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Your attention please!

The following text was published as a book by Prentice Hall in 1994.

It has since gone out of print, and the copyright was formally reversed

back to the authors Kim Waldén and Jean-Marc Nerson in 2001.

We have decided to make the text publicly available with the following provision.

You may download and copy the text at will, but as soon as some part is extracted

and presented out of context of the complete book, there must always be a clearly

visible reference to the book and its authors, and also a pointer to the website

www.bon-method.com (since the printed book is no longer available) so that any

reader of the partial content can quickly find the original full text. The text of this

provision must also be included in any such presentation.

Seamless

Object-Oriented

Software

Architecture

Analysis and Design of Reliable Systems

Kim Waldén

Jean-Marc Nerson

Printed version

September 6, 1994

Contents

Series editor’s preface xi

Preface xiii

Part I Introduction 1

1 Object-oriented software development 3

1.1 Introduction 3

1.2 Seamlessness 7

1.3 Reversibility 7

1.4 Software contracting 9

2 The BON approach 11

2.1 Introduction 11

2.2 What is not in BON 12

2.3 Other methods 15

2.4 The BON position 16

2.5 Characteristics of the notation 17

2.6 Static and dynamic models 24

Part II The model 27

3 The static model—classes and clusters 29

3.1 Introduction 29

3.2 BON modeling charts 30

3.3 System state information 35

3.4 Typed class interfaces 36

3.5 Class header 37

3.6 Indexing clause 37

v

vi CONTENTS

3.7 Inheritance clause 39

3.8 Class features 39

3.9 Class invariant 47

3.10 Graphical and textual specification 48

3.11 The BON assertion language 49

3.12 The BON predicate logic 52

3.13 Assertions and partial functions 57

3.14 Scalable system description 59

3.15 Class headers 59

3.16 Clusters 60

4 Static relations 65

4.1 Inheritance relations 65

4.2 Client relations 69

4.3 Class vs. object dependencies 86

4.4 Indirect client dependencies 88

4.5 Semantic links 89

5 The dynamic model 90

5.1 Introduction 90

5.2 Dynamic charts 93

5.3 Dynamic diagrams 99

Part III The method 117

6 Issues of a general method 119

6.1 Introduction 119

6.2 Simulating an ideal process 120

6.3 Enterprise modeling 121

6.4 What is analysis and what is design? 122

6.5 Reuse 125

6.6 User centered design 129

6.7 Roles of analysis objects 135

6.8 Modeling a system interface 136

6.9 System usage 140

6.10 A paradigm shift 142

CONTENTS vii

7 The BON process 143

7.1 The BON deliverables 143

7.2 Characteristics of the process 148

7.3 Task 1: Delineate system borderline 152

7.4 Task 2: List candidate classes 160

7.5 Task 3: Select classes and group into clusters 162

7.6 Task 4: Define classes 166

7.7 Task 5: Sketch system behaviors 168

7.8 Task 6: Define public features 171

7.9 Task 7: Refine system 172

7.10 Task 8: Generalize 174

7.11 Task 9: Complete and review system 175

8 BON standard activities 178

8.1 Finding classes 178

8.2 Classifying 189

8.3 Clustering 194

8.4 Defining class features 198

8.5 Selecting and describing object scenarios 203

8.6 Working out contracting conditions 205

8.7 Assessing reuse 212

8.8 Indexing and documenting 215

8.9 Evolving the system architecture 220

Part IV Case studies 229

9 A conference management system 231

9.1 System borderline 232

9.2 Candidate classes in the problem domain 235

9.3 Class selection and clustering 236

9.4 Class definition 242

9.5 System behavior 252

9.6 Formal class description 260

10 A video recorder 270

10.1 System borderline 270

viii CONTENTS

10.2 Candidate classes 274

10.3 Class selection and clustering 275

10.4 Class definition 279

10.5 System behavior 283

10.6 Formal class description 286

10.7 Final static architecture 288

11 Relational and object-oriented coexistence 289

11.1 From data storage to object persistence 290

11.2 Object models and relational models 293

11.3 A relational database wrapper 295

11.4 Interfacing an existing relational schema 303

11.5 Querying a persistent object model 312

11.6 Persistent object management 315

11.7 Automatic generation of SQL statements 324

11.8 Full static architecture 331

12 Exercises 332

12.1 Clustering a problem domain 332

12.2 Defining class relationships 334

12.3 Assertions and classification 335

12.4 Dynamic behavior 336

12.5 Prescription and description 337

12.6 Traffic-control system 338

12.7 Dice game 339

12.8 Car rental company 340

12.9 Truck freight 341

12.10 Real-time process control 345

Part V Appendices 347

Appendix A BON textual grammar 349

A.1 Introduction 349

