1. Systems Analysis and Design I
Session 6
Object Interaction

2. Recap
Further discussion of class diagrams (Section 7.5.2, pp 198 – 201)
Stereotypes: entity, boundary, control; association class
Software and Specification Reuse
Section 20.2 (pp. 585 – 586)
Section 8.2 (pp. 234 – 237)
Section 8.4 (pp. 246 – 247)
Section – (pp. 252 – 253)
Adding Further Structure (to Class Diagrams)
Section – (pp. 237 – 244)
Section (pp. 409 – 410)
In this lecture you will learn … 2 2

3. Today Object Interaction and Collaboration CRC Cards
Section 9.2 (pp. 260 – 262)
CRC Cards
Section 7.6 (pp. 215 – 218)
Communication Diagrams
Section 7.4 (pp. 194 – 197)
Section 9.4 (pp. 280 – 284)
Sequence Diagrams
Section 9.3 – (pp. 262 – 276)
Model Consistency
Section 9.7 (p. 289)
In this lecture you will learn … 3 3

4. Object Interaction & Collaboration
How do you communicate, interact, and collaborate?
Message Passing
Objects communicate by sending messages
This is a metaphor!
When an object sends a message to another object, an operation is invoked in the receiving object
The Campaign object sends a message to each of its Advert object
currentAdvertCost = anAdvert.getCost()
The aim of modelling object interaction is to determine the most appropriate scheme of message passing between objects to support a particular user requirement 4 4

5. Object Interaction & Collaboration
Appropriate distribution of object responsibility improves software modularity
Each class tends not to be unduly complex, and as a result is easier to develop, to test and to maintain.
Each class is relatively small and self-contained, and as a result has a much greater potential for reuse.
The system is more resilient to changes in its requirements
(see next slide)
A modular system is easier
to be maintained and upgraded
to achieve high reliability
to be implemented in small, manageable increments 5 5

6. Real-world requirements Application that caters for these requirements
Equivalent areas of change—a highly resilient system.
A small change in requirements causes a much greater change in software—not a resilient system. 6 6

7. CRC Cards Class–Responsibility–Collaboration (CRC) cards
An effective technique for exploring the possible ways of allocating responsibilities to classes and the collaborations that are necessary to fulfil the responsibilities
Brainstorm the classes
Allocate each class to a team members
For each use case, role play the interaction to distribute responsibilities among classes:
each object identifies the object that he/she thinks is most appropriate to take on a needed responsibility for collaboration
each object should be as lazy as possible, refusing to take on any responsibility unless persuaded by its fellow objects 7 7

8. Format of CRC Cards Class Name: Collaborations Responsibilities
Responsibilities of the class are listed in this section.
Collaborations with other classes are listed here, together with a brief description of the purpose of the collaboration. 8 8

9. CRC Cards: Use Case Example
Use Case: Add a new advert to a campaign
A campaign can consist of many adverts. Details of each
advert are entered into the system with a target completion
date and estimated cost.
Glossary:
Campaign: Adverts are organised into campaigns in order to
achieve a particular objective....
Client: A company or organisation that wishes to obtain the
services to develop and manage an advertising campaign,
and design and produce adverts for the campaign

10. Advert provides advert details. Advert constructs new object.
Class Name
Client
Responsibilities
Collaborations
Provide client information.
Campaign provides campaign details.
Provide list of campaigns.
Class Name
Campaign
Responsibilities
Collaborations
Provide campaign information.
Provide list of adverts.
Add a new advert.
Advert provides advert details.
Advert constructs new object.
Class Name
Advert
Responsibilities
Collaborations
Provide advert details.
Construct adverts. 10 10

11. Dynamic Analysis with UML
Sequence Diagrams
Communication Diagrams
Interaction Overview Diagrams
Timing Diagrams
In UML 1.x,
communication diagrams are called collaboration diagrams
interaction overview diagrams and timing diagrams do not exist 11 11

