1. Use-Cases

2. Outline
Introduction
Use Case Diagrams
Writing Use Cases

3. Use Cases Register User
Illustration
Register User
admin
Use case in diagram
Use Case in script
A use case is a contract of an interaction between the system and an actor.
A full use-case model comprise of:
A diagram, describing relations between use-cases and actors.
A document describing the use case in details
Use cases are essentially a contract, which can be used for testing.
More use cases in the cellphone example:
Cell migration
HTML sites viewing
integration …

4. Use Case Diagram Objective
Create a semi-formal model of the functional requirements
Analyze and define:
Scope
External interfaces
Scenarios and reactions

5. Outline Introduction Use Case Diagrams Writing Use Cases
Guidelines for Effective Use Cases

6. What’s in a Use Case Model?
Diagram
Use Case
Defines a sequence of actions performed by a system that yields an observable result of value to the actor
Actor
Someone or something outside the system acting in a role that interacts with the system. Could be a human or another system.
Actor
Detailed textual actor and Use Case report descriptions (99% of the effort)
Summarised as Use Case Model diagram (1% of the effort)
Use Case

7. A Simple Example
Example
Actors
System boundary
Association
Use Case

8. Use Case Model: Identify
Example use case:
Name: Submit Timesheet
Brief Description: The consultant enters details of the number of hours worked and the activities worked on, then confirms and submits the timesheet.
Each Actor and Use Case needs a brief description.
Actor
Briefly state its role, goals and responsibilities in relation to the system.
Use Case
Briefly summarise what takes place in the use case, giving an indication of its scope.
Example actor:
Name: Consultant
Brief Description: The consultant updates their daily timesheet and expense information and gets status reports.
Consultant
System
Project Manager
Submit Timesheet
Complete
Timesheets
Use Case Name
The name should be:
Brief
Unique
Written as an active verb
Meaningful
You should be able to tell what the actor wants to achieve in the use case from its name.
Project Accounting

9. Linking Use-Cases
Three mechanism are used:
Include
Extend
Inheritance

10. Alistair Cockburn “Writing Effective Use Cases”
Use-Case Levels
Base Use Case: Used directly by the user
Perform Sale
User goals
Sub-functionality
Choose Products
Fill-in billing info
One of the main utilizations for linking mechanisms is to distinguish between use-case levels.
Alistair Cockburn “Writing Effective Use Cases”

11. The “Include” Construct
Include is used when:
Decomposing complicated behavior
Centralizing common behavior
The base use case explicitly incorporates the behavior of another use case at a location specified in the base.
Example
Perform Sale
Fill-in billing info
<<include>>

12. Extend – Graphical Representation
The base use case can incorporate another use case at certain points, called extension points.
Note the direction of the arrow
The base use-case does not know which use-case extends it
Example
<<extend>>
Product is a gift
Gift wrap Products
Perform Sale
After checkout

13. Example: Amazon Shopping Cart Product Page Review Writing
TODO: add a search page before the product page
Think of Amazon, which functions are there. List them, and try to create a use-case from them.
Review Writing

14. Example – cont’d Write Review Rank Supplier Search Product
«extend»
Search Product
View Product Details
«include»
Write Review
After page generation
Navigate Deals
«extend»
«include»
Add to cart
«extend»
Checkout
«include»
«extend»
user is not
a member
Important points: Note that weak relations between use-cases are not mentioned. For instance, add to cart provides products to checkout, but the relation is not modeled. Also, the hierarchy between use-cases originates from the user perspective: Even though “View Product Details” is the central component from the programmers point of view, the user wants to search products or to see deals, and that the top use-case.
Use extend when:
You want to leave the important UC as simple as possible
For flexibility: we want to be able to add more features to important UCses.
When the extending UC is rare, it does not always happen.
Login
Register
Handle Order Status
«include»

15. Outline Introduction Use Case Diagrams Writing Use Cases
Guidelines for Effective Use Cases

16. Structure of a Use Case Specification
Name
Actors
Trigger
Preconditions
Post conditions
Success Scenario
Alternatives flows

