Design Patterns Explained

32 Part I • An Introduction to Object-Oriented Software Development
Different ways of
showing class
information
There are variations on these themes. For example, to say something
contains something else can mean that
• The contained item is a part of the containing item (like an
engine in a car).
• I have a collection of things that can exist on their own (like
airplanes at an airport).
The first example is called composition while the second is called
aggregation. 1
Figure 2-1 illustrates several important things. First, each rectangle
represents a class. In the UML, I can represent up to three things in a
class:
• The name of the class
• The data members of the class
• The methods (functions) of the class
Figure 2-1 The Class Diagram — its three variations.
I have three different ways of showing these.
• The leftmost rectangle shows just the class' name. I would use this
type of class representation when more detailed information is
not needed.
1. Gamma, Helm, Johnson, and Vlissides (the Gang of Four) call the first "aggrega tion"
and the second "composition"— exactly the reverse of the UML. However, the
Gang of Four book was written before the UML was finalized. The presented
definition is, in fact, consistent with the UML's. This illustrates some of the motivation
for the UML; before it came out there were several different modeling languages,
each with its own notation and terms.