Object-Oriented Software Engineering
Practical Software Development using UML and Java
Chapter 5,
Modelling with Classes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 2
5.1 What is UML?
The Unified Modelling Language is a standard graphical language
for modelling object oriented software
? At the end of the 1980s and the beginning of 1990s,the first object-
oriented development processes appeared
? The proliferation of methods and notations tended to cause
considerable confusion
? Two important methodologists Rumbaugh and Booch decided to
merge their approaches in 1994.
— They worked together at the Rational Software Corporation
? In 1995,another methodologist,Jacobson,joined the team
— His work focused on use cases
? In 1997 the Object Management Group (OMG) started the process
of UML standardization
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 3
UML diagrams
? Class diagrams
— describe classes and their relationships
? Interaction diagrams
— show the behaviour of systems in terms of how
objects interact with each other
? State diagrams and activity diagrams
— show how systems behave internally
? Component and deployment diagrams
— show how the various components of systems are
arranged logically and physically
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 4
UML features
? It has detailed semantics
? It has extension mechanisms
? It has an associated textual language
— Object Constraint Language (OCL)
The objective of UML is to assist in software
development
— It is not a methodology
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 5
What constitutes a good model?
A model should
? use a standard notation
? be understandable by clients and users
? lead software engineers to have insights about the
system
? provide abstraction
Models are used:
? to help create designs
? to permit analysis and review of those designs,
? as the core documentation describing the system,
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 6
5.2 Essentials of UML Class Diagrams
The main symbols shown on class diagrams are:
? Classes
- represent the types of data themselves
? Associations
- represent linkages between instances of classes
? Attributes
- are simple data found in classes and their instances
? Operations
- represent the functions performed by the classes and their
instances
? Generalizations
- group classes into inheritance hierarchies
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 7
Classes
A class is simply represented as a box with the name of
the class inside
? The diagram may also show the attributes and operations
? The complete signature of an operation is,
operationName(parameterName,parameterType …),returnType
R e c t a n g l e
he ig ht, i nt
w i dt h,i nt
ge t Area ( ),i nt
res iz e(in t,in t )
R e c t a n g l e
he ig ht
w i dt h
ge t Area
res iz e
R e c t a n g l e
he ig ht
w i dt h
R e c t a n g l e
ge t Area
res iz e
R e c t a n g l e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 8
5.3 Associations and Multiplicity
An association is used to show how two classes are
related to each other
? Symbols indicating multiplicity are shown at each end of
the association
0,3., 8 *
E m p l o y e e
*
* *1., *
*0., 1
S e c r e t a r y
O f f i c e
P e r s o n
C o m p a n y
E m p l o y e e C o m p a n y
M a n a g e r
B o a r d O f D i r e c t o r s
B o a r d O f D i r e c t o r s
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 9
Labelling associations
? Each association can be labelled,to make explicit the
nature of the association
*
su p e r v is o r
*1., *
*
w o r k sF o r
*
a l l o c a t e d T o
0., 1
b o a r d M e m b e r
0,3., 8 *
E m p l o y e e
S e c r e t a r y
O f f i c e
P e r s o n
C o m p a n y
E m p l o y e e C o m p a n y
M a n a g e r
B o a r d O f D i r e c t o r s
B o a r d O f D i r e c t o r s
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 10
Analyzing and validating associations
? One-to-one
— For each company,there is exactly one board of
directors
— A board is the board of only one company
— A company must always have a board
— A board must always be of some company
C o m p a n y B o a rd O f D i re c t o rs
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 11
Analyzing and validating associations
? Many-to-many
— A secretary can work for many managers
— A manager can have many secretaries
— Secretaries can work in pools
— Managers can have a group of secretaries
— Some managers might have zero secretaries,
— Is it possible for a secretary to have,perhaps
temporarily,zero managers?
*
s u p e r v is o r
*1,, *S e c re t a r y M a n a g e r
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 12
Analyzing and validating associations
? One-to-one
— For each company,there is exactly one board of
directors
— A board is the board of only one company
— A company must always have a board
— A board must always be of some company
C o m p a n y B o a rd O f D i re c t o rs
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 13
Analyzing and validating associations
Avoid unnecessary one-to-one associations
Avoid this do this
P e r s o n
n a m e
a d d r e s s
e m a i l
b i r t h d a t e
P e r s o n
n a m e
P e r s o n I n f o
a d d r e s s
e m a i l
b i r t h d a t e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 14
A more complex example
? A booking is always for exactly one passenger
— no booking with zero passengers
— a booking could never involve more than one
passenger.
