UML Weekend Crash Course™ - To Parent Directory

Session 11—The Class Diagram: Aggregation and Generalization 119
What makes aggregation unique More importantly, what makes aggregation beneficial
Aggregation describes a group of objects in a way that changes how you interact with them.
The concept is aimed at protecting the integrity of a configuration of objects in two specific
ways.
First, aggregation defines a single point of control in the object that represents the
assembly. This ensures that no matter what others might want to do to the members of the
assembly, the control object has the final word on whether the actions are allowed. This
assignment of control may be at many levels within the aggregation hierarchy. For example,
an engine might be the controller of its parts, but the car is the controller of the engine.
Second, when an instruction is given that might effect the entire collection of objects,
the control object dictates how the members will respond. So for all intentions, the assembly
appears to function like a single object. When I push the gas pedal, telling the car I
want to accelerate, the entire car assembly (with its thousands of parts) accelerates, not
just the gas pedal.
Elements of composition
Composition is used for aggregations where the life span of the part depends on the life
span of the aggregate. The aggregate has control over the creation and destruction of the
part. In other words, the member object cannot exist apart from the aggregation. Draw this
stronger form of aggregation simply by making the aggregation diamond solid (black).
In Figure 11-3, the team example uses aggregation, the hollow diamond. Players are
assembled into a team. But if the Team is disbanded, the players live on (depending of
course on how well they performed). The Book example uses composition, the solid diamond.
A Book is composed of Chapters. The Chapters would not continue to exist elsewhere
on their own. They would cease to exist along with the Book.
Team
Aggregation
0..1 9..9
Player
Book
Composition
1..1 1..*
Chapter
Figure 11-3 How to represent a composition relationship in the UML
Note how the multiplicity provides some clues on the distinction between aggregation
and composition. On the Team example in Figure 11-3, each Player may or may not be a
member of a Team (0..1). This tells me that a Player may exist independent from the Team.
The Book example says that a Chapter must be associated with one and only one Book
(1..1). This tells me that a Chapter cannot exist independent of the Book, so it must be a
composition relationship.