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Figure 3.26 shows the internal composition of the class Car . The car has an
engine, e that drives the two front wheels. Moreover, there are two rear wheels. In
contrast to the rear wheels, the role of the front wheels has a relationship to the
engine, e , in the context of a car. This role specifi cation ensures that undesirable
object constellations are prevented ( Figure 3.27 ). Figure 3.26 shows the wheels in
dashed notation, because the car doesn ??™ t own them any more; it uses an aggregation
to merely reference them. The special designation of the objects with slashes
in their names were be explained in Section 3.4.1.
3.4.1 Role
3.4 The Composite Structure Diagram
FIGURE 3-26
The composite structure diagram for ???car.???
Defi nition
A role describes a structure in the context of a class.
class Composite structure Car
Car
rear:Wheel
front:Wheel e:Engine
2
2
a:axle
166 CHAPTER 3 UML??”Unifi ed Modeling Language
Attributes specify the structure of a class. An attribute ??™ s type is a class. This class
describes the structure of the attribute.


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