? A Passenger can have any number of Bookings
— a passenger could have no bookings at all
— a passenger could have more than one booking
**P a s s e n g e r S p e c i f i c F l i g h tB o o k i n g
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 15
Association classes
? Sometimes,an attribute that concerns two associated
classes cannot be placed in either of the classes
? The following are equivalent
Registration
grade
Student CourseSection* *
Registration
grade
Student CourseSection* *
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 16
Reflexive associations
? It is possible for an association to connect a class to
itself
Course * isMutuallyExclusiveWith
*
*
prerequisite
successor *
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 17
Directionality in associations
? Associations are by default bi-directional
? It is possible to limit the direction of an association by
adding an arrow at one end
* NoteDay
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 18
5.4 Generalization
Specializing a superclass into two or more subclasses
? The discriminator is a label that describes the criteria
used in the specialization
A n i m a l A n i m a l
h a b i t a t t y p e O f F o o d
H e r b i v o r eC a r n i v o r eL a n d A n i m a lA q u a t i c A n i m a l
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 19
Avoiding unnecessary generalizations
R o c k R e c o r d i n gB l u e s R e c o r d i n gC l a s s i c a lR e c o r d i n gJ a zzR e c o r d in gM u s i c V i d e o
V i d e o R e c o d i n g A u d i o R e c o r d i n g
R e c o r d in g
rockbluesclassicaljazzmusic video
video audio
RecordingCategory
*
subcategorydescription
Recording * hasCategory
subcategory subcategorysubcategorysubcategorysubcategory
:RecordingCategory,RecordingCategory
:RecordingCategory,RecordingCategory,RecordingCategory,RecordingCategory:RecordingCategory
9th Symphony
:Recording
Let it be
:Recording
The BeatlesBeethoven
title
artist
Inappropriate hierarchy of
classes,which should be
instances
Improved class diagram,
with its corresponding
instance diagram
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 20
Handling multiple discriminators
A n i m a l
h a b i t a t
L a n d A n i m a lA q u a t i c A n im a l
A q u a t i c C a r n i v o r e A q u a t i c H e r b i v o r e L a n d C a r n i v o r e L a n d H e r b i v o r e
t y p e O f F o o d t y p e O f F o o d
? Creating higher-level generalization
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 21
? Using multiple inheritance
? Using the Player-Role pattern (in Chapter 6)
Handling multiple discriminators
Animal
habitat typeOfFood
HerbivoreCarnivoreLandAnimalAquaticAnimal
AquaticCarnivore AquaticHerbivore LandCarnivore LandHerbivore
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 22
Avoiding having instances change class
S t u d e n t
a t t e n d a n ce
P a r t T im e S t u d e n tF u ll T i m e S t u d e n t
? An instance should never need to change class
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 23
5.5 Instance Diagrams
? A link is an instance of an association
— In the same way that we say an object is an instance
of a class
Carla:Employee
Ali:Employee
Wayne:Employee
OOCorp:Company OOCorp's Board:
UML inc's BoardUML inc:Company
Pat:Employee
Terry:Employee
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 24
Associations versus generalizations in
instance diagrams
? Associations describe the relationships that will exist
between instances at run time,
— When you show an instance diagram generated from
a class diagram,there will be an instance of both
classes joined by an association
? Generalizations describe relationships between classes
in class diagrams,
— They do not appear in instance diagrams at all,
— An instance of any class should also be considered
to be an instance of each of that class’s superclasses
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 25
5.6 More Advanced Features,Aggregation
? Aggregations are special associations that represent
‘part-whole’ relationships,
— The ‘whole’ side is often called the assembly or the
aggregate
— This symbol is a shorthand notation association
named isPartOf
*
* Region
VehiclePart
Country
Vehicle
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 26
When to use an aggregation
As a general rule,you can mark an association as an
aggregation if the following are true:
? You can state that
— the parts ‘are part of’ the aggregate
— or the aggregate ‘is composed of’ the parts
? When something owns or controls the aggregate,then they
also own or control the parts
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 27
? A composition is a strong kind of aggregation
— if the aggregate is destroyed,then the parts are
destroyed as well
? Two alternatives for addresses
Composition
* RoomBuilding
E m p l o y e eE m p l o y e e
ad dres s, A ddres s
A d d r e s s
s t ree t
m un ic ip al it y
regi on
c o unt ry
po s t al C od e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 28
Aggregation hierarchy
* *
*
WheelTransmissionEngineFrame
DoorBodyPanelChassis
Vehicle
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 29
Propagation
? A mechanism where an operation in an aggregate is
implemented by having the aggregate perform that
operation on its parts
? At the same time,properties of the parts are often
propagated back to the aggregate
? Propagation is to aggregation as inheritance is to
generalization,
— The major difference is:
- inheritance is an implicit mechanism
- propagation has to be programmed when required
* L i n e S e g m e n tP o l y g o n
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 30
Interfaces
An interface describes a portion of the visible behaviour
of a set of objects.
? An interface is similar to a class,except it lacks instance
variables and implemented methods
?interface?
Cashier
withdraw
deposit
Machine
ATMEmployee
Person Machine
ATMEmployee
Person
Cashier Cashier
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 31
Notes and descriptive text
? Descriptive text and other diagrams
— Embed your diagrams in a larger document
— Text can explain aspects of the system using any
notation you like
— Highlight and expand on important features,and
give rationale
? Notes,
— A note is a small block of text embedded in a UML
diagram
— It acts like a comment in a programming language
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 32
Object Constraint Language (OCL)
OCL is a specification language designed to formally
specify constraints in software modules
? An OCL expression simply specifies a logical fact (a
constraint) about the system that must remain true
? A constraint cannot have any side-effects
— it cannot compute a non-Boolean result nor modify
any data.
? OCL statements in class diagrams can specify what the
values of attributes and associations must be
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 33
OCL statements
OCL statements can be built from:
? References to role names,association names,attributes
and the results of operations
? The logical values true and false
? Logical operators such as and,or,=,>,< or <> (not
equals)
? String values such as,‘a string’
? Integers and real numbers
? Arithmetic operations *,/,+,-
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 34
An example,constraints on Polygons
LinearShape
startPoint,Point
1..*
edge LineSegment
Path Line Polygon
RegularPolygon
endPoint,Point
{startPoint <> endPoint}
{ordered}
{edge->size=1}
{edge->forAll(e1,e2 |
e1.length = e2.length)}
{edge->forAll(e1,e2 |
e1 <> e2
implies e1.startPoint <> e2.startpoint
and e1.endPoint <> e2.endpoint)}
length, int
length
{length =
edge.length->sum}
{edge->first.startPoint =
edge->last.endPoint}
a LinearShape is any shape
that can be constructed of line
segments (in contrast with
shapes that contain curves).
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 35
5.7 Detailed Example,A Class Diagram for
Genealogy
? Problems
— A person must have two parents
— Marriages not properly accounted for
2
ch i ld
P e r s o n
na m e
pl ac eOf Bi r t h
da t e Of B irt h
pl ac eOf D ea t h
da t e Of D eat h
pl ac eOf M arr i ag e
da t e Of M a r rai ge
da t e Of D iv orc e
*
p a r e n t
0., 1
0., 1 w i f e
h u sb a n d
{ h u sb a n d, s e x
= # m a l e }
{ w i f e, s e x
= # f e m a l e }
{ p a r e n t - > f o r A ll ( p 1,p 2,
p 1 < > p 2
i m p li e s p 1, s e x < > p 2, se x) }
s ex
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 36
Genealogy example,Possible solutions
Person
name
placeOfBirth
dateOfBirth
placeOfDeath
dateOfDeath
Union
placeOfMarriage
dateOfMarriage
dateOfDivorce
parents
0..1
child
*
*
*
partner 0..2
sex
Person
name
placeOfBirth
dateOfBirth
placeOfDeath
dateOfDeath
Union
placeOfMarriage
dateOfMarriage
dateOfDivorce
parents
0..1
child
*
child* malePartner0..1child
**
femalePartner 0..1
Woman Man
{partner->forAll(p1,p2 |
p1 <> p2
implies p1.sex <> p2.sex)}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 37
5.8 The Process of Developing Class
Diagrams
You can create UML models at different stages and with
different purposes and levels of details
? Exploratory domain model:
— Developed in domain analysis to learn about the
domain
? System domain model:
— Models aspects of the domain represented by the
system
? System model:
— Includes also classes used to build the user interface
and system architecture
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 38
包含表示领
域中事务的
元素
只为被真正
实现的事物
建模
包含不表示领域中
的事物，但构建完
整系统需要的元素
Exploratory
domain model
Yes No No
System domain
model
Yes Yes No
System model Yes Yes Yes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 39
System domain model vs System model
? The system domain model omits many classes that are
needed to build a complete system
— Can contain less than half the classes of the system.
