1. IMSE 11 - UML Class Diagrams
a modeling language is the graphical notation used in developing a system
UML is known as the Unified Modeling Language because it unifies the earlier graphical notations of:
Booch Jacobson and Rambaugh (the 3 amigos)
who collaborated to produce UML
other important O-O precursors:
Coad and Yourdon (OOA, OOD)
Wilfs-Brock (responsibility-driven design)
Cunningham and Beck (class-responsibility-collaboration cards)
UML is very young (1998) and is now the industry-wide standard modeling language for O-O systems

2. Familiar and Unfamiliar Concepts in UML
UML caters for many of the general OO concepts we are already familiar with, e.g.
classes with attributes and operations or methods
relationships between classes (inheritance (known as generalisation in UML) aggregation, association)
cardinality of relationships, known as multiplicity in UML
abstract classes
some concepts we haven’t met in earlier lectures are provided by UML, e.g.
multiple classification, dynamic classification, composition, derived attributes, qualified association, parameterised classes

3. UML Diagrams UML uses diagrams and symbols of many types, e.g.
class diagrams
interaction diagrams:
sequence diagrams
collaboration diagrams
use case diagrams
package diagrams
state diagrams
activity diagrams
deployment diagrams
many of the diagrams are developed in and belong to the analysis stage of software development
for the high level design of a system we will focus on class diagrams

4. Class Diagrams describe the types of objects in the system
show the attributes and operations
show the static relationships between the objects
main types of relationships:
- generalisation - “type of”
- association e.g. project leader and company car
- aggregation and composition - “part of”
also show multiplicity, navigability, derived attributes, visibility (public, private, protected), multiple classification, discriminators, dynamic classification etc, etc ...

5. Generalisation ... ... UML’s term for inheritance is generalisation
Student
reg no.
name
address
change address
a clear triangular arrowhead symbolises generalisation
superclass
subclass
BABC Student
BUS279 gp
...
HND Student
COH208 gp
...
HND students are a type of student

6. Association and Roles an association has 2 roles, e.g:
- the role from project leader to company car
- the role from company car to project leader
a role may be labeled with a name:
- a project leader “has the use of” a company car
- a company car “is allocated to” a project leader
the name of the role can be the same as the target class
Project Leader
name
...
Company Car
reg. no. …
...
has the use of
is allocated to

7. Multiplicity multiplicity is UML’s term for cardinality
multiplicity is shown on class diagrams
it states how many objects can participate in a relationship
1 precisely one
n n is any number
* the range zero to infinity
1 .. * the range one to infinity
zero or one
n1 .. n2 the range n1 to n2 (one number to another)
e.gs. car and engine
book and pages 1 *

8. Aggregation the “part of” relationship e.g. car and engine
symbolised by a clear diamond
some analysts would also include a company and its employees
the multiplicity is shown, a car and engine have a 1 to 1 relationship of association
engine
car 1
aggregation

9. Composition stronger version of aggregation
the parts live and die with the whole
symbolised by a filled-in diamond
2 ways of representing composition:
pages
book
book
pages *
composition 1 *
multiplicity = * i.e. any no. from 0 upwards

10. Navigability
does the project leader class have an attribute for storing a company car registration number?
or does the company car class have an attribute for holding a project leader’s name?
or both?
to describe this we say that the association between project leader and company car has a navigability
navigabilities are shown by an arrow, which may be uni-directional or bi-directional

11. Navigabilities
the direction of the association arrow reveals the class that knows
(or is responsible for) which object it will be associated with
Project Leader
name
car reg. no..
...
Project Leader
name
...
Project Leader
name
car reg. no.
...
Company Car
reg. no. ..
Company Car
reg. no.
proj ldr’s name
...
Company Car
reg. no.
proj ldr’s name ..

12. Derived Attributes date of birth is an attribute of person
age is a derived attribute as it is not stored, but can be derived from date of birth
derived attributes are symbolised by a forward slash
Person
date of birth
/ age
derived attribute

13. Visibility
the attributes and operations of a class may have public, private or protected visibility
public - can be used by any other class
private - can be used only by objects of this class
protected - can be used by this class and its subclasses
UML notation for each:
+ public e.g. + name
- private e.g. - salary
# protected e.g. # display supervisor
subtle differences exist at implementation stage, depending on the programming language used

14. Multiple Classification
classification - the relationship between an object and its type
single classification - an object belongs to a single type which may inherit from supertypes
multiple classification - an object may be described by several types not necessarily connected by inheritance
e.g. a person may also be described as
- a man or a woman
- and a patient
- and a doctor or a nurse
note that multiple classification is completely different to multiple inheritance, e.g. a hovercraft
each hovercraft is an instance of the single type hovercraft, even though it has more than one supertype

15. Example of Multiple Classification
Female
Male
Doctor
Nurse
Person
Patient

16. Discriminators male and female are subtypes of person
each person cannot be both male and female
each person cannot be both a doctor and a nurse
we show this on a class diagram by joining the generalisation lines into one triangular arrowhead, and labelling the line with a discriminator
so several subtypes share a discriminator
and if a discriminator is labeled {complete} then an instance of the supertype (e.g. person) must be an instance of one of the subtypes (e.g. female)
note that sex is a complete discriminator but job is not - as every person is either male or female, but not every person is a doctor or nurse

17. Example of Multiple Classification
discriminator
sex {complete}
Female
Male
Person
Patient
Doctor
Nurse
job <<dynamic>>
Legal combinations:
female, patient, doctor
male, nurse
male, patient
Illegal combinations:
patient, doctor (must include male or female as sex is a complete discriminator)
female, doctor, nurse (contains 2 types from the discriminator job)

18. Dynamic Classification
on the class diagram showing the example of multiple classification the discriminator “job” is marked as <<dynamic>>
this shows that a person’s job in the system will be able to change (nurses may become doctors and vice versa)
so objects are allowed to change type within the subtype structure
dynamic classification allows this, static classification does not
useful for bank accounts
- when a bank account is overdrawn it changes behaviour
- operations such as withdraw and close are overridden