However, before making this quantum leap, object-oriented methods still have to prove themselves with respect to more established software development paradigms. True, for small tasks the war is over. Object-oriented programs are more compact than classic structured programs. It is easier to whip them together using powerful class libraries. Inheritance allows ``differential programming'', the modification in a descendant class of what is wrong with a parent class, while inheriting all of its good stuff. User interfaces, which are often sizable fractions of small systems, can be put together easily from object-oriented libraries.
Delivering large object-oriented software systems routinely and cost effectively is still a significant challenge. To quote Ed Yourdon: ``A system composed of 100,000 lines of C++ is not to be sneezed at, but we don't have that much trouble developing 100,000 lines of COBOL today. The real test of OOP will come when systems of 1 to 10 million lines of code are developed.1''.
To be fair and accurate, systems of 100,000 lines of C++ and those of 1,000,000 lines of COBOL are often of the same order of magnitude in complexity.
The development of large systems is qualitatively different from that of small systems. For instance, a multinational banking conglomerate may want a system supporting around-the-clock access to the major stock markets in the world. They may additionally want to integrate accounts for all worldwide customers, providing fault-tolerant distributed transaction services. The banking conglomerate cannot realize this system by relying exclusively on a bundle of smart programmers. Instead, as enshrined by the structured paradigm, analysis and design must precede pure implementation activities. OO methods are known by experience to scale up to such large systems. For example, Hazeltine  reports a project with ``about 1000 classes, 10 methods per class, involving an average of 40 persons over 2 years.''
This book is intended to help the reader better understand the role of analysis and design in the object-oriented software development process. Experiments to use structured analysis and design as precursors to an object-oriented implementation have failed. The descriptions produced by the structured methods partition reality along the wrong dimensions. Classes are not recognized and inheritance as an abstraction mechanism is not exploited. However, we are fortunate that a multitude of object-oriented analysis and design methods have emerged and are still under development. Core OO notions have found their home place in the analysis phase. Abstraction and specialization via inheritance, originally advertised as key ingredients of OO programming, have been abstracted into key ingredients of OO analysis (OOA). Analysis-level property inheritance maps smoothly on the behavior inheritance of the programming realm.
A common selling point of the OO paradigm is that it is more ``natural'' to traverse from analysis to implementation. For example, as described in , developers at Hewlett-Packard who were well versed in the structured paradigm reported that the ``conceptual distances'' between the phases of their project were smaller using OO methods. Classes identified in the analysis phase carried over into the implementation. They observed as well that the defect density in their C++ code was only 50% of that of their C code.
However, more precise characterizations of why this might be so and how best to exploit it are still underdeveloped. The black art mystique of OO methods is a major inhibitor to the widespread acceptance of the OO paradigm. Hence we have devoted ample attention to the process aspects of development methods.
Will this book be the last word on this topic? We hope not. The object-oriented paradigm is still developing. At the same time, new challenges arise. Class libraries will have to be managed. Libraries for classic computer science concepts can be traversed using our shared, common knowledge. But access to application specific domain libraries seems to be tougher. Deciding that a particular entry is adequate for a particular task without having to ``look inside'' the entry is a challenge. Will the man-page style of annotations be sufficient? Our experience with man-pages makes us doubt that it will be.
Further horizons in OO are still too poorly understood to be exploited in the construction of reliable systems. For example, some day methods may exist for routinely developing ``open systems'' of ``smart'' active objects inspired by the pioneering work of Hewitt.2 While active object models do indeed form much of the foundation of this book, their furthest-reaching aspects currently remain the focus of research and experimental study.
Some quotes from this manifesto paper : ``This paper proposes a modular ACTOR architecture and definitional method for AI that is conceptually based on a single kind of object: actors... The formalism makes no presuppositions about the representation of primitive data structures and control structures. Such structures can be programmed, micro-coded or hard-wired in a uniform modular fashion. In fact it is impossible to determine whether a given object is `really' represented as a list, a vector, a hash table, a function or a process. Our formalism shows how all of the modes of behavior can be defined in terms of one kind of behavior: sending messages to actors.'' We briefly discuss open systems of actors in Chapter 22.