— Should be developed to be used independently of
particular sets of
- user interface classes
- architectural classes
? The complete system model includes
— The system domain model
— User interface classes
— Architectural classes
— Utility classes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 40
Suggested sequence of activities
? Identify a first set of candidate classes
? Add associations and attributes
? Find generalizations
? List the main responsibilities of each class
? Decide on specific operations
? Iterate over the entire process until the model is
satisfactory
— Add or delete classes,associations,attributes,
generalizations,responsibilities or operations
— Identify interfaces
— Apply design patterns (Chapter 6)
Don’t be too disorganized,Don’t be too rigid either.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 41
Identifying classes
? When developing a domain model you tend to discover
classes
? When you work on the user interface or the system
architecture,you tend to invent classes
— Needed to solve a particular design problem
— (Inventing may also occur when creating a domain
model)
? Reuse should always be a concern
— Frameworks
— System extensions
— Similar systems
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 42
A simple technique for discovering domain
classes
? Look at a source material such as a description of
requirements
? Extract the nouns and noun phrases
? Eliminate nouns that:
— are redundant
— represent instances
— are vague or highly general
— not needed in the application
? Pay attention to classes in a domain model that represent
types of users or other actors
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 43
Identifying associations and attributes
? Start with classes you think are most central and
important
? Decide on the clear and obvious data it must contain and
its relationships to other classes.
? Work outwards towards the classes that are less
important.
? Avoid adding many associations and attributes to a class
— A system is simpler if it manipulates less
information
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 44
Tips about identifying and specifying valid
associations
? An association should exist if a class
- possesses
- controls
- isconnected to
- isrelated to
- isa part of
- has as parts
- isa member of,or
- has as members
some other class in your model
? Specify the multiplicity at both ends
? Label it clearly.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 45
Actions versus associations
? A common mistake is to represent actions as if they
were associations
*
LibraryPatron
borrow Loan
borrowedDate
dueDate
returnedDate
Bad,due to the use of associations
that are actions
*
*
return
CollectionItem
*
*
LibraryPatron
CollectionItem
*
*
Better,The borrow operation creates a Loan,and
the return operation sets the returnedDate
attribute.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 46
Identifying attributes
? Look for information that must be maintained about each
class
? Several nouns rejected as classes,may now become
attributes
? An attribute should generally contain a simple value
— E.g,string,number
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 47
Tips about identifying and specifying valid
attributes
? It is not good to have many duplicate attributes
? If a subset of a class’s attributes form a coherent group,
then create a distinct class containing these attributes
*
*
P e r s o n
n a m e
a d d re s s e s
a d d r e s se s
P e r s o n
n a m e
s t re e t 1
m u n ic ip a l it y 1
p ro v O r S t a t e 1
c o u n t ry 1
p o s t a l C o d e 1
s t re e t 2
m u n ic ip a l it y 2
p ro v O r S t a t e 2
c o u n t ry 2
p o s t a l C o d e 2
P e r s o n
n a m e
A d d r e s s
s t re e t
m u n ic ip a l it y
p ro v O r S t a t e
c o u n t ry
p o s t a l c o d e
t y p e
B a d d u e t o
a p l u ra l
a t t r i b u t e
B a d d u e t o t o o m a n y
a t t r i b u t e s,a n d i n a b i l i t y
t o a d d m o r e a d d r e s s e s
G o o d s o l u t i o n, T h e
t y p e i n d i c a t e s w h e t h e r
i t i s a h o m e a d d re s s,
b u s i n e s s a d d re s s e t c,
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 48
An example (attributes and associations)
*
supervisor
RegularFlight
time
flightNumber
*
*
Passenger
*
*
*
SpecificFlight
date
name
employeeNumber
Employee
jobFunction
Booking
seatNumber
name
number
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 49
Identifying generalizations and interfaces
? There are two ways to identify generalizations:
— bottom-up
- Group together similar classes creating a new superclass
— top-down
- Look for more general classes first,specialize them if
needed
? Create an interface,instead of a superclass if
— The classes are very dissimilar except for having a
few operations in common
— One or more of the classes already have their own
superclasses
— Different implementations of the same class might
be available
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 50
An example (generalization)
*
supervisor
RegularFlight
time
flightNumber
*
*
PassengerRole
*
*
*
SpecificFlight
date
Person
name
idNumber
0..2
EmployeeRole
jobFunction
Booking
seatNumber
PersonRole
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 51
Allocating responsibilities to classes
A responsibility is something that the system is required to do.
? Each functional requirement must be attributed to one of the classes
— All the responsibilities of a given class should be clearly
related.
— If a class has too many responsibilities,consider splitting it into
distinct classes
— If a class has no responsibilities attached to it,then it is
probably useless
— When a responsibility cannot be attributed to any of the existing
classes,then a new class should be created
? To determine responsibilities
— Perform use case analysis
— Look for verbs and nouns describing actions in the system
description
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 52
Categories of responsibilities
? Setting and getting the values of attributes
? Creating and initializing new instances
? Loading to and saving from persistent storage
? Destroying instances
? Adding and deleting links of associations
? Copying,converting,transforming,transmitting or
outputting
? Computing numerical results
? Navigating and searching
? Other specialized work
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 53
An example (responsibilities)
— Creating a new
regular flight
— Searching for a
flight
— Modifying
attributes of a
flight
— Creating a
specific flight
— Booking a
passenger
— Canceling a
booking
*
supervisor
RegularFlight
time
flightNumber
*
*
PassengerRole
*
*
*
SpecificFlight
date
*
*Person
name
idNumber
0..2
EmployeeRole
jobFunction
Booking
seatNumber
PersonRole
Airline
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 54
Prototyping a class diagram on paper
? As you identify classes,you write their names on small
cards
? As you identify attributes and responsibilities,you list
them on the cards
— If you cannot fit all the responsibilities on one card:
- this suggests you should split the class into two related
classes.
