Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 6

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Title: Software Engineering, CPSC-4360-01, CPSC-5360-01, Lecture 6

1
Software Engineering, CPSC-4360-01,
CPSC-5360-01,Lecture 6
2
Review of Last Lecture

Design
Major activities during design phase.
Tools
Class Diagram.
Object Diagram.

3
Overview of This Lecture

Class Diagram
Association Class, Reification
N-Ary Association
Qualified Association
Interface
Interactions diagrams
Collaborations, classifier and association roles
Interaction diagrams, object creation and
destruction
Role multiplicity and iterated messages
Multi-objects
Conditional messages, messages to self

4
Where are we now?

In depth look at the relevant tools
Class Diagram
Interaction Diagram

5
Association Review

Annotation for Association

Role Name The role of the instance in the
relationship. Represented as reference name
Multiplicity Number of instances associated
Association Name Description of the relationship
6
Association Review

Role names can also help to distinguish multiple
associations, or self association.

7
Properties of Links

There are cases where information about the link
need to be kept.

Example
A student takes a module and gets a mark for it.
The mark only makes sense if we know the student
and the module.
The mark is not simply an attribute of either
class.

8
Association Class

Association Classes can be added.
Behave like a combination of both association and
class
Association Can connect two classes.
Class Allow attribute to be stored.
Syntax dashed line between association and class
icon.

Association Class
9
Association Class Example

Example
John took CPSC4360, and scored 25 marks.

John Student
CPSC4360 Module

The Takes instance is uniquely defined by both
JohnStudent and CPSC4360Module.
Follow Up
What if John retakes CPSC4360, and scored 99
marks?

10
Reification

Another technique to capture association
information is reification.
Reification means Replacing an association with
a Class.

Apply Reification. An intermediate class is added
11
Reification Example

Draw an object diagram for the two examples given
previously
John takes CPSC4360 and score 25
John retakes CPSC4360 and score 99
The reification has the property of allowing
students to take a module more than once, in case
they failed first time.
This property was not possible for previous
slide, when there is exactly one link between a
student and a module.
Question to ponder
Is reification and association class the same?

Draw the Object Diagram before Lecture
12
Association Class Property

Association classes are classes
So they can participate in associations.
Example MarkSheet lists all students taking a
module with the marks they got.

Can participate in the relationship just like a
normal class
13
N-ary Association

Associations can connect more than two classes

A 3-way association could be used to store marks.
Multiplicity at a given association end defines
the number of instances of the class at that end
that can be linked to a tuple of instances, one
from each of the other ends of association.
Example specifies that each pair of student
and module object can be linked to zero or more
Attempt objects.

14
Qualified Association

In the real world, objects are usually
identified by a unique value (also known as key).
Example
University uses Matriculation Number to identify
a student.

Problems
Is matric number unique? Is it represented as
such?
How do we know the matric can be used as a key?

15
Qualified Association

Syntax
The key is known as a qualifier.
The qualifier is written in a square box attached
to the class used to identify the other party.
The multiplicity is changed to reflect the
relationship with the qualifier instead of the
class.

Changed Multiplicity
Qualifier
16
Qualified Association

Semantics
The set of qualifiers is unique in the context of
the attached class
E.g., the University has a set of unique matric
values.
The multiplicity reflects the relationship
between the identified class and the qualifier
E.g., a Student is given exactly one matric in
the University.
E.g., an University object will use matric to
uniquely identify zero or one Student (why zero?).

17
Qualified Association Example

Draw an object diagram to show
A student John in LU university with matric. no.
007.
A student Helen in LU university with matric. no.
1234.

18
Interface

An interface in UML is a named set of operations.
Interfaces are used to characterize the behaviour
of an entity.
shown as a stereotyped class

Generalization can be defined between interfaces.

19
Realizing an Interface

A class realizes an interface if it provides
implementations of all the operations.
Similar to the implements keyword in Java.
UML provides two equivalent ways of denoting this
relationship

ltltinterfacegtgtList
CircularLinkedList
CircularLinkedList

Both represent CircularLinkedList implements all
operations defined by the List interface.

20
Interface Dependency

A class can be dependent on an interface.
This means that it makes use of the operations
defined in that interface.
E.g., the Restaurant class makes use of the List
interface

21
Interface Dependency

If the class realizing the interface is known,
the dependency can be specified by

This information can also be attached to the role
name (known as the interface specifier)

Restaurant
CircularLinkedList
bookingList List
22
Evolution of Class Diagram

Class Diagram evolves with the development
process
Starts out as a Domain Model
Classes with name and attribute only.
Refined to an Analysis Class Model
Classes with name, attribute and simplified
operation.
Refined to a Design Class Model
Classes with name, attribute and operation.
Type information and Scope defined for attribute
and operation.

