1. Use Cases CS/SWE 421 Introduction to Software Engineering Dan Fleck
(Slides adapted from Dr. Stephen Clyde with permission)
Additional information:
Coming up: Introduction

2. Introduction
Use Case: “... a typical interaction between a user and a computer system”, Booch
Here, “user” is anything that needs or invokes the functionality of the system
“Computer system” is the system being modeled
Use cases can capture and document the user-visible functionality of a system
Use cases capture how the system will benefit the user
Each use case achieves a discrete goal for the user
Examples of modeling languages:
Data Flow Diagrams
Entity Relationship Diagrams
Flow Charts
Context Diagrams
State Charts
Object-oriented Systems Modeling (OSM)
The term model is actual a more appropriate term for what Booch, et al.m informal refer to as a modeling language.
Coming up: Example Use Case Diagram

3. Example Use Case Diagram
Coming up: Goals

4. Goals
Use cases help everyone come to a common understanding of what the system should do
Developers
End-users
Domain Experts
Use-cases are a communication tool for the design (not the implementation)
Use cases represent a functional requirement of your system (as a whole)
Examples of modeling languages:
Data Flow Diagrams
Entity Relationship Diagrams
Flow Charts
Context Diagrams
State Charts
Object-oriented Systems Modeling (OSM)
The term model is actual a more appropriate term for what Booch, et al.m informal refer to as a modeling language.
Coming up: Use Case Diagrams

5. Use Case Diagrams
Use Case Diagrams provide a visual way to document user goals and explore possible functionality
Three primary modeling components:
Actors
Use Cases
Relationships between use cases
Record class grades
Review Transcripts
Teacher
Student
Authorized
Staff Worker
Coming up: Actors

6. Actors
Actors are things outside the system that need to interact with the system
Actors carry out use cases
Actors are represented as stick figures
Although users are actors, not all actors are users
Actors can be external software systems
External hardware (sensors, actuators, etc.)
Actors can be people that need the functionality of the system, but may not be the ones who actually invoke the software commands
Coming up: Hints for Finding Actors

7. Hints for Finding Actors
Who or what will use the main functionality of the system?
Who or what will provide input to this system?
Who or what will use output from this system?
Who will need support from the system to do their work?
Are there any other software systems with which this one needs to interact
Are there any hardware devices used or controlled by this system?
Coming up: Hints for Modeling Actors

8. Hints for Modeling Actors
An actor can be a role that a user plays with respect to the system
A single person may play different roles
A single actor may perform many use cases
A use case may be performed by many actors
Show external systems as actors only when they are the ones who need a use case
Coming up: Relationships Between Actors

9. Relationships Between Actors
Actors cam be related by generalization/specialization
Actors are classifiers (not individual users)
Student
Do this when very obvious
Graduate Student
Coming up: Use Cases

10. Use Cases
Each use case represents something the user needs to do with the system – a goal
A use cases is given a short name and textual description
Use cases can be large or small from a conceptual perspective
Use cases can relate to each other via dependencies, such as
<<extends>>
<<includes>>
Generalization or <<refines>> (“is a”)
Validate User (General) --- Specialization: retinal scan, key-card scan
Extends - Hire Employee --- Hire international employee
Includes - process data --- login to system
Coming up: Use Case Relationships

11. Use Case Relationships
Includes
Extends
Generalization
Validate User (General) --- Specialization: retinal scan, key-card scan
Extends - Hire Employee --- Hire international employee
Includes - process data --- login to system
Extends adds functionality usually for abnormal case, generalization/specialization replaces functionality
After a while you realize extends and generalization are not too different. Just know generalization and includes… forget about extends (the difference is only in intent)
Coming up: Use-Case Relationships

12. Use-Case Relationships
Includes Dependency: Defines how one use case can invoke behavior defined by another use case
Alter Student Grade
<<includes>>
Record Grades for a Section
Teacher
Coming up: Use-Case Relationships

13. Use-Case Relationships
Extends dependency: defines a use-case that is a variation of another, usually for handling an abnormal situation
Alter Student Grade
<<extends>>
Alter student grade for a class taken more than a year ago
Authorized
Staff Worker
Coming up: Use-Case Relations

14. Alter Student Grade for a Graduate Course
Use-Case Relations
Generalization: Defines one use case as a generalization of another.
Alter Student Grade
Alter Student Grade for a Graduate Course
Teacher
Coming up: Hints for Finding Use Cases

15. Hints for Finding Use Cases
Try listing actors first and then look at the activities each needs to perform and then try to express the goal that represent these activities
although this will uncover many valuable use cases, it will not find them all
Try listing external events and then look at what the system needs to do in response to each one.
This technique will find some additional use cases, but not all
Be patient, allow the use cases to unfold
Don’t over do it – Use Case Diagram should be broad-brush characterizations of user goals
Coming up: Hints for Modeling Use Cases

16. Hints for Modeling Use Cases
Establish the context of a user goal by identifying the actors
For each actor, consider the behavior that it expects or requires the system to provide
Name these common behaviors as use cases
Factor common behavior into new use cases
Relate the use cases using the extend, includes, and refines dependencies
Adorn uses cases with notes
Coming up: Benefits of Use Cases

17. Benefits of Use Cases
Use cases diagrams capture user-visible functions
Identifying actors help capture who needs the system functionality
Relationships between use cases document opportunities for reuse
Use cases provide a basis planning and scheduling incremental development
Use cases can provide a basis for system testing
Coming up: In Class Exercise

18. In Class Exercise Lets create a use case diagram for iPod
Television set
Elevator
ATM
Online Scrabble game
Word Processor
Coming up: Use cases for CS421

19. Use cases for CS421 Show system boundary Show Actors outside boundary
Use extend, include, generalization/specialization where appropriate
Typically one diagram for your project is sufficient
Coming up: Use cases for CS421

20. Use cases for CS421 Show system boundary
(Warning: skip this if not supported by the tool – like Netbeans!)
Coming up: Use cases for CS421

21. Use cases for CS421 Use Case Description about slide #14
For each use-case (oval) in your diagram include the use-description text described in the slide for Chapter 5, titled:
Use Case Description
about slide #14
Coming up: Questions

22. Questions
Who might be interested in reviewing or using use case diagrams?
When in the development life cycle should we employ use cases?
What do use cases have to do with object-orientation?
What level of use-case granularity is best?
How many use cases are enough?
Can other modeling activities help in discovering use cases?
When in the development life cycle do we stop referring to or refining the use cases?
What should the text description of use case contain?
Coming up:

23. The following slides were removed over time.
Backup Slides
The following slides were removed over time.
Coming up: Extends vs. Includes vs. Generalization