? Move the cards around on a whiteboard to arrange them
into a class diagram.
? Draw lines among the cards to represent associations and
generalizations.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 55
Identifying operations
Operations are needed to realize the responsibilities of
each class
? There may be several operations per responsibility
? The main operations that implement a responsibility are
normally declared public
? Other methods that collaborate to perform the
responsibility must be as private as possible
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 56
An example (class collaboration)
Airplane
addLinkToSpecificFlight [a2,d3]
deleteLinkToSpecificFlight [d2]
SpecificFlight
+ specifyAirplane [a1]
+ createFlightLog [b1]
+ makeBooking [c1]
+ changeAirplane [d1]
+ findCrewMember [e1]
EmployeeRole
+ getName [e2]
FlightLog
FlightLog [b2]
Booking
Booking [c2]
PassengerRole
addLinkToBooking [c4]
*
*
0..1
*
*
**
crewMember
0..1
addLinkToBooking [c3]
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 57
Class collaboration ‘a’
Making a bi-directional link between two existing objects;
e.g,adding a link between an instance of SpecificFlight
and an instance of Airplane.
1,(public) The instance of SpecificFlight
— makes a one-directional link to the instance of
Airplane
— then calls operation 2.
2,(non-public) The instance of Airplane
— makes a one-directional link back to the instance
of SpecificFlight
A i rp l a n e
a d d L i n k T o S p e c i f i c F l i g h t [ a 2,d 3 ]
S p e c i fi c F l i g h t
+ s p e c i f y A i r p l a n e [ a 1 ]
* 0,, 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 58
Class collaboration ‘b’
Creating an object and linking it to an existing object
e.g,creating a FlightLog,and linking it to a
SpecificFlight.
1,(public) The instance of SpecificFlight
— calls the constructor of FlightLog (operation 2)
— then makes a one-directional link to the new
instance of FlightLog.
2,(non-public) Class FlightLog’s constructor
— makes a one-directional link back to the
instance of SpecificFlight.
S p e c ific F lig h t
+ c r e a t e F l i g h t L o g [ b 1 ]
F lig h t L o g
F l i g h t L o g [ b 2 ]
0,, 10,, 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 59
Class collaboration ‘c’
Creating an association class,given two existing objects
e.g,creating an instance of Booking,which will link a
SpecificFlight to a PassengerRole.
1,(public) The instance of PassengerRole
— calls the constructor of Booking (operation 2).
2,(non-public) Class Booking’sconstructor,among its other actions
— makes a one-directional link back to the instance of
PassengerRole
— makes a one-directional link to the instance of
SpecificFlight
— calls operations 3 and 4.
3,(non-public) The instance of SpecificFlight
— makes a one-directional link to the instance of Booking.
4,(non-public) The instance of PassengerRole
— makes a one-directional link to the instance of Booking.
S p e c i f i c F l i g h t
+ m ak eB o ok i ng [ c 1]
B o o k in g
Bo ok i ng [ c 2 ]
P a s s e n g e r R o le
ad dLi nk T oB o ok in g [ c 4]
* * ad dLi nk T oB o ok in g [ c 3]
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 60
Class collaboration ‘d’
Changing the destination of a link
e.g,changing the Airplane of to a SpecificFlight,from
airplane1 to airplane2
1,(public) The instance of SpecificFlight
— deletes the link to airplane1
— makes a one-directional link to airplane2
— calls operation 2
— then calls operation 3.
2,(non-public) airplane1
— deletes its one-directional link to the instance of
SpecificFlight.
3,(non-public) airplane2
— makes a one-directional link to the instance of
SpecificFlight.
A i r p l a n e
ad dLi nk T oSp ec if i c F l i ght [ a 2,d3 ]
de le t e Li nk T oSpe c i f ic F l ig ht [ d2]
S p e c i fi c F l i g h t
+ c h ang eAi r p la ne [ d1]
* 0., 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 61
Class collaboration ‘e’
Searching for an associated instance
e.g,searching for a crew member associated with a
SpecificFlight that has a certain name.
1,(public) The instance of SpecificFlight
— creates an Iterator over all the crewMember links
of the SpecificFlight\
— for each of them call operation 2,until it finds a
match.
2,(may be public) The instance of EmployeeRole returns
its name.
S p e c ific F lig h t
+ f in d C re w M e m b e r [ e 1 ]
E m p lo y e e R o le
+ g e t N a m e [ e 2 ]
* *
c r e w M e m b e r
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 62
5.9 Implementing Class Diagrams in Java
? Attributes are implemented as instance variables
? Generalizations are implemented using extends
? Interfaces are implemented using implements
? Associations are normally implemented using instance variables
? Divide each two-way association into two one-way associations
— so each associated class has an instance variable.
? For a one-way association where the multiplicity at the other
end is ‘one’ or ‘optional’
— declare a variable of that class (a reference)
? For a one-way association where the multiplicity at the other
end is ‘many’:
— use a collection class implementing List,such as Vector
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 63
Example,SpecificFlight
class SpecificFlight
{
private Calendar date;
private RegularFlight regularFlight;
private TerminalOfAirport destination;
private Airplane airplane;
private FlightLog flightLog;
private ArrayList crewMembers;
// of EmployeeRole
private ArrayList bookings
...
}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 64
Example,SpecificFlight
// Constructor that should only be called from
// addSpecificFlight
SpecificFlight(
Calendar aDate,
RegularFlight aRegularFlight)
{
date = aDate;
regularFlight = aRegularFlight;
}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 65
Example,RegularFlight
class RegularFlight
{
private ArrayList specificFlights;
...
// Method that has primary
// responsibility
public void addSpecificFlight(
Calendar aDate)
{
SpecificFlight newSpecificFlight;
newSpecificFlight =
new SpecificFlight(aDate,this);
specificFlights.add(newSpecificFlight);
}
...
