Design Patterns Explained

36 Part I • An Introduction to Object-Oriented Software Development
Indicating the
number of things
another object has
the contained object's lifetime being controlled by its containing
object. The appropriate use of constructor and destructor methods
is useful here to help facilitate object creation and destruction.
Figure 2-5 The Class Diagram with a Note.
Notes in the UML.
In Figure 2-5, there is a new symbol: the Note. The box contain ing
the message "open diamonds mean aggregation" is a note. They
are meant to look like pieces of paper with the right corner folded
back. You often see them with a line connecting them to a
particular class indicating they relate just to that class.
Class Diagrams show the relationships between classes. With composition
and aggregation, however, the relationship is more specifically
about objects of that type of class. For example, it is true
Airports have Aircraft, but more specifically, specific airports
have specific aircraft. The question may arise— "how many aircraft
does an airport have?" This is called the cardinality of the relationship.
I show this in Figures 2-6 and 2-7.