This presentation covers simple class models with the UML, precise modeling of associations, modeling techniques, and constraining models.

1. Arab Open University
2nd
Semester, 2006-2007
M301
Unit 4.3
Classes and Associations
reem.attas@arabou.org.sa

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Topic Road Map
 Simple class models with the UML
Precise modeling of associations
More modeling techniques
Constraining models

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Class Model
Describes the objects and the
relationships between them that are
needed to implement the required
functionality.

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Searching for Classes
 Tangible objects: the physical things in the domain.
 Ex. rooms, bills, books, vehicles.
 Roles: the roles played by people in the domain.
 Ex. employee, guest, member.
 Events: the circumstances, episodes, interactions,
happenings or significant incidents.
 Ex. reservation of a room, the registration of a vehicle, orders,
deliveries and transactions.
 Organizational units: the groups to which people belong.
 Ex. accounts department, a production team or a maintenance
crew.

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… Searching for Classes
The focus on looking for nouns in a
problem description design does
emphasize that you are concentrating on
things.
In a traditional procedural model, the first
area of concern would instead have been
actions and activities, which are described
by verbs.

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Object Diagram
Model that show a number of concrete
instances of a set of relevant classes and
the relationships between them.
Each object is an instance of a class.
Each link is an instance of an association.

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… Object Diagram

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Object Diagram with Role Names

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Class Diagrams
A model that expresses generalizations
about all valid object diagrams.
Box represent class.
Line represent association.

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Class and some instances

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Associations
 The relationships between classes  How
classes might be related?
 A Guest is related to:
 A Hotel by making a reservation.
 A Room by currently occupying a room or having stayed
in a room previously.
 A Room is related to a Hotel by being physically
located in a Hotel.

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… Associations

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Multiplicities
How many of one sort of thing can be
related to another sort of thing 
determine how many objects are required.
minimum .. maximum

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… Multiplicities

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… Multiplicities

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Naming Associations

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Role Names

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Two Associations

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Several Associations

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Recursive Relationship

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Topic Road Map
Simple class models with the UML
 Precise modeling of associations
More modeling techniques
Constraining models

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Aggregation
A whole–part relationship.
 Ex. The whole (an invoice) is made up from the
parts (the invoice lines).
It is possible to add, remove or replace
one of the ‘parts’ and still have a
meaningful relationship: an invoice would
still be an aggregation of invoice lines if
another invoice line were to be added.

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Composition
A chess-board and its squares are much
more intimately related than an invoice
and its invoice lines because a chess
board cannot exist unless it has 64
squares.
If you were to remove (or add) a square
you would no longer have a recognizable
chess-board.

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… Composition

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Navigation
Put an open arrowhead at one or both
ends of an association to indicate that it is
possible to reach one class from another
following the direction of the arrow.

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… Navigation
An instance of the class Invoice can send
messages to instances of the class
InvoiceLine, but not the other way round.

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Qualified Associations
If the system you are modeling includes a
mechanism for identifying objects
uniquely, you can use qualified
associations to record this information.
In cases where objects can be uniquely
identified by combinations of two or more
attributes, all those attributes should move
into the qualifier.

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… Qualified Associations

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Derived Associations
Associations can be derived from others
already in that diagram.
By preceding a role name with a slash,
you indicate that an association is
derivable.

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… Derived Associations

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Topic Road Map
Simple class models with the UML
Precise modeling of associations
 More modeling techniques
Constraining models

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Generalization and Specialization

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… Generalization and Specialization
Superclass  more general element.
Subclass  more specific element.
A subclass has the same attributes as the
superclass, though it may define some
additional ones.

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Specialization of Account

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Interface
the set of operations that specify the
service that a class provides.
 Ex. The class Account has the interface {credit
(amount: Money), debit (amount: Money),
getBalance ()} while the class SavingsAccount
has the interface {credit (amount: Money), debit
(amount: Money), getBalance (), addInterest ()}.

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Multiple Levels of Specialization

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Violating the is-a rule

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Inventing a Superclass
<<abstract>>  never be used to create
instances.

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Polymorphism
The ability to use instances of various
different classes through a shared
interface.
The interface simply identifies a set of
operations by naming each one. It does
not provide any implementation.

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… Polymorphism

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Dependency

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Topic Road Map
Simple class models with the UML
Precise modeling of associations
More modeling techniques
 Constraining models

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Constraints
Invariants: Boolean expressions that must
always remain true.
 Ex. valid account numbers need to be within a
certain range.

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… Constraints

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Object Constraint Language (OCL)
Combines logical expressions with set
notation to allow a more rigorous
specification of a constraint.

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Constraints across Associations

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Controlling the Use of Different
Accounts

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Constraint on Related Objects

49. TMA4 – Q3

50. Thank You!