23
Interaction Diagrams
24
Interaction Diagrams

When a system is running, objects interact by
passing messages.
The messages define the systems behaviour, but
they are not shown on static diagrams (such as
class diagrams).
UML defines two types of diagram for showing
interactions
collaboration diagram.
sequence diagram.

25
Using Object Diagram Interaction

A message can be added to an Object Diagram.
Syntax
An arrow with message name and parameter (
)
Example
The Bank performs Fund Transfer by withdrawing
from one account, and deposit to another account.

message
Object Diagram with Message
26
Problems with Object Diagrams

Object diagrams show a specific scenario
They show specific objects, not the general case
Can we withdraw from a2 and deposit in a1
instead?
They show a fixed number of objects and links
Can we withdraw from a1 and deposit into a1 again
(for whatever reason)?
They cannot show alternative functionality
What if the withdraw causes overdraft in a1? Can
we proceed with the deposit?
In a nutshell, we need something more general.

27
Solution to this Problem

A more general method is needed to specify
behaviour.
Collaborations in UML in general do not show
individual objects, but rather the roles that
objects can play in the interaction.
An object diagram used to illustrate a
collaboration is known as a collaboration
instance set.

28
Classifier Roles

Define collaborations using classifier roles
Represent any object of a class.
Can have a name describing the role.
Syntax
object_name /role_name base_class

Specify that any object of Account class can be
substituted.
/Debtor describes that money is taken out from
this account.

29
Classifier Role

Syntax Guide
The classifier role is not underlined, to
distinguish from the object diagram usage.
The object_name can be used to label a classifier
role instead of a role name, when the role is not
clear/ not important.
The Object Diagram on Slide 25 is a collaboration
instance set of the Collaboration Diagram on
Slide 28
Substitute an object a1 for the /Debtor role.
Substitute an object a2 for the /Creditor role.

30
Roles and Objects

Objects can play different roles in interactions.
An object can be substituted for a role if
it is an instance of the base class of the role
or one of its subclasses.
In a given interaction, an object playing a
particular role will not normally make use of all
the features provided by the base class of the
role
E.g., an Account object in the /Debtor role will
only receive withdraw() message, but not
deposit() message.

31
Association Role

Similar to Classifier Role, an Association Role
generalizes the links in the Object Diagram.
An association role connecting two classifier
roles indicates that objects playing those roles
Can establish links to each other.
Can exchange messages during interactions.
Question
What are the possible ways to set up a
communication (link) between two objects?

32
Association Stereotypes

There are five ways to establish a link between
two objects
Base Association
Parameter
Local Instantiation
Global Variable
Self-directed Link
In UML, five corresponding stereotypes are used
to denote the above
ltltAssociationgtgt
ltltParametergtgt
ltltLocalgtgt
ltltGlobalgtgt
ltltSelfgtgt

33
Association Role Base Association

The most common kind of association.
The association is defined between the
corresponding classes (i.e., show up in the class
diagram).
More permanent compared to other kind of
association.
Usually kept as an attribute in the class.
Syntax
Label the association role with stereotype
ltltAssociationgtgt.
Example

Class Diagram
34
Base Association Example
ltltAssociationgtgt
ltltAssociationgtgt

The interaction is possible because the Bank
object holds the IsHeldAt to the two account
objects.
As this is the most common case, the
ltltassociationgtgt stereotype is usually omitted.

35
Association Role Parameter

One object is passed to another as a parameter of
a message.
In programming languages, this is implemented by
passing a reference to the object.
The object receiving the message knows the
identity of the parameter object, and can, in the
method body, send messages to that object.
This link is temporary, being available while the
operation is executing.
Syntax
Label with the stereotype ltltparametergtgt.

36
Parameter Example

class A
//No association to class B
void methodA(B objB)
objB.methodB( )

During the execution of methodA(), an object of
class A can pass a message to an object of class
B, because the reference is passed as parameter.
When methodA() terminates, the link will be over.

methodB( )
A
B
ltltparametergtgt
Collaboration Diagram
37
Association Role Local Instantiation

Implementations of operations can create local
instances of any class.
Sending messages to these objects during the
execution of the operation is now possible.
A link corresponding to a local variable only
lasts for the duration of an operation call.
Syntax
Label with the stereotype ltltlocalgtgt.

38
Local Instantiation Example

class A
//No association to class B
void methodA( )
B objB new B()
objB.methodB( )

During the execution of methodA(), an object of
class A can pass a message to an object of class
B, because a local object is created.
When methodA() terminates, the link will be gone.