A.2 The syntax notation 350

A.3 BON specification 352

A.4 Informal charts 352

CONTENTS ix

A.5 Static diagrams 354

A.6 Class interface description 355

A.7 Formal assertions 357

A.8 Dynamic diagrams 358

A.9 Notational tuning 359

A.10 Lexical components 359

Appendix B BON textual examples 364

B.1 BON specification 364

B.2 Informal charts 365

B.3 Static diagrams 367

B.4 Dynamic diagrams 374

Appendix C BON quick reference 381

Appendix D Other approaches 386

Appendix E Glossary of terms 388

References 411

Index 425

x CONTENTS

Series editor’s preface

A rumor has been spreading for some time among people that follow progress in

object-oriented analysis and design: “Wait for BON!” Those not in the know

would ask what in the world BON could be. Indeed, the publicity around the

Business Object Notation has been modest—an article in the Communications of

the ACM, presentations at a workshop or two, public seminars in Europe and

North America, tutorials at TOOLS and other conferences—but it was enough to

attract the attention of many O-O enthusiasts who were dissatisfied with the

limitations of first-generation analysis methods. In the meantime, BON was

being taught to many practitioners, applied in numerous industrial projects, and

repeatedly polished as a result.

As this book finally reaches publication it is certain to cause a major advance

in the field of object-oriented methods. Its most remarkable feature is the

thoroughness with which it applies object-oriented principles, unencumbered by

leftovers from earlier methods. Going O-O all the way is not a matter of

dogmatism, but the secret for obtaining the real benefits of the method, following

in particular from two principles developed at length in the book: seamless

development, the removal of artificial gaps and mismatches between successive

software development activities; and reversibility, the recognition that at any step

of the development process, including implementation and maintenance, it must

be possible to update the results of earlier phases such as analysis and design,

and still maintain full consistency between the analysis, design, implementation

and maintenance views. By ensuring seamlessness and reversibility it is possible

to obtain a continuous software development process, essential to the quality of

the resulting products.

This book is also one of a select few in the OOAD literature that pays serious

attention to the question of software reliability, by using some elements of

formal reasoning, in particular assertions, as a way to specify semantic properties

of a system at the earliest possible stage.

Following the presentation of the model and method in parts I, II, and III, a

large section of the book (part IV) is devoted to a set of in-depth case studies and

to exercises, drawn for the most part from projects in which the authors acted as

xi

xii SERIES EDITOR’S PREFACE

consultants. This abundant practical material will help readers apply the ideas of

BON to their own application areas.

From now on, no one will be able to claim knowledge of object-oriented

analysis and design who has not read Kim Waldén and Jean-Marc Nerson.

Bertrand Meyer

Preface

In the past few years, object-oriented techniques have finally made the passage

from the programming-in-the-small island to the mainland of programming-in￾the-large. Accompanying this transition has been a change in the role and

perception of software methods: in addition to their well-established use in the

earliest stages of a project—requirements analysis and system specification—

they are increasingly viewed as providing the intellectual support needed across

the entire software construction process, through design and implementation to

maintenance and reengineering. The object-oriented approach is best suited to

achieve this seamlessness of the software development process, without which it

would not be possible to meet the quality and productivity challenges that

confront the software industry.

This book shows how a consistent set of object-oriented abstractions can be

applied throughout the process, based on three major ideas: seamlessness,

reversibility, and contracting.

Seamlessness, as in the first word of the title, follows from the observation

that the similarities between the tasks to be carried out at the various steps of a

project far outweigh their inevitable differences, making it possible to obtain a

continuous process that facilitates communication between the various actors

involved, ensures a direct mapping between a problem and its software solution,

and results in a high level of quality for the final product.

