1. IS0514Slide 1 IS0514 Lecture Week 4 Use Case Modelling (2)

2. IS0514Slide 2 Today's Lecture Revision – use cases Use case scenarios What is a use case description? Use / misuse of use cases

3. IS0514Slide 3 Exercise 1 Issues to consider.... In groups of 3-4 spend 5 minutes discussing –What do Use Case Diagrams show you? –How do they help in understanding problems? –What is missing from Use Case Diagrams? At the end of this time please be ready to share your findings

4. IS0514Slide 4 Exercise 1 - Answer What do Use Case Diagrams show you? 1.Who / what uses a system 2.What high level functions are required How do they help in understanding problems? 1.Provide a high level view of required functionality 2.This enables discussion of what is required without any detail What is missing from Use Case Diagrams? 1.The detailed requirements 2.How processes work

5. IS0514Slide 5 Use Case Description Use Case Description is –A detailed description of a use case –Will be text Typically in the form of a use case template (more later) –Can include: Models -Activity diagrams (next year) User interface designs (next year) Other documents that help describe what happens in a use case (next year) Describes what is required to satisfy the use case

6. IS0514Slide 6 Use Case Modeling One Approach Develop Scenarios Develop Use Case Diagram Develop Use Cases (Descriptions) Scenarios are different paths through a use case This is the process suggested by the unified process

7. IS0514Slide 7 Use Case Modeling Identify actors Identify use cases Link actors to use cases Show the system boundary Identify relationships between use cases Consider scenarios Write use case descriptions Validate with stakeholders

8. IS0514Slide 8 Scenarios Unfortunately this is a widely used and often ambiguous term –You will come across the term scenario to mean a domain description –Sometimes it is synonymous with use case In UML the term scenario refers to a single path through a use case A use case usually has several associated scenarios (although it can have only one)

9. IS0514Slide 9 Identifying Scenarios Consider all the possibilities in the use case Identify the normal (primary) scenario associated with the use case –What typically happens –Ignore any complications Identify the exceptions to the normal action:- –Alternative scenarios Things that could happen –Exceptional scenarios Things that could be considered errors that need to be caught Each scenario should be presented as a series of simple numbered steps in text format

10. IS0514Slide 10 Simple Scenario – Make Tea Normal Scenario: 1.Switch on kettle to boil water. [A1: Kettle empty] 2.Place tea bag and milk in mug. 3.When kettle has boiled pour boiling water into mug. 4.Let tea brew and then remove tea bag and put tea bag in the bin. Alternative Scenarios: A1. Kettle empty 1.Fill Kettle with water. 2.Use case continues from step 1. Exceptional Scenarios: E1. No milk 1.Sorry, have tea without milk. 2.Use case terminates. What other scenarios are there?

11. IS0514Slide 11 Exercise 2 – Use Case Scenarios Remember Space Invaders One use case was “move left” Identify –Normal Scenario –Alternative Scenario –Exceptional Scenarios http://www.neave.com/games/invaders/

12. IS0514Slide 12 Exercise 2 - Use Case Scenarios (for the Move Left use case) Normal Scenario1.Player indicates that the base moves to left 2.The system moves the base left Alternative Scenario1.Player indicates that the base moves to left 2.A bullet arrives where the base has moved to 3.The base explodes Exceptional Scenario1.Player indicates that the base moves to left 2.The base is at the extreme left of the screen 3.The base stays where it is

13. IS0514Slide 13 Use Case (Description) includes : Title/Heading Summary Actors Trigger condition Primary Scenario Alternative Scenarios Exceptional Scenarios Options: –pre-conditions –post-conditions –assumptions

14. IS0514Slide 14 Exercise 3 Look at the example use case description “Move Left” Attempt to write a use case description “Move Right”

15. IS0514Slide 15 Exercise 3 - Answer Very similar to “Move Left”

16. IS0514Slide 16 Exercise 4 Attempt to write a use case description “Fire Laser” in your own time.

17. IS0514Slide 17 The use of Use Cases Use cases are used to capture functional requirements Most use case modeling will happen during the early part of a project They drive the rest of development –You build what the client wants! Used as part of planning, testing and evaluation New Use Cases will continue to emerge as project iterates

18. IS0514Slide 18 Possible problems with Use Case modeling Danger of building a system which is not object oriented –Avoid function decomposition i.e. developing a top-down function based system –Need other views Class Diagrams / Communication / Collaboration diagrams Danger of mistaking requirements for design –You are analysing not designing - avoid technical detail Possibility of missing requirements if too much emphasis is placed on actors Incomplete picture –Non-functional requirements –Usability requirements

19. IS0514Slide 19 Use Case Modelling A More Realistic Approach Develop Scenarios Develop Use Case Diagram Develop Use Case Descriptions Iterative?

20. IS0514Slide 20 Extracting information from Use Case modeling Candidate objects Attributes Operations Forms a basis for subsequent modeling activities To be used later in class diagrams, interaction sequence diagrams, statecharts, communication diagrams, etc.

21. IS0514Slide 21 This weeks reading ESSENTIAL READING Dennis A, Wixom B, and Tegarden D (2005) System Analysis and Design with UML version 2 second edition, Wiley Pages 171-209 Further reading Bennett, S., McRobb, S. and Farmer, R. (2002) Object- Oriented Systems Analysis and Design using UML, 2nd Edition, McGraw-Hill Pages 133-146 http://www.omg.org http://alistair.cockburn.us/usecases/usecases.html

22. IS0514Slide 22 Summary Use case diagram High level view of functional requirements Use case description Detailed view of functional requirements Use case template Use case scenarios –Normal / Alternative / Exceptional Use / misuse of use cases Next Week –Introduction to Object-Orientation