. My favorite ones are Inline Method and Extract Method.

Chapter 7: Moving Features Between Objects
Sometimes you need to move things from one object to another. This chapter discusses the art of
moving features between objects. 8 total refactorings are discussed and detailed. My favorite
from this chapter is Extract Class.

Chapter 8: Organizing Data
A very large chapter that discusses in meticulous detail 16 refactorings that will make it much
easier to work with data. One thing that becomes very obvious in this chapter is that certain
refactorings can go either way. What I mean is illustrated by these two: Change Value to
Reference and Change Reference to Value. So some refactorings are not just one way deals. It
just depends on the situation.

Chapter 9: Simplifying Conditional Expressions
This is a very useful chapter since conditional logic is a common occurrence in the programming
world. Because conditional logic has a tendency to get very complex, this chapter has 8
refactorings that will help you simplify things.

Chapter 10: Making Method Calls Simpler
The 15 refactorings in this chapter help teach us how to make method calls easier to deal with.
They range from the very simple Rename Method to the more complex Replace Constructor with
Factory Method.

Chapter 11: Dealing with Generalization
Here are 12 refactorings dealing with the situations that arise from generalization. Inheritance,
Delegation, and Interfaces are some of the topics discussed.

Chapter 12: Big Refactorings
Kent Beck co-wrote this chapter with Mr. Fowler. They discuss what they call the 4 Big
Refactorings: Tease Apart Inheritance, Convert Procedural Design to Objects, Separate Domain
from Presentation, and Extract Hierarchy. These refactorings are of a more all-encompassing
type than the smaller individual refactorings from the preceding chapters. The co-authors do a
great job at putting in a nutshell what would normally take very long explanations.

Chapter 13: Refactoring, Reuse, and Reality
William Opdyke writes this chapter. He discusses his experiences with refactoring as well as
other subjects like why developers are reluctant to refactor and reducing the overhead of
refactoring. This chapter is an excellent 'putting it all together' chapter, and really helps put into
perspective the ideas that the book teaches.

Chapter 14: Refactoring Tools
Don Roberts and John Brant co-author this chapter. They discuss, as the chapter title would
indicate, refactoring tools.

Chapter 15: Putting It All Together
Kent Beck gives a quick 4-page wrap up.

One other thing I would change about the book is that I would want there to be examples in other
languages besides Java. I have practically no Java skills. For me the book would have been an
easier and faster read if it would have had examples in VB.net. Fortunately I understand enough
to get the idea of what is being taught, and that is the most important point.

Well as I said above, this book is really what I would consider a 'hidden treasure'. The things
discussed will help many people write better, more understandable code for years to come. I
would give it a 9.5 out of 10. It is well worth the {price}

24 of 24 found the following review helpful:

Making tired old code betterMay 17, 2000
By Jamie Oglethorpe
The basic thesis of this book is that, for various reasons, real programs are poorly designed. They get that way for a variety of reasons. Initially well designed, extending the program may lead to software decay. Huge methods may result from unanticipated complexity. Refactoring, according to Fowler, is a function preserving transformation of a program. The transformations are reversible, so the intention is to improve the program in some way.

Fowler suggests refactoring a program to simplify the addition of new functionality. The program should also be refactored to make it easier for human readers to understand at the same time.

He also insists that each step is small and preserves functionality, and on frequent unit testing with a comprehensive test suite.

Half of the book consists of a catalogue of refactorings. He gives each refactoring a memorable name, such as "Replace Type Code with Subclasses". He illustrates the design transformation with a pair of UML class diagrams, and has a standard set of sections: Motivation, Mechanics and Example.

The Motivation is a prose section that describes and justifies the refactoring, showing the relationship to other refactorings.

The Mechanics is a sequence of steps needed to carry out the refactoring, shown as a list of bullet points He expands on some points.

The Example is where the value of this book lies. Fowler takes a fragment of Java code, and takes us step by step through the refactoring. The code is small enough that he can show it all each step of the way without overwhelming us, but is large enough to be realistic.

The code is clear enough for non-Java programmers to follow. He explains his code well enough for the book to function as a Java tutorial where the meaning of the code is not obvious. One or two of the refactorings are specific to the Java object model, and do not apply to other languages. Other languages would benefit from similar treatment, but there are very few language-specific refactorings.

The book is very much of the Design Patterns movement, with frequent references to patterns. The aim of a factoring may be to achieve a particular pattern, or it may take advantage of a particular pattern. The book can be used as a tutorial on Design Patterns.