Reversibility means that the seamless procedure must work in both directions:

if one modifies a system that has already reached the implementation phase—a

frequent case in practice—it must be possible to reflect the modification back to

the higher levels of design, specification, and analysis. Without such

reversibility the products of these earlier stages would soon become obsolete,

raising disturbing questions about their very role in the software process. Since

current object-oriented methods are still dominated by hybrid approaches—that

is to say, encumber the application of object-oriented principles with techniques

drawn from non-object-oriented analysis methods and with constructs drawn

from non-object-oriented languages—reversibility has so far been almost absent

from the concerns of the object-oriented literature.

xiii

xiv PREFACE

The contract model was introduced to a wider audience as early as 1988 by

Bertrand Meyer in his magnificent introductory book Object-Oriented Software

Construction (OOSC), which quickly became, and still is, the standard reference

on basic object-oriented concepts. In a sense, the present book is a continuation

of OOSC, carrying some of its software engineering ideas to their logical

conclusion in the area of analysis and design. The result is a method called BON

(Business Object Notation) which contains a set of concepts and corresponding

notations to support object-oriented modeling centered around the three

principles of seamlessness, reversibility, and software contracting.

In the rapidly growing field of object-oriented software development, many

subfields have now accumulated enough experience and techniques to warrant

books of their own. When presenting our ideas, we therefore had to make a

choice: either to cover most of the interesting areas and remain shallow, or to

limit the scope and leave room for more substance. We chose the latter, and

BON concentrates on the basic application-independent ideas of general analysis

and design of software systems.

We have also refrained from including yet another explanation of the basic

object-oriented concepts. There are two main reasons for this. First, the

concepts may be simple enough to define, but understanding their implications in

a deeper sense takes much longer. Therefore, a short overview will not be

enough for those who do not already understand the concepts, while a more

substantial discussion will add significant volume and be utterly boring to

experienced readers. Second, in the general spirit of this book, we believe good

texts should be reused rather than rewritten each time they are needed.

So we will assume that the meaning of classes, instances (objects),

polymorphism, dynamic binding, etc., is already familiar to the reader. If not,

we recommend the OOSC book cited above (a significantly revised second

edition is to appear during 1994). As a small compensation for a basic overview,

we have included a reasonably extensive glossary of terms with brief

explanations of the most important BON concepts and many of the familiar

object-oriented terms. It can be used by the novice as a starting point for further

reading, and by the knowledgeable reader to find out more precisely what flavor

we have chosen for widely used terms whose meaning is not fixed in the object￾oriented literature.

BON was initiated in 1989 by Jean-Marc Nerson, then chief developer of the

ISE Eiffel 2.2 environment, who presented early ideas in a tutorial at the second

TOOLS conference held in Paris in 1990. The ideas were picked up at Enea

Data in Sweden by Kim Waldén, then technically responsible for the company’s

Eiffel distribution in Scandinavia, who started to teach courses on BON in

Sweden shortly after. This was the beginning of a collaboration which gradually

led to a joint development of the notation and method. The BON technique was

PREFACE xv

applied in several industrial developments, and Jean-Marc published three

articles in 1991−92. However, we soon realized that more substantial

documentation on the subject would be necessary to make the method available

to a wider audience, and in 1993 we made the bold decision to jointly publish a

book on BON (bold, because the only time available for this work was evenings

and weekends).

As is often the case with visions whose fulfillment requires far more resources

than are available, it is best not to understand the full extent of the work

beforehand—then you can just go ahead and do it anyway, which is what

happened with this book although at the expense of our friends and families.

However, we found that writing about something you believe in wholeheartedly

has the mysterious capacity of extending the number of daily hours well beyond

the standard 24 (in itself a possible subject of another book). If we succeed in

communicating to our readers just a small fraction of the joy involved in writing

this book, the effort will have been worthwhile.

Scope of the book

The book is intended for software professionals as well as for students at the

graduate and undergraduate levels. We believe it can be read by anyone who has

acquired a general understanding of the problems of software engineering, and

who has some inclination for abstract thinking.

The knowledgeable software engineer used to dealing with practical solutions

may discover that it is not all that easy to keep analysis and design models free

from premature implementation decisions. On the other hand, to achieve a deep

understanding of the technology, it is probably even more important for the high￾level analyst to occasionally take an object-oriented design (not necessarily

large) all the way through implementation. Never to do this is somewhat like

trying to become a mountaineer without ever climbing: there is little replacement

for watching polymorphism and dynamic binding in live action.

Book structure

The book consists of an introduction, three main parts, and five appendices. The

main parts treat in order: the concepts and notations of BON; the BON process

for producing analysis and design models; and a practical part with three case

studies and exercises.

The introduction (chapters 1−2) discusses the general principles which have

guided the development of BON and positions the method relative to other

approaches.

The model part (chapters 3−5) explains the static and dynamic models of BON

and the corresponding notation. Untyped modeling charts are used for the very