1. The Object-Oriented Approach to Requirements
ITEC 3010 “Systems Analysis and Design, I”
LECTURE 7:
The Object-Oriented Approach to Requirements
[Prof. Peter Khaiter]

2. Lecture Outline Unified Modeling Language (UML)
Object-Oriented Requirements (OOR)
System Activities
Use Case Diagram
Activity Diagram
System Sequence Diagram
State Machine Diagram
Integrating Object-Oriented Models

3. Unified Modeling Language (UML)
• OOA considers an IS as a set of objects that work together to carry out the function. OO analysis involves two steps:
(1) Definition the set of objects that will make up the system and describing the interactions or communications among the various objects (these interactions take the form of messages between objects)
(2) Describing the internal processes that go on within each object to respond to messages from other objects, and to initiate messages to other objects
• UML is a notation used in OO modeling
• A preliminary version of UML was presented in 1995
• In 1997 UML was presented to the Object Management Group (OMG) as a standard modeling technique
• OMG (est. 1989) is a consortium of software vendors, developers and organization with the goal to provide a common architectural framework for OO applications –

4. Object-Oriented Requirements (OOR)
Object-oriented system requirements are specified and documented through process of building models
Modeling process starts with identification of use cases and problem domain classes (things in users’ work environment)‏
Business events trigger elementary business processes (EBP) that new system must address as use cases
Use cases define functional requirements

5. OOR - Models
OO models (or diagrams) to define system requirements are called use case models:
• Use case diagram shows the various user roles and how they will use the system. It is an extension of the event table. It is a convenient way to document the function that the system must support.
• Sequence diagram shows inputs and outputs and sequence of messages between an external actor and the system objects during a use case or scenario.
• The flow of information between the objects is in form of messages
OO models (or diagrams) to define classes of objects and their states are called domain models :
• Domain class diagram (see lecture 5) – identifies and classifies the objects that will make up the new system along with their properties (or attributes)
• State machine diagram describes the states and behavior of each object: life of an object in state and transitions

6. The System Activities— A Use Case/Scenario View
Use case model used to identify and define all elementary business processes that system must support
Use cases are identified at two levels:
Overview level (event table and use case diagram of all the use cases)
Detailed level (for each use case – use case description, activity diagram, sequence diagram)
Use case – an activity a system carried out, usually in response to a user request
Actor (person, other system, or device that receive services from the system and who actually interact with the system)
Role played by user
Outside automation boundary

7. Techniques for Identifying Use Cases (Review from Chapter 5)‏
Identify user goals
Each goal at the elementary business process (EBP) level is a use case
EBP – task performed by one user in one place and in response to business event that adds measurable business value, and leaves system and data in consistent state
Event decomposition technique (event table)‏
CRUD analysis technique (create, read/report, update, delete) to ensure coverage

8. Use Case Diagram
Graphical UML diagram that summarizes information about actors and use cases
Simple diagram shows overview of functional requirements
Symbols: use case is symbolized by an oval with the name of the use case inside; stick figure represents an actor (i.e., a role)
Two approaches to use case diagrams
By subsystem
By actor

9. Simple Use Case with an Actor

10. Use Case Diagram with Automation Boundary and Alternate Actor Notation

11. All Use Cases Involving Customer as Actor
Direct access
to the system

12. A use case diagram of the CSS for RMO organizes by subsystems

13. Use Cases of RMO Order Entry Subsystem
A packaged notation is used: a tabbed rectangle that groups similar
component together

14. <<Includes>> Relationship
Documents situation in which one use case requires the services of a common subroutine
Another use case is developed for this common subroutine
A common use case can be reused by multiple use cases
E.g., two use cases “Create new order” and “Update order” may need to validate the customer account. A common use case may be defined to carry out this function

15. Example of Order-Entry Subsystem with <<Includes>> Use Cases

16. Use Case Diagram vs. Structured Techniques
• Objective of the use case diagram is to provide an overview of the system (including actors and the functions they perform)
– A use case diagram is like a context diagram in the sense of defining the scope of the system
– However, individual use cases appear more like a DFD fragment in that they identify an individual function that the system must support
• Difference with structured techniques
– The use case diagram begins by defining the automation boundary while in DFD this boundary is often not defined until the entire process has been detailed
– In a DFD an external agent is always the original source or destination of the information and may not necessarily be the one interacting with the system;
– In a use case diagram, an actor is the one who actually interacts with the system whether or not that actor is the original source of information