The object oriented paradigm, and this book, may impact different software professionals in different ways:
A team-based development project is almost mandatory for effective learning in semester courses, but raises logistics problems common to any software engineering course. Projects should neither be so big that they cannot be at least partially implemented by a team of about three students within the confines of a semester, nor be so small that they evade all systems-level development issues. Successful projects have included a version of ftp with an InterViews based interface, and a simple event display system for Mach processes. To implement projects, students who do not know an OO programming language will need to learn one long before the end of the course. One way to address this is to teach the basics of OOP in a particular language and other implementation pragmatics early on, independently of and in parallel with the topics covered in this book. For example, in a course meeting two or three times per week, one class per week could be devoted to programming and pragmatics. As the semester continues, this class could focus on project status reports and related discussions. This organization remains effective despite the fact that the contents of the different classes are often out of synch. By simplifying or eliminating project options corresponding to the contents of Chapters 22 through 26, final implementation may begin while still discussing how these issues apply to larger efforts.
This text does not aim at defining yet another OO ``method''. Instead, we aim to give a minimum set of notions and to show how to use these notions when progressing from a set of requirements to an implementation. We reluctantly adopt our own minimal (graphical) analysis and (textual) design notations to illustrate basic concepts. Our analysis notation ( OAN) and design language ( ODL) are ``lightweight'' presentation vehicles chosen to be readily translatable into any OO analysis, design, and programming languages and notations you wish to use. (Notational summaries may be found in the Appendix 27.)
Most of what we have written in this book is not true. It is also not false. This is because we are in the prescriptive business. Software development is a special kind of process. We describe in this book a (loosely defined) algorithm for producing a system. Algorithms are not true or false. They are appropriate for a task or not. Thus this book should contain a correctness proof that demonstrates that the application of its methods invariably yields a desired system. However, due to space limitations, we have omitted this proof.
More seriously, we have tried to decrease the fuzziness that is inherent in prescriptive text by giving precise textual descriptions of the key notions in the respective methods. We have tried to be precise as well, in describing how analysis output carries over into design and how the design output gets massaged into an implementation. Thus we obtain checks and balances by integrating the methods across the development phases. This, of course, provides only a partial check on the correctness of the methods. Their application will be their touchstone.
We are opinionated regarding formal techniques. We want to offer software developers ``formality a la carte''. Developers may want to avoid rigorous ``mathematical'' precision as one extreme, or may want to provide correctness proofs of a target system against the requirements as another extreme. We leave this decision to the developer. As a consequence for us, we avoid introducing notions for which the semantics are not crystal clear. As a result, the developer can be as formal as desired. Since we were at times unable to come up with concise semantics, we have omitted some modeling and design notions that are offered in other accounts.
While we have tried to provide a solid foundation for the core concepts, we are convinced as well that a true formalist can still point out uncountably many ambiguities. On the other side, we simply do not understand most of the material in this book sufficiently well to trivialize it into recipes.
Although one can read this book while bypassing the exercises, we do recommend them. Some exercises ask you to operationalize the concepts in this book. Others are quick ``thought questions'', sometimes even silly sounding ones, that may lead you into territory that we have not explored.
Together, we thank our editors Alan Apt and John Wait, and reviewers Jim Coplien, Lew Creary, Desmond D'Souza, Felix Frayman, Watts Humphrey, Ralph Johnson, and Hermann Kaindl.
DdC thanks Alan Apt for being a persistent initiator to this enterprise. He has an amazing ability to exploit your vanity and lure you in an activity that is quite hazardous to plain family life. On the way he gives encouragement, blissfully ignoring the perilous situation in which the writer has maneuvered him/herself. Donna Ho provided an initial sanity check. Patricia Collins expressed very precisely her concerns about domain analysis.
DL thanks others providing comments and advice about initial versions, including Umesh Bellur, Gary Craig, Rameen Mohammadi, Rajendra Raj, Kevin Shank, Sumana Srinavasan, and Al Villarica. And, of course, Kathy, Keith, and Colin.
We remain perfectly happy to take blame for all remaining omissions, stupidities, inaccuracies, paradoxes, falsehoods, fuzziness, and other good qualities. We welcome constructive as well as destructive critiques. Electronic mail may be sent to email@example.com.
Next: Chapter 1