Page 53 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Unit 4: Advanced Class Modelling – I

Self Assessment Notes

Fill in the blanks:
11. The ..................... of an association is the number of roles for each link.
12. ..................... association is defined as an association among n classes.

4.4 Aggregation

Aggregation is a kind of association, between a whole, called the assembly, and its parts, called
the components. Aggregation is often called the “a-part-of” or “parts-explosion” relationship
and may be nested to an arbitrary number of levels. Aggregation bears the transitivity property:
If A is part of B and B is part of C, then A is part of C.
Aggregation is also antisymmetric: If A is part of B, then B is not part of A. Transitivity lets you
compute the transitive closure of an assembly—that is, you can compute the components that
directly and indirectly compose it. Transitive closure is a term from graph theory; the transitive
closure of a node is the set of nodes that are reachable by some sequence of edges.
Aggregation is drawn like an association with a small diamond added next to the assembly end.

Example: A book consists of front matter, multiple chapters, and back matter. Front
matter, in turn, consists of a title page and a preface; back matter consists of multiple appendixes
and an index. This is shown in figure 4.12.

Figure 4.12: Aggregation is a Kind of Association with
Additional Semantic Properties

Appendix
Having presented this example, we hasten to add that aggregation does not specify order.
An assembly is a collection of components without any implied order. A book has a well-known
and simple order of parts; the back matter follows the chapters which follow the front mater. If
the model had to capture component order, you would have to include comments.

An aggregation relationship is essentially a binary association, a pairing between the assembly
class and a component class. An assembly with many kinds of components corresponds to many
aggregations. We define each individual pairing as an aggregation so that we can specify the
multiplicity of each component within the assembly. This definition emphasizes that aggregation
is a special form of association. An aggregation can be qualified, have roles, and have link
attributes just like any other association.
Aggregation can be fixed, variable or recursive.

A fixed aggregate has a fixed structure; the number and types of subparts are predefined.
A variable aggregate has a finite number of levels, but the number of parts may vary.
A recursive aggregate contains, directly or indirectly, an instance of the same kind of
aggregate; the number of potential levels is unlimited.

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