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?" he asked.
One advisor, an Electrical Engineer, answered first. "It is a toaster,"
he said.
The king asked, "How would you design an embedded computer for it?"
The advisor: "Using a four-bit microcontroller, I would write a simple
program that reads the darkness knob and quantifies 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 software-developer, 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 redesign the
toaster completely 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 omelette classes.
"The ham-and-cheese-omelette 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.
"We must not forget the user interface. The lever that lowers the food
lacks versatility, and the darkness knob is confusing. Users won't buy
the product unless it has a user-friendly, graphical interface. When the
breakfast-cooker is plugged in, users should see a cowboy boot on the
screen. Users click on it, and the message 'Booting UNIX v.8.3' appears
on the screen. (UNIX 8.3 should be out by the time the product gets to
the market.) Users can pull down a menu and click on the foods they want
to cook.
"Once we have made the wise decision of initially specifying the software
in the design phase, all that remains is to pick an adequate hardware
platform for the implementation phase. An Intel Pentium with 48MB of
memory, a 1.2GB hard disk, and a SVGA monitor should be sufficient. If
you select a multitasking, object-oriented language that supports
multiple inheritance and has a built-in GUI, writing the program will be
a snap."
The king wisely had the software-developer beheaded, and they all lived
happily ever after.
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?" he asked.
One advisor, an Electrical Engineer, answered first. "It is a toaster,"
he said.
The king asked, "How would you design an embedded computer for it?"
The advisor: "Using a four-bit microcontroller, I would write a simple
program that reads the darkness knob and quantifies 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 software-developer, 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 redesign the
toaster completely 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 omelette classes.
"The ham-and-cheese-omelette 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.
"We must not forget the user interface. The lever that lowers the food
lacks versatility, and the darkness knob is confusing. Users won't buy
the product unless it has a user-friendly, graphical interface. When the
breakfast-cooker is plugged in, users should see a cowboy boot on the
screen. Users click on it, and the message 'Booting UNIX v.8.3' appears
on the screen. (UNIX 8.3 should be out by the time the product gets to
the market.) Users can pull down a menu and click on the foods they want
to cook.
"Once we have made the wise decision of initially specifying the software
in the design phase, all that remains is to pick an adequate hardware
platform for the implementation phase. An Intel Pentium with 48MB of
memory, a 1.2GB hard disk, and a SVGA monitor should be sufficient. If
you select a multitasking, object-oriented language that supports
multiple inheritance and has a built-in GUI, writing the program will be
a snap."
The king wisely had the software-developer beheaded, and they all lived
happily ever after.