17. Use Case Diagram vs. Structured Techniques (cont’d)
Example: in the RMO case, a customer may call an RMO clerk and make an order.
The DFD would identify the external entity as the customer; the clerk’s activity would be embedded in a process “Enter customer order”.
In the use case diagram, the clerk would be identified as the actor who uses the system to enter or create a customer order
The use case diagram does not indicate data flows, i.e. information flowing into and out of the system is not shown (this is done with the next level of modeling – the sequence diagrams)
Important to identify every possible role that will use the system.

18. Developing a Use Case Diagram
Two main approaches to developing a use case diagram:
First approach: start with an event table and analyze each event to determine:
• How the system is used to support the event
• The actors who initiate the event
• Other use cases that will need to be invoked
• Generally, each event will become a use case
Second approach: identifying all the actors who use the system and the EBPs with the user goal technique
Actors must actually contact the system
Assume perfect technology condition (no technical activities!)
Actors – are roles played by users (e.g., order clerk, manager)
Identify goal for each actor (e.g., “accept a return”)
Finalize with a CRUD analysis to ensure completeness (every class has sufficient use cases to support its all methods) – a cross-check!

19. Scenarios
• A use case only shows that an actor interacts with the system to carry out a business activity
• There may be a whole series of individual steps to accomplish the use case (these steps are described with a narrative called a flow of activities) - steps within a use case
• A use case may have several alternative internal activities. In the RMO example, for the use case “Create new order” there are at least two sequences when:
A customer creates telephone order through clerk (a clerk interacts with the system), or
A customer creates web order (a customer interacts with the system)
• A particular sequence of activities within a use case is called scenario. It represents a unique path through the use case
E.g., two possible scenarios for the RMO use case “Create new order”: in the first instance, the second actor “Order clerk” is involved who is actually interfacing with the system

20. Two scenarios for RMO
“Create new order”

21. Activity Diagrams
Used to document workflow of business process activities for each use case or scenario
Standard UML 2.0 diagram as seen in Chapter 4
Can support any level of use case description; a supplement to use case descriptions
Helpful in developing system sequence diagrams

22. Activity Diagram— Telephone Order Scenario

23. Activity Diagram— Web Order Scenario

24. Identifying Inputs and Outputs— The System Sequence Diagram
Interaction diagram – a communication diagram or a sequence diagram
System sequence diagram (SSD) is type of UML 2.0 interaction diagram
Used to model input and output messaging requirements for a use case or scenario
Shows sequence of interactions as messages to/from actors and internal objects during flow of activities
Emphasis: how the actor interacts with the system by entering input data and receiving output data
Object notation instead of class notation is used to show that message is sent to an individual object, not the class
System is shown as one object: a “black box”

25. Object and class names in SSD

26. SSD Notation
Lifeline or object lifeline is a vertical dashed line under object or actor to show the sequence of messages over time: from top to bottom
Message is labelled on arrows to show messages sent to or received by actor or system, including input data: request operation in the destination object
Actor is role interacting with the system with messages
Object is the component that interacts with actors and other objects

27. System Sequence Diagram (SSD) Notation

28. SSD Notation
Activation line

29. Vertical line under object or actor If vertical line dashed
SSD Lifelines
Vertical line under object or actor
Shows passage of time
If vertical line dashed
Creation and destruction of thing is not important for scenario
Long narrow rectangles
Activation lifelines emphasize that object is active only during part of scenario

30. SSD Messages
Internal events identified by the flow of objects in a scenario
Requests from one actor or object to another to do some action
Invoke a particular method
Input message – solid line
Return message – dashed line
Complete syntax:
* [true/false condition] return-value := message-name (parameter list) 

31. SSD for the RMO Look up item
availability

32. Repeating Message

33. Developing a System Sequence Diagram
Begin with detailed description of use case from fully developed form or activity diagram
Identify input messages
Describe message from external actor to system using message notation
Identify and add any special conditions on input message, including iteration and true/false conditions
Identify and add output return messages

