Class Relationships

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Class Relationships

• Lecture Oo09
• Association Relationships

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References

• Ambler, S., The Object Primer, Cambridge Univ.
  Press, 2001, Section 5.7, 5.8

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Teaching Points

• Association Relationships
• Aggregation
• Composition

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Review

• How can we get the effect of run-time binding in
  non-OO languages?
• e.g. Who_AM_I()

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Class Diagrams

• Provide a static view of the design
• The types of objects in the design and the static
  relationship between them
• Three perspectives
• Conceptual
• Specification
• Implementation

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Association Relationships

• Associations usually represent objects of one
  class making use of objects of another class

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Association Relationships

• Conceptual
• conceptual relationship
• Specification
• responsibilities
• methods implied
• Implementation
• visibility (pointers)
• navigability

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Roles
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Multiplicity (Cardinality)

• 1 only one
• 1.. one or more
• 0..1 zero or one
• 0.. zero or more (also )

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Aggregation

• A kind of association used to model whole-part
  relationships
• The whole is a container or composite
• The things being aggregated can be called parts
  or components

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Aggregation
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Benefits of showing Aggregation

• Clarifies whether the part can exist independent
  of the whole
• impacts create/delete dependencies
• Assists in identifying the creator
• Operations such as copy, move, delete, etc.
  should often be propagated to the parts
• Parts hidden within the whole can provide better
  encapsulation

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Composition

• A kind of aggregation
• Stronger relationship than plain aggregation
• The part object may belong to only one whole
• The parts live and die with the whole
• In implementation the component is structurally
  part of (a field of) the whole

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Composition
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Aggregation vs. Association

• It is not always obvious when to use association
  and when to use aggregation
• There are differing views in the OO world
• Aggregation often starts out as association, but
  as the semantics of the problem are refined an
  association can become aggregation
• More important at the implementation perspective

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When to use Aggregation

• The lifetime of the part is bound within the
  lifetime of the aggregate - there is a create
  delete dependency of the part on the whole.
• There is an obvious whole-part physical or
  logical assembly

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When to use Aggregation

• Some properties of the composite propagate to the
  parts, such as its location
• Operations applied to the composite propagate to
  the parts, such as destruction, movement,
  recording, etc.
• If in doubt, leave it out.

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Composite Pattern (Review)

• May be used to describe
• File system
• Graphics objects
• Aircraft Components
• etc.

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Teaching Points

• Association Relationships
• Aggregation
• Composition