12. Sequence Diagrams
A sequence diagram shows an interaction between objects arranged in a time sequence.
Sequence diagrams can be drawn at different levels of detail and to meet different purposes at several stages in the development life cycle.
Sequence diagrams are typically used to represent the detailed object interaction that occurs for one use case or for one operation. 12 12

13. Sequence diagram Frame label Sequence diagram is enclosed in a frame
:Client
:Campaign
:Advert
getName
listCampaigns
listAdverts
Advert
newAd:Advert
addNewAdvert
Lifeline
Activation or Execution
Object creation
:CampaignManager
sd Add a new advert to a campaign
loop
Interaction Operator
Interaction Constraint
Combined Fragment
(loop)
[For all client’s campaigns]
getCampaignDetails
[For all campaign’s adverts]
getAdvertDetails
Frame label
Sequence diagram
is enclosed in a
frame
“sd” is used to determine the interaction diagram; interaction

14. Sequence Diagrams: Basic elements
Objects (or subsystems, components or other connectable elements) involved in interaction appear horizontally across the page
Vertical dimension represents time
Each object is represented by a lifeline.
Once a message is received, the operation that has been invoked begins to execute
The period of time during which an operation executes is called the execution occurrence or activation of an operation
shown by a rectangle on the relevant lifeline.
Messages are usually shown by a solid horizontal arrow
A synchronous message or procedural call is shown with a full arrowhead,
causes the invoking operation to suspend execution until the focus of control has been returned to it.
It is optional to show reply messages (with dashed arrows)
Why? Assume that control is returned to the originating object at the end of the activation in a destination object (except for asynchronous messages). 14 14

15. Sequence Diagrams: Example
Figure 9.4 on p. 265
:LifelineA
:LifelineB
Synchronous (blocking) message
sd Interaction Name
msg a
Send message
Event Occurrence
(msg.sendEvent)
Receive messaged Event Occurrence
(msg.recieveEvent)
start of
Execution Occurrence
end of
Active
State on lifeline showing pre-condition
Message reply showing return of control 16 16

16. Boundary & Control Classes
Most use cases imply at least one boundary object that manages the dialogue between the actor and the system
The control object is represented by the lifeline :AddAdvert and this manages the overall object communication.

17. Control lifeline Boundary lifeline :Client :Campaign :Advert
listCampaigns
listAdverts
Advert
newAd:Advert
:CampaignManager
sd Add a new advert to a campaign
loop
:AddAdvertUI
:AddAdvert
addNewAdvert
getClient
selectClient
selectCampaign
getCampaignDetails
startInterface
[For all clients]
showClientCampaigns
showCampaignAdverts
createNewAdvert
[For all client’s campaigns]
[For all campaign’s adverts]
getAdvertDetails
Boundary lifeline

18. Sequence Diagrams: Reflexive Message
:Client
:Campaign
:Advert
getName
listCampaigns
checkCampaignBudget
getCampaignDetails
getCost
:CampaignManager
sd Check campaign budget
loop
getOverheads
[For all client’s campaigns]
[For all campaign’s adverts]
Reflexive message
A reflexive message that an object sends to itself is shown by a message arrow that starts and finishes at the same object lifeline.
Figure 9.7
on p. 268
An object can send a message to itself. 19 19

19. Sequence Diagrams: Flow Control
Sequencing is shown by arranging messages vertically in the order of their occurrences
Recall: the vertical dimension represents time.
Selection (branching) is shown by a combined fragment rectangle with the interaction operator ‘alt’ (a short form of alternatives)
The alt combined fragment has two (or more) compartments known as operands.
Each operand corresponds to one of the alternatives in the combined fragment and should have an interaction constraint to indicate under what conditions it executes.
Iteration (looping) is shown by a combined fragment rectangle with the interaction operator ‘loop’
The interaction in the loop combined fragment repeats as long as the guard condition in the interaction constraint evaluates as true. 22 22

