Structural Modeling: Class Diagrams

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Structural Modeling Class Diagrams
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Class Diagrams

• Provide static view of system
• Most common type of diagram in OOAD
• Forms basis for component and deployment diagrams
• Shows
• Classes
• Interfaces
• Collaboration
• And their relationships

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Classes

• Definition of the word class
• Oxford a number of individuals having a
  character or feature in common
• Webster a number of persons or things regarded
  as forming a group by reason of common
  attributes, characteristics, qualities or traits

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Classes in everyday life

• Based on similar values for a particular, or
  group of attributes
• All purple dinosaurs all green apples all
  Jesuits
• Based on the range of an attribute
• Everyone who is between 18-21 years of age
• Based on similar behavior
• Everyone who sleeps in class everyone in the
  ECIS program

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All about Classes

• Based on the notion of abstraction
• Starts with identification of all common
  characteristics of a set of elements
• All objects belong to a class
• Objects represent an instance of a class
• Software objects are built from a class through
  instantiation

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Representation of a Class
CLASS NAME
Attributes
Operations ( )
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Representation of a Class
MOTORCYCLE
Color Maximum Speed Engine
Start ( ) Brake ( )
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Purpose of Class Diagrams

• To model the vocabulary of the system
• What lies within and without the system
• To model simple collaborations
• To visualize the relationship between classes
• To model a logical database schema
• Three major types of constructs in class diagrams
• Inheritance structure
• Association structure
• Aggregation structure

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Additional Concepts

• Aggregation
• A relationship between two classes where the
  instances of one class are, in some way,
  components, members , or contents of the
  instances of the other class (from Brown, 2002)
• Types
• Component (assembly)
• Container (contents)
• Collection (Member)

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Aggregation Component
Car
Body
Transmission
Engine
Consists of
Car Part
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Class Hierarchies
Vehicle
Land
Air
Bike
Car
Helicopter
Plane
(referred to as Generalization)
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Problems with Classification
Animal
Two-legged
Four-legged
Carnivore
Herbivore
Carnivore
Herbivore
Why should I classify this way instead of
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Class Hierarchies
Animal
Herbivore
Carnivore
4 legs
2 legs
4 legs
2 legs
This way?
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Multiple Generalization
Animal
Herbivore
Carnivore
Two Legged
Four legged
Dog
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Subclass Partitioning

• Disjoint vs. Overlapping
• Based on mutually exclusive subgroups
• Incomplete vs. Complete
• Coverage of the entire set

Powered Vehicle
ANIMAL
Disjoint and incomplete
Overlapping and complete
Plane Car Truck
Herbivore Carnivore
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Relationships Between Classes

• Associations
• Objects are linked
• Classes are associated
• Represented as

(a link)
SU University
David Student
University
Student
(an association)
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Multiplicity

• Sometimes called the cardinalityof a
  relationship
• The number of instances of each class that are or
  can be involved in a given association

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University
Student

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Other Examples

Driver
Bus


Customer
Product


Student
Course

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Advisor
Student

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Roles and multiplicity
1
University
Student

0..1
University
Teacher

May be replaced by..
teacher
employer

0..1
University
Person

1
student
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An example
Aggregation (composed of..)
COMPANY
located
OFFICES
DEPARTMENT
Address Stringvoice number
Name Name
constraint
association


roles
subset
generalization
manager
member
1..
1
PERSON
HEADQUARTERS
Name NameempID IntegerTitle String
interface
Getphoto() Getsound() Getcontact() Getpersrec()
PersonnelRecord
taxId string Salary currency
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Generalization vs. Aggregation

• Can sometimes be confusing
• Distinction can be made based on verb used
• Generalization
• The phrase IS A KIND OF
• A car is a kind of vehicle
• Aggregation
• The phrase HAS A/AN
• A car has an engine

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An Example
BURNING LAMP
LAMP
ELECTRIC LAMP
BASE
WIRE
SWITCH
FLOURESCENT
INCANDESCENT
SOCKET
BULB