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Appendix The Use of Patterns in This Book
My first "nice car," which I was given shortly after college, was an eight-year-old Peugeot. Sometimes called the "French Mercedes," this car was well crafted, was a pleasure to drive, and had been very reliable. But by the time I got it, it was reaching the age when things start to go wrong and more maintenance is required.
Peugeot is an old company, and it has followed its own evolutionary path over many decades. It has its own mechanical terminology, and its designs are idiosyncratic; even the breakdown of functions into parts is sometimes nonstandard. The result is a car that only Peugeot specialists can work on, a potential problem for someone on a grad student income.
On one typical occasion, I took the car to a local mechanic to investigate a fluid leak. He examined the undercarriage and told me that oil was "leaking from a little box about two-thirds of the way back that seems to have something to do with distributing braking power between front and rear." He then refused to touch the car and advised me to go to the dealership, fifty miles away. Anyone can work on a Ford or a Honda; that's why those cars are more convenient and less expensive to own, even though they are equally mechanically complex.
I did love that car, but I will never own a quirky car again. A day came when a particularly expensive problem was diagnosed, and I had had enough of Peugeots. I took it to a local charity that accepted cars as donations. Then I bought a beat-up old Honda Civic for about what the repair would have cost.
Standard design elements are lacking for domain development, and so every domain model and corresponding implementation is quirky and hard to understand. Moreover, every team has to reinvent the wheel (or the gear, or the windshield wiper). In the world of object-oriented design, everything is an object, a reference, or a message—which, of course, is a useful abstraction. But that does not sufficiently constrain the range of domain design choices and does not support an economical discussion of a domain model.
To stop with "Everything is an object" would be like a carpenter or an architect summing up houses by saying "Everything is a room." There would be the big room with high-voltage outlets and a sink, where you might cook. There would be the small room upstairs, where you might sleep. It would take pages to describe an ordinary house. People who build or use houses realize that rooms follow patterns, pattern ............

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