UML Weekend Crash Course™ - To Parent Directory

108
Saturday Morning
The answer to each question goes next to the class that describes the objects that the
question is counting. The answer to “How many People . . .” goes on the end of the association
near the Person class. The answer to “How many Cars . . .” goes on the end of the association
next to the Car class.
Multiplicity is expressed in a couple of ways. The most common is a range defining the
minimum number of objects allowed and the maximum number of objects allowed in the
format
Minimum . . Maximum
You must use integer values for the minimum and maximum. But, as you have probably
found in your own applications, sometimes you don’t know the upper limit or there is no
actual upper limit. The UML suggests the use of an asterisk to mean no upper limit. Used by
itself, the asterisk can also mean the minimum is zero and there is no upper limit, or zero
or more.
You may also encounter a situation when a range is not appropriate. If you had to define
how many cylinders are in the engines you service, you might want to say, “I only work on
4–, 6–, and 8-cylinder engines.” For these situations, the UML suggests a comma-separated
list (for example, 4,6,8).
You can simplify the notation when the minimum and maximum values are the same by
using a single value. So instead of writing 4..4, you can just write 4. This is a nice shortcut,
but beware! The most common place to use this shortcut is when the multiplicity is 1..1.
Unfortunately, the shortcut encourages people to overlook or ignore the possibility that the
minimum is zero, that is, the relationship is optional.
I’ll use the example of a car and an engine. That’s easy! Each car has one engine, right
Well, what about cars on an assembly line During the first n stages of assembly, there is no
engine in the car. In this case, the multiplicity should be 0..1 to allow the car to exist
before it has an engine installed. Frankly, I have found very few instances when the minimum
multiplicity should be 1. My rule of thumb is to set the minimum to 0 until I have
positive proof that the one object cannot exist with the other object.
Most software failures are because of small, difficult-to-find errors like the
difference between 0 and 1. Most of those errors are caused by assumptions
Tip or failures to think critically about the details. I once witnessed an explosion
in a power plant caused by a one-character mistake in a program. Like
they say, “The devil is in the details.”
Here’s a summary list of the options for specifying multiplicity followed by some
examples.
Values separated by two periods (..) mean a range. For example, 1..3 means between
1 and 3 inclusively; 5..10 means between 5 and 10 inclusively.
Values separated by commas mean an enumerated list of possibilities. For example,
4,6,8 means you may have 4 objects or 6 objects or 8 objects of this type in the
association.
Asterisk (*) when used alone means zero or more, no lower or upper limit.
Asterisk (*) when used in a range (1..*) means no upper limit — you must have at
least one but you can have as many more as you want.