requirements analysis 4. 1 use case i - 2005b504.ppt © copyright de montfort university 2000 all...

Requirements Analysis 4. 1 Use Case I - 2005b504.ppt

© Copyright De Montfort University 2000All Rights Reserved

INFO2005Requirements Analysis

Use-Cases I

Department of Information Systems



Learning Objectives

Introduce the concept of use cases Examine various styles of specification

for use cases Use cases and RUP Consider the use of generalisation. Identify some of the problems with use

cases



Use Cases

Developed at Ericsson by Ivar Jacobson to help build long-lived, flexible telecoms systems

Premise: “the most volatile part of a system is its users’ behaviour”

Conclusion: “Model systems from the point of view of its interactions with its users”

Use cases were part of the Objectory method (OOSE) , and now in UML



Use Cases

A Use Case is a textual “description of a set of sequences or actions (that a system performs) that yields an observable result of value to a particular actor”– An Actor is a User in a role – Users need not be human, can be other

systems, devices, subsystems etc.



Use Cases Use Cases may also be

represented diagrammatically. The use case specification need

not be purely textual but may include state charts, activity diagrams, collaboration diagrams and sequence diagrams.



Use case diagram

The elements of a use case may surrounded by a rectangle that represents the containing systems or classifier

UpdateMedicalHistory

Doctor

PrintPrescription

Actor Use case



Actors and Use Cases Actors

“They represent everything that needs to exchange information with the system. Since the actors represent what is outside the system, we do not describe them in detail.”

Use Cases“When a user uses the system, she or he will perform a behaviorally related sequence of transactions in a dialogue with the system. We call such a special sequence a use case.”

Jacobson, p.127.



Use Cases & Instances Use cases may involve several processes,

but the users expect to be able to carry them all out together.

Use cases represent the general description of what happens when an actor performs a particular task with the system.

Each particular use of a use case is a use instance and each instance may take a different route through use case.



Use Cases - applicability Use cases represent the functionality of

system or classifier such as subsystem or class.

A classifer is – A mechanism that describes

behavioral and structural features. Classifiers include interfaces classes, datatypes, and components.

RUP 2000



Use Cases - applicability

This new definition (UML v1.3) of a Use Case gives it a wider role than its previous “system-level” definition

Use cases can also be used to describe “business processes”– As Jacobson suggests in The Object

Advantage?



Use Case Descriptions

Each Use Case has additional details entered in the data dictionary which describe it.

This includes a text description and a list of the Actors who use it.



Use Case Specification

Rational Rose 2000



An Example Use Case Diagram

DepositMoney Withdraw

Money

Transfer BetweenAccounts

Bank Customer

An ATM application which offers three use cases is shown.The possible use of a use case by an Actor (Bank Customer in the example below) is shown by an association linking the actor and the use case.



Example Use Case Specification for Withdraw Money

“The Bank Customer enters her card and is asked to enter her pin number. The ATM verifies her ID with the system.

If the ID is verified, the ATM asks the Bank Customer to select an option. If the withdraw option is selected the ATM asks the Customer to enter the amount to be withdrawn from Account. The ATM requests the withdrawal amount from the banking system. The system asks the Account to validate the request and, if possible, withdraws the amount.

The system then asks the Dispenser to dispense the appropriate amount.”



Alternative Specification for Withdraw Money

The use case can also be shown as a step by step breakdown of action between the actor and the system (or classifier to be precise).

Bank Customer

1. The Bank Customerinserts her card

ATM

2. The ATM asks theCustomer to enter her pin

3. Customer enters pinnumber

4. Systems verifies her ID5. If the ID is verified, theATM asks the BankCustomer to select anoption.



Use Case Formats

The format used for WithdrawMoney is described by Alaistair Cockburn as the ‘casual form’ in Writing Effective Use Cases.

He describes various formats - format used should reflect the needs of the project.



Use Case Formats

Context of use Scope Level: Summary, User-goal,

Subfunction Primary Actors Stakeholders & Interests

Cockburn, Alistair, Writing Effective Use Cases



Use Case Formats Precondition Minimal Guarantees Success Guarantees Trigger Main Success Scenario Extensions Technology and Data Variations List Related Information

Cockburn, Alistair, Writing Effective Use Cases



Class Discussion: Use Cases

MakesSaleOf_StockItem

MakesSaleOf_StockItem

CheckOutOperator

CheckOutOperator

Customer

Which is right…

and why?



Generalisation

‘A generalization from an actor A to an actor B indicates that an instance of A can communicate with the same kind of use case instances as an instance of B.’ - UML 1.3

For example, the Manager actor can communicate with the same use cases as the StaffMember actor



Generalization - Actors

Actor B

Actor A

Generalization

Multiplicity RecordSick

Manager

*1

ClockOn

ClockOff

StaffMember

*

1

*

1

*

*

1

1 *

1



Use Cases - Multiplicity

The multiplicity on the previous diagram shows the multiplicity between actor instances and use case instances. – Eg. A Manager instance may communicate

with zero or more instances of the RecordSick use case

– An instance of RecordSick may be communicated from only one Manager instance



Use Case-driven Development Use cases drive the whole

development process in the RUP.– The result of the requirements workflow.

– The basis of decisions regarding architecture and component identification and design.

– Source for analysis and design use case realisations (i.e. the models involved).

– The basis of test scripts and test procedures.– The foundation of what is described in the user

manuals.



Use Case-driven Development Using use cases involves

– Problem statement and/or domain information is mapped to Use Case specification and diagrams

– Candidate classes are extracted from Use Cases

– Collaboration diagram developed to show collaborations

– Class diagram(s) are built up.



Problems with Use Cases

How broad is a Use Case? How deep is a Use Case? How many Use Cases should there be in

some typical system? Can be confused with DFDs Can lead to “main” sub-programs

through misinterpretation of “control” objects



Summary

Introduce the concept of use cases Examine various styles of specification

for use cases Use cases and RUP Consider the use of generalisation. Identify some of the problems with use

cases



References Cockburn, Alistair, Writing Effective Use Cases,

Addison-Wesley, 2000 Bennett, S., McRobb, S. & Farmer, R. Object-Oriented

Systems Analysis and Design using UML McGraw-Hill 1999, (Ch 5)

Jacobson, I., Booch, G. and Rumbaugh, J. (1999), The Unified Software Development Process, Addison-Wesley, Reading Mass. (Ch 7)

Jacobson I, Christerson M, Jonsson P and Overgaard G (1992), Object-Oriented Software Engineering: A Use Case Driven Approach. (Ch 6)

Rational Unified Process 2000

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