OOA2'1

1
Use Cases and Scenarios

• Sources
• "Object Models", Peter Coad
• "Object-Oriented Software Engineering", Ivar
  Jacobson
• UML Distilled, Martin Fowler

2
Product Mapping
Functional Requirement
Data Requirement
Use Case
helps id
breaks down into one or more
mapped to
helps id
mapped to
Use Case Path
ids
Domain Class
used to implement
Concrete Analysis Scenario
Event Trace
tested by
Class Design
ids details for
used to implement
Concrete Design Scenario
Message Trace
tested by
Code
ids context for
3
Defining System Purpose and Features

• Brief statement of the system purpose (25 words
  or less)
• Why this system
• Why now
• List of Features
• Types
• Log important information
• Conduct business
• Analyze business results
• Interact with other systems
• Identify best and worst features of comparable
  systems
• Prioritize
• Concentrate on the top 10

4
Example Small Library SystemPurpose and Features

• The purpose of the Library System is to assist
  borrowers using the library and the library
  managing check-out and check-in of media items.
• Specific features of the Library System will
  include
• maintain a catalog of all media items to assist
  locating information
• maintain a history of checked-out media items for
  a borrower and for a media topic
• maintain a history of media checked-in and out,
  per media topic and borrower
• produce late notifications and fines for over-due
  media

5
Defining Use Cases

• Actors - what exists outside the system
• Actor term used by Jacobson and Rumbaugh. Maps
  more to Coad's "Participant".
• Use Cases - what should be performed by the
  system

6
Actors (External "Participants"/"Roles")

• Roles played by prospective users of the system
• Models anything that needs to exchange
  information with the system
• Can be a person
• Can be a communicating system
• An external object can play the role of one or
  more different actors

7
Use Cases

• Defines the functionality within the system
• Complete course of events between the actor and
  the system
• Locating Use Cases
• What are the main tasks of each actor?
• Will the actor have to read/write/modify system
  information?
• Will the actor have to input outside information?
• Does the actor wish to be informed about
  unexpected changes?
• Samples
• Locate Media
• Add Media
• Check-out Media
• Check-in Media
• Remove Media
• Add New Borrowers
• Remove Borrowers
• Issue Overdue Notices
• Pay Fines

8
Defining Use Cases

• Start with identification and then add details
• Use Cases are made up of one basic course and
  several alternate courses
• Basic Course - most important course for a use
  case
• Alternate Courses - variants to the basic course
  for special error conditions
• Add details to the alternate courses only after
  model stabilizes
• Add details independently with relation to time

9
Example Use Case Locate Media

• Primary Path (Media Located) The librarian and
  borrower (researchers) will both search for media
  topics in the library through a computer display.
  The topics can be located by type, title, author,
  keywords in the description as well as specific
  media type selections (ex software format). The
  results of the search are displayed to the
  researcher in summary format. The detailed
  descriptions of each media topic can then be
  displayed as well as printed.
• Alternate Path (Media not Located) The system
  does not locate the media. The researcher is
  informed of this result and can then edit the
  previous query or cancel the request.

10
Example Use Case Add Media

• Primary Path (Add Media for New Topic) The
  librarian will interact with a computer display
  to enter in a new media topic and copy. The
  librarian will enter the title, author, and
  description for the media topic and serial
  numbers for each new copy. The librarian will
  also be able to enter the following information
  for specific types of media topic
• Books pages
• Software format and version
• Video Tapes format and rating
• The new media is entered into the library
  system and is now available for borrowers to
  search for and check-out.
• Alternate Path (Add Media for Existing Topic)
  The librarian will search for the media topic by
  title and author and add the new serial numbers
  for the additional copies.

11
Defining Use Case Relationships

• Uses
• extracted common behavior
• breakdown of complex behavior
• Extends
• additional sub-course of events
• complex alternate path

12
Library Use Case Model (Jacobson)
13
Library Use Case Model (UML)
Issue Late Notices
ltltextendsgtgt
Add Borrowers
ltltextendsgtgt
Update Borrowers
External Librarian
Remove Borrowers
ltltextendsgtgt
ltltextendsgtgt
ltltextendsgtgt
Credit Payments
Checkout Media
Checkin Media
ltltextendsgtgt
ltltextendsgtgt
ltltextendsgtgt
Update Media
ltltextendsgtgt
ltltusesgtgt
Add Media
External Browser
Locate Media
Remove Media
14
Demonstrating Coverage with Scenarios

• Scenarios
• UML Model
• Sequence Diagram
• Collaboration Diagram
• Content
• Analysis Events (Event Trace)
• Detailed Design Methods (Method Trace)
• Method
• Select Key Use Case Paths
• Start with Problem Domain
• Work backwards to Human Interaction
• Work forwards to (external) System Interfaces
• Add Data Management where necessary
• Act out interactions

15
Sequence Diagram Event Trace
objects
time
OutputDevice
Device Player
External Listener
1 Initialize ( )
Message
2 Play ( )
3 Audio ( )
4 Terminate ( )
Return Message (optional)
Activation (focus of control)
Label
16
Scenario Add Media Topic

• The librarian will add a new book to the library
  by creating a new book media topic through a card
  catalog window. In creating the media copy
  (book), the librarian will supply parameters of
  title, description, author, and pages. An
  existing topic is searched for in the existing
  topics. The successful addition is reported to
  the librarian. The librarian can then add a new
  media copy for each copy of the book (refer to
  Add New MediaCopy).

17
Scenario Add Media Copy

• The librarian first requests the media topic from
  the card catalog window. A set of search criteria
  is filled in and used to locate the media topic
  in the card catalog. Once found, the media topic
  can have many operations performed on it. In this
  instance, the librarian requests that a new copy
  be added. The librarian will supply a bar code
  for the new copy. The media topic adds the new
  copy and the librarian is informed of the
  successful completion.

18
Some Points

• Don't ask an object what type it is - let the
  object worry about its kind
• Search for an object and then interact with it -
  simplifies intervening objects
• suggests interacting with desired object rather
  than cascading messages
• Allow sensing objects act rather that poll for
  status
• Events in a scenario must match class model
• Simplify complex or multiple event interactions
• Limited detail/concern (defer to design)
• creation and deletion of objects
• locating objects

19
Allocating Requirements

• System/Software Requirements
• Extract Essential Details
• Allocation
• Static Requirements - Subsystem/Domain Class
• Functional Requirements - Use Case/Use Case Path
• Design Requirements
• Between subsystems or organizations
• Allocation
• Static Requirements - Unit/Design Class
• Functional Requirements - Event Trace