}
Practical Software Development using UML and Java
Chapter 5,
Modelling with Classes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 2
5.1 What is UML?
The Unified Modelling Language is a standard graphical language
for modelling object oriented software
? At the end of the 1980s and the beginning of 1990s,the first object-
oriented development processes appeared
? The proliferation of methods and notations tended to cause
considerable confusion
? Two important methodologists Rumbaugh and Booch decided to
merge their approaches in 1994.
— They worked together at the Rational Software Corporation
? In 1995,another methodologist,Jacobson,joined the team
— His work focused on use cases
? In 1997 the Object Management Group (OMG) started the process
of UML standardization
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 3
UML diagrams
? Class diagrams
— describe classes and their relationships
? Interaction diagrams
— show the behaviour of systems in terms of how
objects interact with each other
? State diagrams and activity diagrams
— show how systems behave internally
? Component and deployment diagrams
— show how the various components of systems are
arranged logically and physically
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 4
UML features
? It has detailed semantics
? It has extension mechanisms
? It has an associated textual language
— Object Constraint Language (OCL)
The objective of UML is to assist in software
development
— It is not a methodology
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 5
What constitutes a good model?
A model should
? use a standard notation
? be understandable by clients and users
? lead software engineers to have insights about the
system
? provide abstraction
Models are used:
? to help create designs
? to permit analysis and review of those designs,
? as the core documentation describing the system,
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 6
5.2 Essentials of UML Class Diagrams
The main symbols shown on class diagrams are:
? Classes
- represent the types of data themselves
? Associations
- represent linkages between instances of classes
? Attributes
- are simple data found in classes and their instances
? Operations
- represent the functions performed by the classes and their
instances
? Generalizations
- group classes into inheritance hierarchies
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 7
Classes
A class is simply represented as a box with the name of
the class inside
? The diagram may also show the attributes and operations
? The complete signature of an operation is,
operationName(parameterName,parameterType …),returnType
R e c t a n g l e
he ig ht, i nt
w i dt h,i nt
ge t Area ( ),i nt
res iz e(in t,in t )
R e c t a n g l e
he ig ht
w i dt h
ge t Area
res iz e
R e c t a n g l e
he ig ht
w i dt h
R e c t a n g l e
ge t Area
res iz e
R e c t a n g l e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 8
5.3 Associations and Multiplicity
An association is used to show how two classes are
related to each other
? Symbols indicating multiplicity are shown at each end of
the association
0,3., 8 *
E m p l o y e e
*
* *1., *
*0., 1
S e c r e t a r y
O f f i c e
P e r s o n
C o m p a n y
E m p l o y e e C o m p a n y
M a n a g e r
B o a r d O f D i r e c t o r s
B o a r d O f D i r e c t o r s
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 9
Labelling associations
? Each association can be labelled,to make explicit the
nature of the association
*
su p e r v is o r
*1., *
*
w o r k sF o r
*
a l l o c a t e d T o
0., 1
b o a r d M e m b e r
0,3., 8 *
E m p l o y e e
S e c r e t a r y
O f f i c e
P e r s o n
C o m p a n y
E m p l o y e e C o m p a n y
M a n a g e r
B o a r d O f D i r e c t o r s
B o a r d O f D i r e c t o r s
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 10
Analyzing and validating associations
? One-to-one
— For each company,there is exactly one board of
directors
— A board is the board of only one company
— A company must always have a board
— A board must always be of some company
C o m p a n y B o a rd O f D i re c t o rs
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 11
Analyzing and validating associations
? Many-to-many
— A secretary can work for many managers
— A manager can have many secretaries
— Secretaries can work in pools
— Managers can have a group of secretaries
— Some managers might have zero secretaries,
— Is it possible for a secretary to have,perhaps
temporarily,zero managers?
*
s u p e r v is o r
*1,, *S e c re t a r y M a n a g e r
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 12
Analyzing and validating associations
? One-to-one
— For each company,there is exactly one board of
directors
— A board is the board of only one company
— A company must always have a board
— A board must always be of some company
C o m p a n y B o a rd O f D i re c t o rs
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 13
Analyzing and validating associations
Avoid unnecessary one-to-one associations
Avoid this do this
P e r s o n
n a m e
a d d r e s s
e m a i l
b i r t h d a t e
P e r s o n
n a m e
P e r s o n I n f o
a d d r e s s
e m a i l
b i r t h d a t e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 14
A more complex example
? A booking is always for exactly one passenger
— no booking with zero passengers
— a booking could never involve more than one
passenger.