39
Association Role Global Variable

If any global variables exist and are visible, an
object can send messages to an object stored in
such a variable.
Example in Java
Nested class
Example in C
Global object pointer
Syntax
Label with the stereotype ltltglobalgtgt

40
Global Variable Example

class OuterA
B objB
class A //Nested class
//No association to class B
void methodA( )
objB.methodB( )

During the execution of methodA(), an object of
class A can pass a message to an object of class
B, because an attribute of the parent class is
accessible to all nested classes.
When methodA() terminates, the link remains
(why?).

methodB( )
A
B
ltltglobalgtgt
Collaboration Diagram
41
Association Role Self-Directed

An object can always send messages to itself,
even though no explicit link to self is
defined.
In programming languages, this capability is
provided by defining a pseudo-variable called
this or self.
Syntax
Label with the stereotype ltltselfgtgt.

42
Self-Directed Example

class A
//No association to class A
void methodA( )
this.methodA( )

During the execution of methodA(), an object of
class A can pass a message to itself, because a
self reference (this in Java) is always
available.
When methodA() terminates, the link remains.

A
ltltselfgtgt
methodA( )
Collaboration Diagram
43
Sequence Diagram Review

Review sequence diagram introduced earlier in the
light of classifier role.

Classifier Role is the more formal description
Classifier Role
Classifier Role
Message With parameter
Message ( Para )
Activation Bar
Lifeline
Time Passes
Return value
Return Control and possibly value are returned
44
Sequence Diagrams

Show classifier roles only (no association
roles).
The vertical dimension is the time and the
messages are drawn from top to bottom, in the
order they are sent.
Each role has a lifeline
indicating the period of time during which
objects playing that role exist.
Syntax vertical dashed line below the classifier
role.
Messages are shown as arrows leading from the
lifeline of the messages sender to that of the
receiver.

45
Sequence Diagrams

The period of time during which an object is
processing a message is called activation.
Syntax narrow rectangle whose top is connected
to a message.
When an object finishes processing a message, the
control returns to the sender of the message.
Syntax dashed arrow from the bottom of
activation rectangle back to lifeline of the role
that sent the message.
The messages with solid arrowhead denote
synchronous messages, such as normal procedure
calls (the object that sends the message is
suspended until the called object returns the
control to the caller).

46
Sequence Diagram Simple Example

Suppose statements are to be printed for bank
accounts
Bank passes the relevant Account to a Statement
object for printing.

Sequence Diagram
47
Anything Missing?

The link between classifier roles is not
indicated
How can a statement object contacts the relevant
account object ?
You have to read the diagram carefully to deduce
that the link may be established by the
parameter.
In some cases, such deductions are
impossible/prone to error when the information is
not enough.
A Collaboration Diagram can show the same
exchange, but also includes the association role
used for communication.

48
Collaboration Diagram

Show classifier and association roles.
Compared with diagram on Slide 46, messages also
have
sequence numbers to indicate order
optional returned values with notation.

Collaboration Diagram
Return Value
Sequence Number
49
Collaboration versus Sequence Diagrams

Unlike sequence diagrams, collaboration diagrams
show the association role.
Message sequencing cannot be shown graphically
and messages are numbered to indicate the order
in which they are sent.
Messages can be numbered sequentially, but more
commonly a hierarchical numbering scheme is used
(i.e., to reflect the nesting activation made
explicit in sequence diagrams).

50
Hierarchical Numbering

Within each activation, messages are numbered
sequentially starting from 1.
A unique label can then be generated for each
message by adding the number of the message to
the end of the number of the activation sending
the message.
Syntax use a .
to separate the numbers and
to reflect that another level of nesting of
control flow has been initiated.

51
Hierarchical Numbering. Example
1 because it is the first message
1.1 because it is the first message sent during
the computation of message 1
1.2 because it is the second message sent during
the computation of message 1
2.1.2 because it is the second message sent
during the computation of message 2.1
52
Interaction Diagram Additional Notations

Subsequent examples illustrate the notations for
Object Creation
Object Destruction
Iterated Messages
Multiobjects
Conditional Messages
Message to Self
Should take note of how to portrait certain
interaction in both Sequence Diagram (SD) and
Collaboration Diagram (CD).

53
Sequence Diagram Object Creation

As time is explicitly represented in a sequence
diagram, the object creation is easy to draw

A
class A void methodA( ) B objB new
B() objB.methodB( )
B( )
B
methodB( )
Sequence Diagram
54
Collaboration Diagram Object Creation

In a Collaboration Diagram, new objects and new
associations have to be labeled with new.

