1. a generalization is a relationship between a general thing (the
Generalization (Ch 26)
a generalization is a relationship between a general thing (the
superclass or parent class) and a more specific kind of thing
(the subclass or child class)
example:
a MemberOfStaff is a specialized kind of LibraryMember
LibraryMember
MemberOfStaff
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2. everything a LibraryMember can do, a MemberOfStaff can do
Generalization
example
continued:
everything a LibraryMember can do, a MemberOfStaff can do
a LibraryMember can borrow a book, so can a MemberOfStaff
there are some things that a MemberOfStaff can do that a LibraryMember cannot (the specialization aspect)
the MemberOfStaff has all the attributes the LibraryMember has, and possibly more (again, the specialization aspect)
MemberOfStaff
Nov 2002
R McFadyen

3. A generalization hierarchy
a Generalization example:
a SavingsAccount is a specialized kind of BankAccount
a ChequingAccount is a specialized kind of BankAccount
an Asset may be a BankAccount, RealEstate, or a Security
Asset
RealEstate
BankAccount
Security
SavingsAccount
ChequingAccount
A generalization hierarchy
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R McFadyen

4. The subclass must conform to 100% of the superclass’s: attributes
Text:
All members of a conceptual subclass set are members of their superclass set
100% of the conceptual superclass’s definition should be applicable to the subclass.
The subclass must conform to 100% of the superclass’s:
attributes
associations
A conceptual subclass is a kind of superclass - is-a-kind-of is usually referred to as is-a
A ROT/recommendation
Nov 2002
R McFadyen

5. Generalization/specialization
Student
A constraint can be added to be very specific about an association
Complete vs incomplete
Overlapping vs disjoint.
{complete}
PartTime
FullTime
Person
Student
{overlapping}
Employee
Supervisor
{disjoint}
PartTime
FullTime
Nov 2002
R McFadyen

6. Figure 26.7
Every payment, regardless of whether it is cash, credit, or cheque, has an Amount and it is associated with a Sale
Pays-for
Payment
Sale 1 1
Amount: money
Cash Payment
Credit Payment
Cheque Payment * 1
Only credit payments are associated with credit cards
Only cheque payments are associated with cheques 1 1
CreditCard
Cheque
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R McFadyen

7. Motivation for partitioning a conceptual class into subclasses:
the subclass has additional attributes of interest
the subclass has additional associations of interest
the subclass is operated on, handled, reacted to, or manipulated differently than the superclass or other subclasses
the subclass concept represents an animate thing that behaves differently than the superclass or other subclasses
Examples
a chequing account will accrue interest differently from a savings account
a full time employee will have different associations from a part time employee
Nov 2002
R McFadyen

8. Figure 26.7
If every member of a class must be a member of a subclass, then the superclass is considered an abstract class - it is never instantiated. Abstract classes have their name italicized.
Pays-for
Payment
Sale 1 1
Amount: money
Cash Payment
Credit Payment
Cheque Payment * 1 1 1
CreditCard
Cheque
Nov 2002
R McFadyen

9. ROT: Do not model with subclasses. Model with states.
Watch for objects that are performing roles, or moving from one state to another.
ROT: Do not model with subclasses. Model with states.
Text:
Consider payments that begin as unauthorized and may eventually become authorized.
Do not model as:
Its difficult to mutate an object from one class to another
Payment
Unauthorized Payment
Authorized Payment
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R McFadyen

10. Text: Instead of Payment Unauthorized Payment Authorized Payment Use:
Is-in
Payment
PaymentState 1 1
Unauthorized State
Authorized State
Nov 2002
R McFadyen