? A Passenger can have any number of Bookings
— a passenger could have no bookings at all
— a passenger could have more than one booking
**P a s s e n g e r S p e c i f i c F l i g h tB o o k i n g
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 15
Association classes
? Sometimes,an attribute that concerns two associated
classes cannot be placed in either of the classes
? The following are equivalent
Registration
grade
Student CourseSection* *
Registration
grade
Student CourseSection* *
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 16
Reflexive associations
? It is possible for an association to connect a class to
itself
Course * isMutuallyExclusiveWith
*
*
prerequisite
successor *
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 17
Directionality in associations
? Associations are by default bi-directional
? It is possible to limit the direction of an association by
adding an arrow at one end
* NoteDay
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 18
5.4 Generalization
Specializing a superclass into two or more subclasses
? The discriminator is a label that describes the criteria
used in the specialization
A n i m a l A n i m a l
h a b i t a t t y p e O f F o o d
H e r b i v o r eC a r n i v o r eL a n d A n i m a lA q u a t i c A n i m a l
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 19
Avoiding unnecessary generalizations
R o c k R e c o r d i n gB l u e s R e c o r d i n gC l a s s i c a lR e c o r d i n gJ a zzR e c o r d in gM u s i c V i d e o
V i d e o R e c o d i n g A u d i o R e c o r d i n g
R e c o r d in g
rockbluesclassicaljazzmusic video
video audio
RecordingCategory
*
subcategorydescription
Recording * hasCategory
subcategory subcategorysubcategorysubcategorysubcategory
:RecordingCategory,RecordingCategory
:RecordingCategory,RecordingCategory,RecordingCategory,RecordingCategory:RecordingCategory
9th Symphony
:Recording
Let it be
:Recording
The BeatlesBeethoven
title
artist
Inappropriate hierarchy of
classes,which should be
instances
Improved class diagram,
with its corresponding
instance diagram
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 20
Handling multiple discriminators
A n i m a l
h a b i t a t
L a n d A n i m a lA q u a t i c A n im a l
A q u a t i c C a r n i v o r e A q u a t i c H e r b i v o r e L a n d C a r n i v o r e L a n d H e r b i v o r e
t y p e O f F o o d t y p e O f F o o d
? Creating higher-level generalization
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 21
? Using multiple inheritance
? Using the Player-Role pattern (in Chapter 6)
Handling multiple discriminators
Animal
habitat typeOfFood
HerbivoreCarnivoreLandAnimalAquaticAnimal
AquaticCarnivore AquaticHerbivore LandCarnivore LandHerbivore
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 22
Avoiding having instances change class
S t u d e n t
a t t e n d a n ce
P a r t T im e S t u d e n tF u ll T i m e S t u d e n t
? An instance should never need to change class
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 23
5.5 Instance Diagrams
? A link is an instance of an association
— In the same way that we say an object is an instance
of a class
Carla:Employee
Ali:Employee
Wayne:Employee
OOCorp:Company OOCorp's Board:
UML inc's BoardUML inc:Company
Pat:Employee
Terry:Employee
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 24
Associations versus generalizations in
instance diagrams
? Associations describe the relationships that will exist
between instances at run time,
— When you show an instance diagram generated from
a class diagram,there will be an instance of both
classes joined by an association
? Generalizations describe relationships between classes
in class diagrams,
— They do not appear in instance diagrams at all,
— An instance of any class should also be considered
to be an instance of each of that class’s superclasses
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 25
5.6 More Advanced Features,Aggregation
? Aggregations are special associations that represent
‘part-whole’ relationships,
— The ‘whole’ side is often called the assembly or the
aggregate
— This symbol is a shorthand notation association
named isPartOf
*
* Region
VehiclePart
Country
Vehicle
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 26
When to use an aggregation
As a general rule,you can mark an association as an
aggregation if the following are true:
? You can state that
— the parts ‘are part of’ the aggregate
— or the aggregate ‘is composed of’ the parts
? When something owns or controls the aggregate,then they
also own or control the parts
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 27
? A composition is a strong kind of aggregation
— if the aggregate is destroyed,then the parts are
destroyed as well
? Two alternatives for addresses
Composition
* RoomBuilding
E m p l o y e eE m p l o y e e
ad dres s, A ddres s
A d d r e s s
s t ree t
m un ic ip al it y
regi on
c o unt ry
po s t al C od e
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 28
Aggregation hierarchy
* *
*
WheelTransmissionEngineFrame
DoorBodyPanelChassis
Vehicle
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 29
Propagation
? A mechanism where an operation in an aggregate is
implemented by having the aggregate perform that
operation on its parts
? At the same time,properties of the parts are often
propagated back to the aggregate
? Propagation is to aggregation as inheritance is to
generalization,
— The major difference is:
- inheritance is an implicit mechanism
- propagation has to be programmed when required
* L i n e S e g m e n tP o l y g o n
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 30
Interfaces
An interface describes a portion of the visible behaviour
of a set of objects.
? An interface is similar to a class,except it lacks instance
variables and implemented methods
?interface?
Cashier
withdraw
deposit
Machine
ATMEmployee
Person Machine
ATMEmployee
Person
Cashier Cashier
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 31
Notes and descriptive text
? Descriptive text and other diagrams
— Embed your diagrams in a larger document
— Text can explain aspects of the system using any
notation you like
— Highlight and expand on important features,and
give rationale
? Notes,
— A note is a small block of text embedded in a UML
diagram
— It acts like a comment in a programming language
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 32
Object Constraint Language (OCL)
OCL is a specification language designed to formally
specify constraints in software modules
? An OCL expression simply specifies a logical fact (a
constraint) about the system that must remain true
? A constraint cannot have any side-effects
— it cannot compute a non-Boolean result nor modify
any data.
? OCL statements in class diagrams can specify what the
values of attributes and associations must be
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 33
OCL statements
OCL statements can be built from:
? References to role names,association names,attributes
and the results of operations
? The logical values true and false
? Logical operators such as and,or,=,>,< or <> (not
equals)
? String values such as,‘a string’
? Integers and real numbers
? Arithmetic operations *,/,+,-
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 34
An example,constraints on Polygons
LinearShape
startPoint,Point
1..*
edge LineSegment
Path Line Polygon
RegularPolygon
endPoint,Point
{startPoint <> endPoint}
{ordered}
{edge->size=1}
{edge->forAll(e1,e2 |
e1.length = e2.length)}
{edge->forAll(e1,e2 |
e1 <> e2
implies e1.startPoint <> e2.startpoint
and e1.endPoint <> e2.endpoint)}
length, int
length
{length =
edge.length->sum}
{edge->first.startPoint =
edge->last.endPoint}
a LinearShape is any shape
that can be constructed of line
segments (in contrast with
shapes that contain curves).
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 35
5.7 Detailed Example,A Class Diagram for
Genealogy
? Problems
— A person must have two parents
— Marriages not properly accounted for
2
ch i ld
P e r s o n
na m e
pl ac eOf Bi r t h
da t e Of B irt h
pl ac eOf D ea t h
da t e Of D eat h
pl ac eOf M arr i ag e
da t e Of M a r rai ge
da t e Of D iv orc e
*
p a r e n t
0., 1
0., 1 w i f e
h u sb a n d
{ h u sb a n d, s e x
= # m a l e }
{ w i f e, s e x
= # f e m a l e }
{ p a r e n t - > f o r A ll ( p 1,p 2,
p 1 < > p 2
i m p li e s p 1, s e x < > p 2, se x) }
s ex
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 36
Genealogy example,Possible solutions
Person
name
placeOfBirth
dateOfBirth
placeOfDeath
dateOfDeath
Union
placeOfMarriage
dateOfMarriage
dateOfDivorce
parents
0..1
child
*
*
*
partner 0..2
sex
Person
name
placeOfBirth
dateOfBirth
placeOfDeath
dateOfDeath
Union
placeOfMarriage
dateOfMarriage
dateOfDivorce
parents
0..1
child
*
child* malePartner0..1child
**
femalePartner 0..1
Woman Man
{partner->forAll(p1,p2 |
p1 <> p2
implies p1.sex <> p2.sex)}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 37
5.8 The Process of Developing Class
Diagrams
You can create UML models at different stages and with
different purposes and levels of details
? Exploratory domain model:
— Developed in domain analysis to learn about the
domain
? System domain model:
— Models aspects of the domain represented by the
system
? System model:
— Includes also classes used to build the user interface
and system architecture
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 38
包含表示领
域中事务的
元素
只为被真正
实现的事物
建模
包含不表示领域中
的事物，但构建完
整系统需要的元素
Exploratory
domain model
Yes No No
System domain
model
Yes Yes No
System model Yes Yes Yes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 39
System domain model vs System model
? The system domain model omits many classes that are
needed to build a complete system
— Can contain less than half the classes of the system.
