Use Case Model

04/11/23 CSci 5802 Univ. of Minnesota 1

Use Cases and Use Case Diagram


A Use Case

Represents the functionality from the user’s point of view• Some sort of functionality interface to the users• Answer the question: How can the system

provide observable value to the users, or to fulfill their goals?

Represents the functional requirements Defines the boundaries of the system Written in natural language

• Advantages?• Disadvantages?


Description of a Use Case

Name Participating actors Flow of events Entry condition Exit condition Quality requirement


Example

Name: withdraw Participate actor: customer Flow of event:

1. Customer selects withdraw button 2. The system displays the withdraw screen, and initializes the input amount use case…

… Quality requirement: at any point during the flow

of events, this use case can include the cancel transaction use case. The cancel transaction use case is initialized when the cancel button is selected.


Organizing Flow of Events

Basic course of actions/major scenario Alternative course of actions

• Use to describe the branching Benefits of using such organization?


Scenario vs. Use Case

Scenario• An instance of a use case• Description of a concrete set of actions• Like an execution trace of a program• No branching, no entry condition, no exit

conditions (why?)• The focus is the understandability

Use case• An abstraction that describe all possible

scenarios involved in certain functionality• Like an algorithm• The focus is the completeness


Separation of Concerns

Decomposing use cases• Separating the common cases from the

exceptions and errors handling• Separating common functionality to avoid

redundancy To make decomposition work, relationships

among the use cases needs to be described• Include• Extend• Inheritance


Use-Case Diagram

A UML diagram that represents the relationship between actors and use cases, and among the use cases

Represents an “architectural” view of the requirements


Major Components

Actors• External entities (e.g., user role, another system)

Relationship between actors and use cases• Initiation• Communication

Relationship among different use cases• Enables the decomposition of complex use

cases into smaller ones


Use Case Diagram for E-homework management system

E-homeworkdistributionTA Students

E-homeworksubmission

E-homeworkgrading


Include Relationship

Use case A includes use case B if the flow of events for A contains the flow of events for B

A whole-part relationship Allow use case A to access another common

use case B


Representation

In use case description• At a particular point during the flow of events

• Mention the inclusion at that point• At any point in the flow of events

• Mention the inclusion in the quality requirement In the use case diagram

<<include>>


Example

<<include>>withdraw

Input amount

Cancel transaction

<<include>>

customer


When to Refactor Using Include Relation When a particular flow of events is

repeatedly described in more than on use cases• Purpose of refactoring: reduce redundancy

When a particular flow of events corresponds to a concept in the problem domain• Purpose of refactoring: improving the

understandability When the description of a use case is too

long• Purpose of refactoring: improving the

understandability


Example from “Patterns for Effective Use Cases”


Extension Relation

A use case A extends a use case B if the flow of events in A can occur amid the flow of events in B when certain condition is true

It is like an “interrupt” It is used to separate the exceptional

behavior from the normal behavior• Advantage?


Representation

Use case description• Mentioned extension in the entry condition in

the extending use case• Specifying the condition that would trigger the

extending use case Use case diagram

<<extend>>


Example

Connectiondown

Deposit

withdraw

<<extend>>

<<extend>>

Name: connection down…Entry condition: This use case extends the Deposit and withdraw use case. It is initialized by the system whenever the connection between the customer and the central sever is lost.


Comparing include and extend

Similarity• They are equivalent: both are mechanisms for

composing two use cases to describe a functionality

Difference• The major difference is the location of the

triggering conditions• Suppose that A and B together describe a

functionality– A includes B when A has the triggering condition for B: in

this case, A has to know about B– B extends A when B has the triggering condition for B: in

this case, A does not need to know about B


Include vs. Extend

<<include>

A B

<<extend>>

A B

Move triggering condition for B fromA to B

Move triggering condition for B fromB to A

Guideline: judge to see whether the triggering condition for B is an integral part of A or an integral part for B.


Example: Why extend is better than include here?

Connectiondown

Deposit

withdraw

<<extend>>

<<extend>>

Name: connection down…Entry condition: This use case extends the Deposit and withdraw use case. It is initialized by the system whenever the connection between the customer and the central sever is lost.


Inheritance Relationship

It represent specialization or conceptual classification

A specialized use case can inherit the participating actor, entry and exit conditions, and some courses of actions from the base use case• Enable polymorphism from the client’s

perspective Inheritance can be used to simplify the use

case diagram


Representation

Description• Mention the inheritance of various descriptions

from another use case • Participating actor• Entry/exit conditions• Different courses of actions

Diagram

Authenticatewith finger

print

Authenticate

with passwd

Authenticate


Example

Name: Authenticate with passwordParticipating actor: Inherited from Authenticate use caseFlow of events:

….Entry condition: Inherited from Authenticate use caseExit condition: Inherited from Authenticate use case


What we’ve learned

Use cases and use case diagrams Factoring and organizing use cases using

“include”, “extend”, and “inheritance” relationships

Question: • Are these relationships enough for organizing

the use cases?

Use Case Model

Education

Transcript of Use Case Model