Jim Danley
Fri Jul 20 17:53:57 EDT 2001

Once upon a time, in a kingdom not far from here, a king summoned two
of his advisors for a test. He showed them both a shiny metal box with
two slots in the top, a control knob, and a lever. "What do you think
this is?"

One advisor, an engineer, answered first. "It is a toaster," he
said. The king asked, "How would you design an embedded computer for
it?" The engineer replied, "Using a four-bit microcontroller, I would
write a simple program that reads the darkness knob and quantizes its
position to one of 16 shades of darkness, from snow white to coal black.
The program would use that darkness level as the index to a 16-element
table of initial timer values. Then it would turn on the heating elements
and start the timer with the initial value selected from the table. At
the end of the time delay, it would turn off the heat and pop up the
toast. Come back next week, and I'll show you a working prototype."

The second advisor, a computer scientist, immediately recognized the
danger of such short-sighted thinking. He said, "Toasters don't just
turn bread into toast, they are also used to warm frozen waffles. What
you see before you is really a breakfast food cooker. As the subjects
of your kingdom become more sophisticated, they will demand more
capabilities. They will need a breakfast food cooker that can also cook
sausage, fry bacon, and make scrambled eggs. A toaster that only makes
toast will soon be obsolete. If we don't look to the future, we will
have to completely redesign the toaster in just a few years."

"With this in mind, we can formulate a more intelligent solution to the
problem. First, create a class of breakfast foods. Specialize this class
into subclasses: grains, pork, and poultry. The specialization process
should be repeated with grains divided into toast, muffins, pancakes,
and waffles; pork divided into sausage, links, and bacon; and poultry
divided into scrambled eggs, hard- boiled eggs, poached eggs, fried eggs,
and various omelet classes."

"The ham and cheese omelet class is worth special attention because
it must inherit characteristics from the pork, dairy, and poultry
classes. Thus, we see that the problem cannot be properly solved without
multiple inheritance.  At run time, the program must create the proper
object and send a message to the object that says, 'Cook yourself.' The
semantics of this message depend, of course, on the kind of object, so
they have a different meaning to a piece of toast than to scrambled eggs."

"Reviewing the process so far, we see that the analysis phase has revealed
that the primary requirement is to cook any kind of breakfast food. In the
design phase, we have discovered some derived requirements. Specifically,
we need an object-oriented language with multiple inheritance.  Of course,
users don't want the eggs to get cold while the bacon is frying, so
concurrent processing is required, too."

The king wisely had the computer scientist beheaded, and they all lived
happily ever after.

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