34. Activity Diagram of the Telephone Order Scenario

35. Resulting SSD for the Telephone Order Scenario

36. SSD of the Web Order Scenario for the Create New Order Use case

37. The relationship of use case diagram, class diagram and sequence diagrams
SSD includes objects from the class diagram and actors from the use case diagram

38. Developing a System Sequence Diagram: Steps
1. Identify all the objects and actors that are involved in the scenario (they are only actors have been identified in the use case diagram and objects from the classes that are identified in the class diagram)
2. Based on the flow of activities, identify each message that will be required to carry out the scenario along with both source object (or actor) for the message and the destination object (or actor)
3. Determine whether each message is always sent or sent only under certain conditions
4. Sequence the messages correctly and attach them to the appropriate lifelines of actors and objects
5. Add the formal syntax on the messages for conditions, message names and passed parameters
6. Add response messages and communications

39. A complete SSD for the RMO use case Create new order

40. Just for Fun!

41. Identifying Object Behaviour— The State Machine Diagram (SMD)
SSDs give external view of object behaviour – messages passed around but do not show, what an object does when it gets a message
There is a need to specify internal logic for each object, i.e., a description of the actions that the objects perform themselves
State machine diagram is UML a 2.0 diagram that models object internal behaviour, states and transitions
Complex problem domain classes can be modeled
Each object is an instance of a class, and comes into existence in some manner
During its existence it is “in” certain states and makes transitions from state to state
These states and the changes an object makes from state to state are shown in SMD (two main symbols: state and transitions)

42. Identifying Object Behaviour— The State Machine Diagram
State of an object
A condition that occurs during its life when it satisfies some criterion, performs some action, or waits for an event
Each state has unique name (e.g., “on”, “working”, “loading equipment”, etc.) and is a semipermanent condition or status (because external events can interrupt them)
Action is an activity performed by an object in a particular state
A state is represented by a rectangle with rounded corners (with the name of the state inside)
Any actions that must be performed during the period of the state are placed below the state name in the rectangle
Transition - movement of an object from one state to another state
Considered to be short in duration and cannot be interrupted
Once started, a transition runs to completion by taking the object to the new state (called destination state)

43. Simple SMD for a Printer

44. State Machine Terminology
Pseudostate – the starting point of a state machine, indicated by a black dot
Origin state – the original state of an object from which the transition occurs
Destination state – the state to which an object moves after the completion of a transition
Message event – the trigger for a transition, which causes the object to leave the origin state
Guard condition – a true/false test to see whether a transition can fire
Action expression – a description of the activities performed as part of a transition

45. Composite States and Concurrency—States within a State

46. Concurrent Paths for Printer in the On State

47. Relationships among OO models

48. State Machine Diagram and SSD
•Each column is labelled with the name of a class (the columns identify all of the classes in the system); each row is for one sequence diagram; the Xs in the cells show which classes are involved in which sequence diagram).
• An SMD for a class is based on the entire column for that class
• Every cell with X provides information about the messages to and from the class

49. Rules for Developing State Machine Diagram
Review domain class diagram, select important ones, and list all state and exit conditions
Begin building state machine diagram fragments for each class
Sequence fragments in correct order and review for independent and concurrent paths
Expand each transition with message event, guard-condition, and action-expression
Review and test each state machine diagram

50. States and Exit Transitions for OrderItem

51. Partial State Machine for OrderItem

52. Final State Machine for OrderItem

53. Order Domain Class for RMO— States and Exit Transitions

54. First-Cut State Machine Diagram for Order

55. Second-Cut State Machine Diagram for Order

56. Integrating Object-Oriented Models
Complete use case diagram is needed to understand total scope of new system
Domain model class diagrams should also be as complete as possible for entire system
With iterative approach, construct use case descriptions, activity diagrams, and system sequence diagrams for use cases in iteration
Development of a new diagram often helps refine and correct previous diagrams

57. Relationships Between OO Requirements Models

58. Readings
Today’s lecture: Chapter 7 – “The Object-Oriented Approach to Requirements”
For next lecture: Chapter 8 – “Evaluating Alternatives for Requirements, Environments, and Implementation”
Thank you !!!