A
class A void methodA( ) B objB new
B() objB.methodB( )
1 B( )
new
Bnew
ltltlocalgtgt
2 methodB( )
Collaboration Diagram
55
Sequence Diagram Object Destruction

In languages with automatic garbage collection
(like Java), you cannot explicitly delete an
object.
Instead, remove all references to the object for
it to be collected.
Label the message as ltltdestroygtgt.
Mark the lifeline of the destroyed object with a
cross.

class A void methodA( ) B objB new
B() ... ... ... objB null ... ...
...
A
B
ltltdestroygtgt
Sequence Diagram
Reminder Only the circled part of code is
represented.
56
Collaboration Diagram Object Destruction

In a Collaboration Diagram
Similarly, label the message as ltltdestroygtgt.
Label the destroyed objects and links with
destroyed.

class A void methodA( ) B objB new
B() ... ... ... objB null ... ...
...
A
destroyed
Bdestroyed
ltltlocalgtgt
1 ltltdestroygtgt
Collaboration Diagram
57
Role Multiplicity

The number of objects playing a role can vary
from one occasion to another.
Example (Restaurant Case Study)
Looking for bookings for a particular date will
depend on how many total bookings are there.

Restaurant
Booking
Need to show many booking objects will be
involved.
getDate( )
Return date
Sequence Diagram
58
Role Multiplicity

Role multiplicity can be added to classifier role
to indicate the number of objects involved.
Syntax
Same as the multiplicity notation in class
diagram (e.g., 1..8, , 2.. , etc).

Role Multiplicity Shows that zero or more Booking
objects will be involved
Restaurant
Booking
getDate( )
Return date
Sequence Diagram
59
Role Multiplicity

The notation is the same in both Sequence Diagram
and Collaboration Diagram.
However, the fact that the message is iterated is
still not represented.

Restaurant
Booking

getDate( )
Sent Multiple times
Return date
Sequence Diagram
60
Iterated Messages

We can clarify this by
adding a multiplicity to the affected role.
specifying that the message is iterated.
Syntax
Recurrence consists in written after the
sequence number, possibly followed by an
iteration clause.
There is no formal syntax for iteration clause.
Pseudo code-like condition is usually used, e.g.,
i 1 .. N or i 1 to N.
Example

Denote Recurrence
Denote the Recurrence Condition
1 i 1.. N getDate( )

Restaurant
Booking
Collaboration Diagram
61
Multiobject

A multiobject denotes a collection of objects
It is a role with a multiplicity of zero or more.

Denote Multiobject

It implies an intermediate data structure, e.g.

Restaurant
Booking
Vector
Collaboration Diagram
62
Property of Multiobject

Using multiobject prevents a premature commitment
to a particular data structure
E.g., What if vector is not a good data structure
for this case?
Semantically, a multiobject is a single object
representing a collection of objects.
A single message sent to it implies an operation
involving the collection of objects.
A good example of such operation would be looking
for a particular object in the collection.

63
Multiobject Example

Assume that there is only one booking per date to
simplify the discussion
A single message findBooking(Date) is sent to the
Booking multiobject.
The multiobject inspects all its Booking object
and returns the appropriate booking B (to
indicate that B is not a new object, but one from
the multiobject, a composition link is used).
The Restaurant stores B as a local reference for
subsequent operations.

64
Multiobject Message for all objects

To send a message to all objects in the
multiobject
Send a single message to the multiobject.
The multiobject goes through some iterative
process and sends the message to each object in
the collection.
By convention, such interactions can be
abbreviated by using iterative messages.
Iterated messages to a multiobject are understood
to be sent to individual objects.

Each booking in the multiobject will receive the
Print( ) message
Print( )
Booking
Restaurant
Booking
Collaboration Diagram
65
Sequence Diagram Conditional Message

Conditions can be added to messages to show the
situations when they are sent.
Syntax
Write the condition in preceding a message.

class A void methodA( ) B objB new
B() ... ... ... if ( x lt y )
objB.methodB( )
A
B
x lt y methodB( )
Sequence Diagram
66
Collaboration Diagram Conditional Message

Same syntax can be used for a collaboration
diagram.

class A void methodA( ) B objB new
B() ... ... ... if ( x lt y )
objB.methodB( )
A
B
Collaboration Diagram
67
Alternative Flows

Sequence diagrams can show alternative message
sequences in one diagram
Two or more messages start at the same point
(fork).
They are distinguished by conditions (only one
will be sent).
Return messages come together later (join).
Objects that receive messages may need branching
lifelines to represent alternative possibilities.
Should draw two separate diagrams instead.

68
Sequence Diagram Alternative Flow

Pay attention to
Branching of the messages (Fork).
Joining of the return messages (Join).

B
A
class A void methodA( ) B objB new
B() ... ... ... if ( x lt y )
objB.methOne( ) else objB.methTwo( )
x lt y methOne( )
x gt y methTwo( )
Sequence Diagram
69
Sequence Diagram Message to Self

An object can send message to itself
Invoking another operation of its own.
Usually represents implementation details.

class A void methodA( ) B objB new
B() objB.methOne( ) class B void
methOne( ) methTwo( ) void methTwo(
) ... ... ...
A
B
methOne( )
methTwo( )
Sequence Diagram
70
Collaboration Diagram Message to Self