I''ve been lucky in a lot of ways in my life; one of my great strokes of fortune was being in the right place with the right knowledge to write the first edition of this book in 1997. Back then, the chaotic world of object-oriented (OO) modeling was just beginning to unify under the Unified Modeling Language (UML). Since then, the UML has become the standard for the graphical modeling of software, not just for objects. My fortune is that this book has been the most popular book on the UML, selling more than a quarter of a million copies.Well, that''s very nice for me, but should you buy this book? I like to stress that this is a brief book. It''s not intended to give you the details on every facet of the UML, which has grown and grown over the years. My intention is to find that fraction of the UML that is most useful and tell you just that. Although a bigger book gives you more detail, it also takes longer to read.

And your time is the biggest investment you''ll make in a book. By keeping this book small, I''ve spent the time selecting the best bits to save you from having to do that selection yourself. (Sadly, being smaller doesn''t mean proportionately cheaper; there is a certain fixed cost to producing a quality technical book.) One reason to have this book is to begin to learn about the UML. Because this is a short book, it will quickly get you up to speed on the essentials of the UML. With that under your belt, you can go into more detail on the UML with the bigger books, such as the User Guide Booch, UML user or the Reference Manual Rumbaugh, UML Reference. This book can also act as a handy reference to the most common parts of the UML. Although the book doesn''t cover everything, it''s a lot lighter to carry around than most other UML books.

It''s also an opinionated book. I''ve been working with objects for a long time now, and I have definite ideas about what works and what doesn''t. Any book reflects the opinions of the author, and I don''t try to hide mine. So if you''re looking for something that has a flavor of objectivity, you might want to try something else. Although many people have told me that this book is a good introduction to objects, I didn''t write it with that in mind. If you are after an introduction to OO design, I suggest Craig Larman''s book Larman. Many people who are interested in the UML are using tools. This book concentrates on the standard and on conventional usage of the UML and doesn''t get into the details of what various tools support.

Although the UML did resolve the tower of Babel of pre-UML notations, many annoying differences remain between what tools show and allow when drawing UML diagrams. I don''t say much in this book about Model Driven Architecture (MDA). Although many people consider the two to be the same thing, many developers use the UML without being interested in MDA. If you want to learn more about MDA, I would start with this book to get an overview of the UML first and then move on to a book that''s more specific about MDA. Although the main point of this book is the UML, I''ve also added bits of other material about techniques, such as CRC cards, that are valuable for OO design. The UML is just a part of what you need to succeed with objects, and I think that it''s important to introduce you to some other techniques. In a brief book like this, it''s impossible to go into detail about how the UML relates to source code, particularly as there is no standard way of making that correspondence. However, I do point out common coding techniques for implementing pieces of the UML.

My code examples are in Java and C#, as I''ve found that these languages are usually the most widely understood. Don''t assume that I prefer those languages; I''ve done too much Smalltalk for that! Why Bother with the UML? Graphical design notations have been with us for a while. For me, their primary value is in communication and understanding. A good diagram can often help communicate ideas about a design, particularly when you want to avoid a lot of details. Diagrams can also help you understand either a software system or a business process. As part of a team trying to figure out something, diagrams both help understanding and communicate that understanding throughout a team. Although they aren''t, at least yet, a replacement for textual programming languages, they are a helpful assistant. Many people believe that in the future, graphical techniques will play a dominant role in software development.

I''m more skeptical of that, but it''s certainly useful to have an appreciation of what these notations can and can''t do. Of these graphical notations, the UML''s importance comes from its wide use and standardization within the OO development community. The UML has become not only the dominant graphical notation within the OO world but also a popular technique in non-OO circles. Structure of the Book Chapter 1 gives an introduction to the UML: what it is, the different meanings it has to different people, and where it came from. Chapter 2 talks about software process. Although this is strictly independent of the UML, I think that it''s essential to understand process in order to see the context of something like the UML. In particular, it''s important to understand the role of iterative development, which has been the underlying approach to process for most of the OO community. I''ve organized the rest of the book around the diagram types within the UML.

Chapters 3 and 4 discuss the two most useful parts of the UML: class diagrams (core) and sequence diagrams. Even though this book is slim, I believe that you can get the most value out of the UML by using the techniques that I talk about in these chapters. The UML is a large and growing beast, but you don''t need all of it. Chapter 5 goes into detail on the less essential but still useful parts of class diagrams. Chapters 6 through 8 describe three useful diagrams that shed further light on the structure of a system: object diagrams, package diagrams, and deployment diagrams. Chapters 9 through 11 show three further useful behavioral techniques: use cases, state diagrams (although officially known as state machine diagrams, they are generally called state diagrams), and activity diagrams. Chapters 12 through 17 are very brief and cover diagrams that are generally less important, so for these, I''ve only provided a quick example and explanation. The inside covers summarize the most useful parts of the notation.

I''ve often heard people say that these covers are the most valuable part of the book. You''ll probably find it handy to refer to them as you''re reading some of the other parts of the book. Changes for the Third Edition If you have earlier editions of this book, you''re probably wondering what is different and, more important, whether you should buy the new edition. The primary trigger for the third edition was the appearance of UML 2. UML 2 has added a lot of new stuff, including several new diagram types. Even familiar diagrams have a lot of new notation, such as interaction frames in sequence diagrams. If you want to be aware of what''s happened but don''t want to wade through the specification (I certainly don''t recommend that!), this book should give you a good overview. I''ve also taken this opportunity to completely rewrite most of the book, bringing the text and examples up to date.

I''ve incorporated much that I''ve learned in teaching and using the UML over the past five years. So although the spirit of this ultrathin UML book is intact, most of the words are new.Over the years, I''ve worked hard to keep this book as current as is possible. As the UML has gone through its changes, I''ve done my best to keep pace. This book is based on the UML 2 drafts that were accepted by the relevant committee in June 2003. It''s unlikely that further changes will occur between that vote and more formal votes, so I feel that UML 2 is now stable enough for my revision to go into print. I''ll post information any further updates on my Web site (http://martinfowler.com).

Martin Fowler Melrose, Massachusetts http://martinfowler.com.