17. Triggers What starts the use-case? Examples: Customer reports a claim
Customer inserts card
System clock is 10:00pm

18. Preconditions
What the system needs to be true before running the use-case.
Examples
User account exists
User has enough money in her account
There is enough disk space

19. Post-Conditions A post-condition is the outcome of the use-case.
Examples
Money was transferred to the user account
User is logged in
The file is saved to the hard-disk

20. Success Scenario
The success scenario is the main story-line of the use-case
It is written under the assumption that everything is okay, no errors or problems occur, and it leads directly to the desired outcome of the use-case
It is composed of a sequence of action steps
Example:
Interaction step
Administrator enters course name, code and description
System validates course code
System adds the course to the db and shows a confirmation message
Validation Step
Internal Change Step
(plus) Interaction Step

21. Use-Cases – Common Mistakes
Complex diagram
No system
No actor
Too many user interface details
“User types ID and password, clicks OK or hits Enter”
Very low goal details
User provides name
User provides address
User provides telephone number …

22. Alternative Flows Starting points
Success
Scenario
Used to describe exceptional functionality
Examples:
Errors
Unusual or rare cases
Failures
Starting points
Endpoints
Shortcuts
Shortcuts
Exceptions
Endpoints

23. Alternative Flows - Example
Errors:
“Case did not eject properly”
“Any network error occurred during steps 4-7”
“Any type of error occurred”
Unusual or rare cases
“Credit card is defined as stolen”
“User selects to add a new word to the dictionary”
Endpoints
“The system detects no more open issues”
Shortcuts:
“The user can leave the use-case by clicking on the “esc” key
An alternative success path (user uses a shortcut)
A sequential behavior had ended (End-of-file)
The first stage of a sequential behavior
Actor behaves incorrectly (invalid course ID)
Inaction by an actor (timeout)
Every validation step (system validates…)
Internal system failure, which is part of a normal business scenario (printer is stuck, network is unavailable)
Unexpected and abnormal internal failure (no more memory)

24. Exercise I am the manager of a theatre.
I want to create an automated movie ticket machine.
You are analysts who need to describe what the customer wants as a set of use cases
Simplifying assumptions:
One movie showing at a time
Movie time is same every day, only one time, same price
Only manager can change/add movie
Customer can only buy tickets
Who or what are the actors?
What are the use cases (goals of actors)?

25. Use case diagram for Movie Ticket Machine
Why are there three Actors?
Why three use cases for Customer?
Which use cases look easy to write?

26. Use cases for Manager Use case: Set title Actors: Manager, Machine
1. Manager requests a change of movie title
2. Machine asks manager for new movie title
3. Manager enters movie title
Use case: Set price
1. Manager requests a change of ticket price
2. Machine asks manager for new price for movie title
3. Manager enters ticket price
Alternatives: Invalid price
If manager enters price below $1 or greater than $10
3a. Machine asks manager to reenter price
Use case: Set seats
1. Manager requests a change in number of seats
2. Machine asks manager for number of seats in theatre
3. Manager enters number of seats
Alternatives: Invalid number of seats
If manager enters number less than 20 or greater than 999
3a. Machine asks manager to reenter number of seats

27. Use cases for Customer Use case: # of tickets
Actors: Customer, Machine
1. Customer enters number of tickets
2. Machine displays total balance due
Alternative: Customer wants zero tickets
At step 1, customer enters zero tickets
1a. Display thank you message
1b. Set balance to $0.0
Use case: Return change to customer
1. Customer requests change
2. Machine dispenses money
3. Machine updates customer balance
Use case: Buy tickets
Actors: Customer, Machine
1. Customer requests tickets
2. Machine tells customer to put balance due in money slot
3. Customer enters money in money slot
4. Machine updates customer balance
5. Customer requests tickets
6. Machine prints tickets
7. Machine updates number of seats
Alternative: Insufficient seats
At step 1, if number of tickets requested is less than available seats,
1a. Display message and end use case
Alternative: Insufficient funds
At step 5, if money entered < total cost,
5a. Display insufficient amount entered
5b. Go to step 3

28. In Summary
Disciplined documentation of use case flows makes testing easier
Use case flows combine as use case scenarios
Use Case scenarios become test scenarios
Test scenarios + data = test cases