20. Figure 9.16 on p. 275 alt interaction operator shows branching
:Client
:Campaign
:Advert
getName
listCampaigns
ref
:CampaignManager
Advert
addCostedAdvert
newAd:Advert
newRequest:Request
alt
[else]
sd Add a new advert to a campaign if within budget
List client campaigns
[totalCost <= budget]
Request
Get campaign budget
alt interaction operator
shows branching
Two interaction operands,
one for each alternative
Figure on p. 275 23 23

21. Figure 9.3 on p. 263 Frame label 24 Lifeline Activation or Execution
:Client
:Campaign
:Advert
getName
listCampaigns
listAdverts
Advert
newAd:Advert
addNewAdvert
Lifeline
Activation or Execution
Object creation
:CampaignManager
sd Add a new advert to a campaign
loop
Interaction Operator
Interaction Constraint
Combined Fragment
(loop)
[For all client’s campaigns]
getCampaignDetails
[For all campaign’s adverts]
getAdvertDetails
A sequence diagram
is enclosed in a
frame
Frame label 24 24

22. 25

23. Sequence Diagrams: Object Creation and Destruction
Figure 9.6 on p. 267
listAdverts
deleteAdvert
Object destruction
:Campaign
:Advert
delete
getAdvertDetails X
loop
sd Delete advert
Object creation is shown with the construction message (dashed arrow) going to the object symbol.
Object destruction is indicated by a large X on the lifeline on the destruction point. 26 26

24. Sequence Diagrams: Handling Complexity
Complexity interactions can be modelled using various different techniques
Interaction fragments
Lifelines for subsystems or groups of objects
Continuations
Interaction Overview Diagrams
Interaction occurrences and 27 27

25. Interaction fragments: Example
:Client
:Campaign
:Advert
getName
listCampaigns
:CampaignManager
sd Check campaign budget
ref
Gate showing the message enter this interaction occurrence
List client campaigns
Get campaign budget
ref interaction operator
indicates interaction occurrence that references an interaction fragment
:Client
:Campaign
listCampaigns
getCampaignDetails
sd List client campaigns
loop
Gate showing the message enter this Interaction Fragment
[For all client’s campaigns]
Interaction fragment that is referenced in the “Check campaign budget” sequence diagram 28 28

26. Interaction Fragment example (cont’d)
Interaction fragment that is also referenced in
Check campaign budget sequence diagram
:Campaign
:Advert
getCost
sd Get campaign budget
loop
getOverheads
checkCampaignBudget
:CampaignManager
[For all campaign’s adverts]

27. Lifelines for subsystems or groups of objects
:Client
:Campaign
:Advert
listCampaigns
listAdverts
Advert
newAd:Advert
:CampaignManager
sd Add a new advert to a campaign
loop
:AddAdvertUI
:AddAdvert
addNewAdvert
getClient
selectClient
selectCampaign
getCampaignDetails
startInterface
[For all clients]
showClientCampaigns
showCampaignAdverts
createNewAdvert
[For all client’s campaigns]
[For all campaign’s adverts]
getAdvertDetails
Boundary lifeline
Control lifeline
Lifelines for subsystems or groups of objects
Complex interactions can also be split up using lifelines to represent groups objects and their interactions or to present subsystems.

28. Lifelines for subsystems or groups of objects
:ClientCampaigns
ref ClientCampaignAds
listCampaigns
listAdverts
:CampaignManager
sd Add a new advert to a campaign
:AddAdvertUI
:AddAdvert
addNewAdvert
getClient
selectClient
loop
selectCampaign
[For all clients]
showClientCampaigns
startInterface
showCampaignAdverts
createNewAdvert
Lifelines for subsystems or groups of objects
Lifeline representing the interaction between a group of objects
Complex interactions can also be split up using lifelines to represent groups objects and their interactions or to present subsystems.
Complex interactions can also be split up using lifelines to represent groups objects and their interactions or to present subsystems.

