Design Patterns Explained

Chapter 2 • The UML— The Unified Modeling Language 35
In this diagram, I also show that an Aircraft is either a Jet or a
Helicopter. I can see that Aircraft is an abstract class because its
name is shown in italics. That means that an Airport will have
either Jet or Helicopter but will treat them the same (as Aircraft). Aggregatio
The open (unfilled) diamond on the right of the Airport class
indicates the aggregation relationship.
The other type of has-a relationship is where the containment
means the contained object is a part of the containing object. This type
of relationship is also called composition.
n
Figure 2-4 The Class Diagram showing composition and the uses
relationship.
Figure 2-4 shows that a Car has Tires as parts (that is, the Car is
made up of Tires and other things). This type of has-a relationship,
called composition, is depicted by the filled in diamond. This dia -
gram also shows that a Car uses a GasStation. The uses relationship
is depicted by a dashed line with an arrow. This is also called a
dependency relationship.
Both composition and aggregation involve one obje ct containing
one or more objects. Composition, however, implies the contained
object is a part of the containing object, whereas aggregation
means the contained objects are more like a collection of things.
We can consider composition to be an unshared association, with
Composition and
uses
Composition versus
aggregation