— Should be developed to be used independently of
particular sets of
- user interface classes
- architectural classes
? The complete system model includes
— The system domain model
— User interface classes
— Architectural classes
— Utility classes
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 40
Suggested sequence of activities
? Identify a first set of candidate classes
? Add associations and attributes
? Find generalizations
? List the main responsibilities of each class
? Decide on specific operations
? Iterate over the entire process until the model is
satisfactory
— Add or delete classes,associations,attributes,
generalizations,responsibilities or operations
— Identify interfaces
— Apply design patterns (Chapter 6)
Don’t be too disorganized,Don’t be too rigid either.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 41
Identifying classes
? When developing a domain model you tend to discover
classes
? When you work on the user interface or the system
architecture,you tend to invent classes
— Needed to solve a particular design problem
— (Inventing may also occur when creating a domain
model)
? Reuse should always be a concern
— Frameworks
— System extensions
— Similar systems
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 42
A simple technique for discovering domain
classes
? Look at a source material such as a description of
requirements
? Extract the nouns and noun phrases
? Eliminate nouns that:
— are redundant
— represent instances
— are vague or highly general
— not needed in the application
? Pay attention to classes in a domain model that represent
types of users or other actors
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 43
Identifying associations and attributes
? Start with classes you think are most central and
important
? Decide on the clear and obvious data it must contain and
its relationships to other classes.
? Work outwards towards the classes that are less
important.
? Avoid adding many associations and attributes to a class
— A system is simpler if it manipulates less
information
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 44
Tips about identifying and specifying valid
associations
? An association should exist if a class
- possesses
- controls
- isconnected to
- isrelated to
- isa part of
- has as parts
- isa member of,or
- has as members
some other class in your model
? Specify the multiplicity at both ends
? Label it clearly.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 45
Actions versus associations
? A common mistake is to represent actions as if they
were associations
*
LibraryPatron
borrow Loan
borrowedDate
dueDate
returnedDate
Bad,due to the use of associations
that are actions
*
*
return
CollectionItem
*
*
LibraryPatron
CollectionItem
*
*
Better,The borrow operation creates a Loan,and
the return operation sets the returnedDate
attribute.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 46
Identifying attributes
? Look for information that must be maintained about each
class
? Several nouns rejected as classes,may now become
attributes
? An attribute should generally contain a simple value
— E.g,string,number
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 47
Tips about identifying and specifying valid
attributes
? It is not good to have many duplicate attributes
? If a subset of a class’s attributes form a coherent group,
then create a distinct class containing these attributes
*
*
P e r s o n
n a m e
a d d re s s e s
a d d r e s se s
P e r s o n
n a m e
s t re e t 1
m u n ic ip a l it y 1
p ro v O r S t a t e 1
c o u n t ry 1
p o s t a l C o d e 1
s t re e t 2
m u n ic ip a l it y 2
p ro v O r S t a t e 2
c o u n t ry 2
p o s t a l C o d e 2
P e r s o n
n a m e
A d d r e s s
s t re e t
m u n ic ip a l it y
p ro v O r S t a t e
c o u n t ry
p o s t a l c o d e
t y p e
B a d d u e t o
a p l u ra l
a t t r i b u t e
B a d d u e t o t o o m a n y
a t t r i b u t e s,a n d i n a b i l i t y
t o a d d m o r e a d d r e s s e s
G o o d s o l u t i o n, T h e
t y p e i n d i c a t e s w h e t h e r
i t i s a h o m e a d d re s s,
b u s i n e s s a d d re s s e t c,
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 48
An example (attributes and associations)
*
supervisor
RegularFlight
time
flightNumber
*
*
Passenger
*
*
*
SpecificFlight
date
name
employeeNumber
Employee
jobFunction
Booking
seatNumber
name
number
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 49
Identifying generalizations and interfaces
? There are two ways to identify generalizations:
— bottom-up
- Group together similar classes creating a new superclass
— top-down
- Look for more general classes first,specialize them if
needed
? Create an interface,instead of a superclass if
— The classes are very dissimilar except for having a
few operations in common
— One or more of the classes already have their own
superclasses
— Different implementations of the same class might
be available
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 50
An example (generalization)
*
supervisor
RegularFlight
time
flightNumber
*
*
PassengerRole
*
*
*
SpecificFlight
date
Person
name
idNumber
0..2
EmployeeRole
jobFunction
Booking
seatNumber
PersonRole
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 51
Allocating responsibilities to classes
A responsibility is something that the system is required to do.
? Each functional requirement must be attributed to one of the classes
— All the responsibilities of a given class should be clearly
related.
— If a class has too many responsibilities,consider splitting it into
distinct classes
— If a class has no responsibilities attached to it,then it is
probably useless
— When a responsibility cannot be attributed to any of the existing
classes,then a new class should be created
? To determine responsibilities
— Perform use case analysis
— Look for verbs and nouns describing actions in the system
description
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 52
Categories of responsibilities
? Setting and getting the values of attributes
? Creating and initializing new instances
? Loading to and saving from persistent storage
? Destroying instances
? Adding and deleting links of associations
? Copying,converting,transforming,transmitting or
outputting
? Computing numerical results
? Navigating and searching
? Other specialized work
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 53
An example (responsibilities)
— Creating a new
regular flight
— Searching for a
flight
— Modifying
attributes of a
flight
— Creating a
specific flight
— Booking a
passenger
— Canceling a
booking
*
supervisor
RegularFlight
time
flightNumber
*
*
PassengerRole
*
*
*
SpecificFlight
date
*
*Person
name
idNumber
0..2
EmployeeRole
jobFunction
Booking
seatNumber
PersonRole
Airline
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 54
Prototyping a class diagram on paper
? As you identify classes,you write their names on small
cards
? As you identify attributes and responsibilities,you list
them on the cards
— If you cannot fit all the responsibilities on one card:
- this suggests you should split the class into two related
classes.