29. Sequence diagram referenced in the
:Client
:Campaign
:Advert
listCampaigns
listAdverts
Advert
newAd:Advert
sd ClientCampaignAds
loop
addNewAdvert
getClient
[For all client’s campaigns]
getCampaignDetails
[For all campaign’s adverts]
getAdvertDetails
Sequence diagram referenced in the
Add a new advert to a campaign sequence diagram

30. Using Continuations NB: Dashed arrows are NOT part of UML!
getCost
Within budget
Budget spent
:LifelineA
:LifelineB
:LifelineC
alt
sd Calculate costs
[else]
ref
Identify under spend
[Within budget]
sd Authorise expenditure
authorize
stopExpenditure
Calculate costs
NB: Dashed arrows are NOT part of UML!
Continuations are used to link sequence diagrams

31. Communication Diagrams
Communication diagrams hold the same information as sequence diagrams
Can be drawn at various levels of detail and during different stages in the system development process
Communication diagrams show links between objects that participate in the collaboration
offer a view of object interactions that is easy to relate to the underlying collaborations because of visibility of links between objects (lifelines)
No time dimension, sequence order is captured with sequence numbers
Not suitable for complex interactions 34 34

32. Communication Diagrams: Notation
Message order is captured with sequence numbers, which are written in a nested style to indicate the nesting of control within the interaction that is being modelled:
e.g., message occurs after message 3.1.1, and both are contained with the activation of message 3.1
:Campaign
anAdvert:Advert
getCost
currentAdvertCost = anAdvert.getCost() 35 35

33. Control lifeline Boundary lifeline :Client :Campaign :Advert
listCampaigns
listAdverts
Advert
newAd:Advert
:CampaignManager
sd Add a new advert to a campaign
loop
:AddAdvertUI
:AddAdvert
addNewAdvert
getClient
selectClient
selectCampaign
getCampaignDetails
startInterface
[For all clients]
showClientCampaigns
showCampaignAdverts
createNewAdvert
[For all client’s campaigns]
[For all campaign’s adverts]
getAdvertDetails
Boundary lifeline

34. Communication Diagrams: Example
Figure 9.21 on p. 282
: Advert
4.1.2 *[For all campaign’s adverts]:
getAdvertDetails
5.1.1: addNewAdvert
:AddAdvertUI
:AddAdvert
:Client
:Campaign
:Advert
3.1: showClientCampaigns
3: selectClient
4: selectCampaign
4.1: showCampaignAdverts
4.1.1: listAdverts
5: createNewAdvert
5.1: addNewAdvert
newAd:Advert
2: startInterface
3.1.1: listCampaigns
1 *[For all clients]: getClient
:CampaignManager
sd Add a new advert to a campaign
3.1.2 *[For all client’s campaigns]: getCampaignDetails 37 37

35. Communication Diagrams: Message Labels
Type of message
Syntax example
Simple message.
4: addNewAdvert
Nested call with return value.
The return value is placed in the variable name.
3.1.2: name = getName
Conditional message.
This message is only sent if the condition [balance > 0] is true.
5 [balance > 0]: debit(amount)
Iteration
4.1 *[For all adverts]: getCost 38 38

36. sd Check campaign budget
:Client
:Campaign
:Advert
getName
listCampaigns
checkCampaignBudget
getCampaignDetails
getCost
:CampaignManager
sd Check campaign budget
loop
getOverheads
[For all client’s campaigns]
[For all campaign’s adverts]
:Campaign
:Advert
checkCampaignBudget
1*[For all adverts]: getCost
2: getOverheads
The direction in which a
link can be navigated
may be shown if required.
sd checkCampaignBudget

37. Model Consistency
The communication/sequence diagrams should be mutually consistent with the class diagrams
The allocation of operations to objects must be consistent with the class diagram and the message signature must match that of the operation
Can be enforced through CASE tools
Every sending object must have the object reference for the destination object
Either an association exists between the classes or another object passes the reference to the sender
Message pathways should be carefully analysed.
This issue is key in determining association design 40 40

38. Take Home Messages Object Interaction and Collaboration CRC Cards
Communication Diagrams
Sequence Diagrams
Model Consistency
In this lecture you have learned about … 41 41