I have a small number of complaints. Fowler advocates the use of refactoring while studying code for a code review. One needs to be very sensitive to the feelings of the programmer here, especially if he or she is a novice. The reviewer should read the code with refactoring in mind, and possible refactorings recommended, but it is for the programmer to make the changes.

Reading this book has inspired me to refactor some of my own code. My mistakes underlined the need to take small steps, and to test frequently. I spent a day building a useful Delphi testing framework from the description Fowler gives of the JUnit testing framework. The one category of code that does not seem to lend itself to this approach is some highly coupled parsing code. While I can extract small blocks of code, they remain tightly coupled with each other, and it is hard to give them meaningful names. The answer here may be to use the top down approach of recursive descent, rather than the bottom up approach of refactoring. Perhaps recursive descent can guide refactoring. Refactoring is largely a local approach. One can almost say a pinhole approach. Sometimes a global view is needed.

In summary, I would say that this very good book would be of use to Java programmers who have some understanding and much bafflement. It is very good for us older dogs who have become a little jaded and need some new ideas and motivation.

38 of 41 found the following review helpful:

Don't just read it - buy itJul 18, 2000
By Daniel Moth
One can read good books on a specific technology (COM, UML etc) or on specific programming languages or even on different approaches to software development (RUP, OPEN etc) but every now and then a true classic comes along. Like Design Patterns 4 years ago now refactoring comes along. Every serious OO developer should own both of these books. Get your hands on Refactoring if only to read chapter 3, which summarises all the 'bad smells' that may creep into code. 21 generic examples of what is bad programming and why. The remainder of the book describes numerous techniques (refactorings) for changing existing code in order to remove the 'smell'. Most refactorings are accompanied with some UML, which should be enough to get the idea, and they are then further described in Java. What makes this great a book is that it can be used as a reference very easily since its design was well thought out for this purpose with a comprehensive index and tables matching smells and respective refactorings. If any of this rings a bell to CODE COMPLETE readers it should cause the ideas are very similar but very much updated here. Fowler's writing style makes once again for easy, pleasant reading. Unreservedly recommended.

19 of 19 found the following review helpful:

Don't wait as long as I didMay 13, 2003
By David C. Veeneman
I've known about this book for over a year. Initially, I thought it was about re-engineering legacy systems. I don't do that, so I didn't give it much thought. Over the past year, I have stumbled across repeated references to this book. Everyone seems to cite it, and now I understand why.

It's very easy to fall into 'analysis paralysis' when doing object design. A commonly heard complaint is "I have created 27 different class diagrams, and 42 separate sequence diagrams, but I can't seem to get any code written..." XP's popularity is due, in part, becuase it get's you into action--you begin writing code immediately, instead of creating diagrams for weeks (or months) on end. XP's motto could be "just do it!"

But how does one reconcile this "code first, ask questions later" mentality with an acknowledged need to at least do some design work? In "Refactoring", Martin Fowler provides the answer. His prescription is to create some code to get something working, then look at the code to see how it might be improved-- refactor it.

In Fowler's view, you won't really understand the problem until you have coded it, so instead of spending the next three weeks trying to find the perfect pattern for your next task, forget the pattern, and get some code going. Once you've got something workable, then think about patterns you might back into from your existing code.

Of course, that's a gross oversimplification of the process, but it gives a flavor of the ready-fire-aim process that 'Refactoring' is built around. And it seems to work--even people who don't buy into other core practices of XP seem to have adopted refactoring as a central element in their process.

The catalog of refactorings that the books provides are a first class reference on how to clean up particular problems. But to me, the most valuable part of the book is its first fifty pages.

Fowler starts the book with a simple, but ugly, example, that he proceeds to refactor, step-by-step, into something rather elegant. If you like to learn principles first, you might want to read the second chapter before going through the example, but I found it a very valuable exercise. I recommend coding the example in your language of choice, then refactoring along with Fowler as you work through the example.

There is a temptation to relegate refactoring, like testing, to simply another development technique. But like testing, refactoring is at the core of a development philosophy: "I know I'm not going to get it right on my first pass, so I'll be satisfied with making it as right as I can. Having done that, I'll have a much better idea how to make it better, and I will. But time's a' wasting, so I need to get moving."

This philosophy of continuous improvement allows the developer to get into action fairly quickly, and it reduces the risk of failure-by-delay. Fowler's book is a top-notch resource that will help the developer create more flexible code more quickly. I can recommend 'Refactoring' without reservation.

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