From Artificial Intelligence to Patterns to True Object Orientation

My journey into design patterns had a different starting point than Alan's but we have reached the same conclusions:

• Pattern-based analyses make you a more effective and efficient analyst because they let you deal with your models more abstractly and because they represent the collected experiences of many other analysts.

• Patterns help people to learn principles of object orientation. The patterns help to explain why we do what we do with objects.

I started my career in artificial intelligence (AI) creating rule-based expert systems. This involves listening to experts and creating models of their decision-making processes and then coding these models into rules in a knowledge-based system. As I built these systems, I began to see repeating themes: in common types of problems, experts tended to work in similar ways. For example, experts who diagnose problems with equipment tend to look for simple, quick fixes first, then they get more systematic, breaking the problem into component parts; but in their systematic diagnosis, they tend to try first inexpensive tests or tests that will eliminate broad classes of problems before other kinds of tests. This was true whether we were diagnosing problems in a computer or a piece of oil field equipment.

Today, I would call these recurring themes patterns. Intuitively, I began to look for these recurring themes as I was designing new expert systems. My mind was open and friendly to the idea of patterns, even though I did not know what they were.

Then, in 1994, I discovered that researchers in Europe had codified these patterns of expert behavior and put them into a package that they called Knowledge Analysis and Design Support, or KADS. Dr. Karen Gardner, a most gifted analyst, modeler, mentor, and human being, began to apply KADS to her work in the United States. She extended the European's work to apply KADS to object-oriented systems. She opened my eyes to an entire world of pattern-based analysis and design that was forming in the software world, in large part due to Christopher Alexander's work. Her book, Cognitive Patterns (Cambridge University Press, 1998) describes this work.

Suddenly, I had a structure for modeling expert behaviors without getting trapped by the complexities and exceptions too early. I was able to complete my next three projects in less time, with less rework, and with greater satisfaction by end-users, because:

• I could design models more quickly because the patterns predicted for me what ought to be there. They told me what the essential objects were and what to pay special attention to.

• I was able to communicate much more effectively with experts because we had a more structured way to deal with the details and exceptions.

• The patterns allowed me to develop better end-user training for my system because the patterns predicted the most important features of the system.

This last point is significant. Patterns help end-users understand systems because they provide the context for the system, why we are doing things in a certain way. We can use patterns to describe the guiding principles and strategies of the system. And we can use patterns to develop the best examples to help end-users understand the system.

I was hooked.

So, when a design patterns study group started at my place of employment, I was eager to go. This is where I met Alan who had reached a similar point in his work as an object-oriented designer and mentor. The result is this book.

I hope that the principles in this book help you in your own journey to become a more effective and efficient analyst.

James R. Trott
December, 2000