? Move the cards around on a whiteboard to arrange them
into a class diagram.
? Draw lines among the cards to represent associations and
generalizations.
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 55
Identifying operations
Operations are needed to realize the responsibilities of
each class
? There may be several operations per responsibility
? The main operations that implement a responsibility are
normally declared public
? Other methods that collaborate to perform the
responsibility must be as private as possible
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 56
An example (class collaboration)
Airplane
addLinkToSpecificFlight [a2,d3]
deleteLinkToSpecificFlight [d2]
SpecificFlight
+ specifyAirplane [a1]
+ createFlightLog [b1]
+ makeBooking [c1]
+ changeAirplane [d1]
+ findCrewMember [e1]
EmployeeRole
+ getName [e2]
FlightLog
FlightLog [b2]
Booking
Booking [c2]
PassengerRole
addLinkToBooking [c4]
*
*
0..1
*
*
**
crewMember
0..1
addLinkToBooking [c3]
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 57
Class collaboration ‘a’
Making a bi-directional link between two existing objects;
e.g,adding a link between an instance of SpecificFlight
and an instance of Airplane.
1,(public) The instance of SpecificFlight
— makes a one-directional link to the instance of
Airplane
— then calls operation 2.
2,(non-public) The instance of Airplane
— makes a one-directional link back to the instance
of SpecificFlight
A i rp l a n e
a d d L i n k T o S p e c i f i c F l i g h t [ a 2,d 3 ]
S p e c i fi c F l i g h t
+ s p e c i f y A i r p l a n e [ a 1 ]
* 0,, 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 58
Class collaboration ‘b’
Creating an object and linking it to an existing object
e.g,creating a FlightLog,and linking it to a
SpecificFlight.
1,(public) The instance of SpecificFlight
— calls the constructor of FlightLog (operation 2)
— then makes a one-directional link to the new
instance of FlightLog.
2,(non-public) Class FlightLog’s constructor
— makes a one-directional link back to the
instance of SpecificFlight.
S p e c ific F lig h t
+ c r e a t e F l i g h t L o g [ b 1 ]
F lig h t L o g
F l i g h t L o g [ b 2 ]
0,, 10,, 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 59
Class collaboration ‘c’
Creating an association class,given two existing objects
e.g,creating an instance of Booking,which will link a
SpecificFlight to a PassengerRole.
1,(public) The instance of PassengerRole
— calls the constructor of Booking (operation 2).
2,(non-public) Class Booking’sconstructor,among its other actions
— makes a one-directional link back to the instance of
PassengerRole
— makes a one-directional link to the instance of
SpecificFlight
— calls operations 3 and 4.
3,(non-public) The instance of SpecificFlight
— makes a one-directional link to the instance of Booking.
4,(non-public) The instance of PassengerRole
— makes a one-directional link to the instance of Booking.
S p e c i f i c F l i g h t
+ m ak eB o ok i ng [ c 1]
B o o k in g
Bo ok i ng [ c 2 ]
P a s s e n g e r R o le
ad dLi nk T oB o ok in g [ c 4]
* * ad dLi nk T oB o ok in g [ c 3]
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 60
Class collaboration ‘d’
Changing the destination of a link
e.g,changing the Airplane of to a SpecificFlight,from
airplane1 to airplane2
1,(public) The instance of SpecificFlight
— deletes the link to airplane1
— makes a one-directional link to airplane2
— calls operation 2
— then calls operation 3.
2,(non-public) airplane1
— deletes its one-directional link to the instance of
SpecificFlight.
3,(non-public) airplane2
— makes a one-directional link to the instance of
SpecificFlight.
A i r p l a n e
ad dLi nk T oSp ec if i c F l i ght [ a 2,d3 ]
de le t e Li nk T oSpe c i f ic F l ig ht [ d2]
S p e c i fi c F l i g h t
+ c h ang eAi r p la ne [ d1]
* 0., 1
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 61
Class collaboration ‘e’
Searching for an associated instance
e.g,searching for a crew member associated with a
SpecificFlight that has a certain name.
1,(public) The instance of SpecificFlight
— creates an Iterator over all the crewMember links
of the SpecificFlight\
— for each of them call operation 2,until it finds a
match.
2,(may be public) The instance of EmployeeRole returns
its name.
S p e c ific F lig h t
+ f in d C re w M e m b e r [ e 1 ]
E m p lo y e e R o le
+ g e t N a m e [ e 2 ]
* *
c r e w M e m b e r
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 62
5.9 Implementing Class Diagrams in Java
? Attributes are implemented as instance variables
? Generalizations are implemented using extends
? Interfaces are implemented using implements
? Associations are normally implemented using instance variables
? Divide each two-way association into two one-way associations
— so each associated class has an instance variable.
? For a one-way association where the multiplicity at the other
end is ‘one’ or ‘optional’
— declare a variable of that class (a reference)
? For a one-way association where the multiplicity at the other
end is ‘many’:
— use a collection class implementing List,such as Vector
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 63
Example,SpecificFlight
class SpecificFlight
{
private Calendar date;
private RegularFlight regularFlight;
private TerminalOfAirport destination;
private Airplane airplane;
private FlightLog flightLog;
private ArrayList crewMembers;
// of EmployeeRole
private ArrayList bookings
...
}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 64
Example,SpecificFlight
// Constructor that should only be called from
// addSpecificFlight
SpecificFlight(
Calendar aDate,
RegularFlight aRegularFlight)
{
date = aDate;
regularFlight = aRegularFlight;
}
? Lethbridge/Laganière 2001 Chapter 5,Modelling with classes 65
Example,RegularFlight
class RegularFlight
{
private ArrayList specificFlights;
...
// Method that has primary
// responsibility
public void addSpecificFlight(
Calendar aDate)
{
SpecificFlight newSpecificFlight;
newSpecificFlight =
new SpecificFlight(aDate,this);
specificFlights.add(newSpecificFlight);
}
...
}
