COMP1007 Intro to Systems Requirements © Copyright De Montfort University 2002 All Rights Reserved COMP1007 Intro to System Requirements Lecture 2 Use-Cases.

COMP1007 Intro to Systems Requirements

© Copyright De Montfort University 2002All Rights Reserved

COMP1007 Intro to System Requirements

Lecture 2

Use-Cases I



Learning Objectives

Introduce the concept of use cases Examine various styles of

specification for use cases Consider the use of generalisation. Identify some of the problems with

use cases



Use Cases

Developed at Ericsson by Ivar Jacobson 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 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

Represented diagrammatically. The specification need not be purely

textual – may include state charts, activity

diagrams, collaboration diagrams and sequence diagrams.



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

UpdateMedicalHistory

Doctor

PrintPrescription

Actor Use case

Doctor

Use Case Diagram



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 behaviourally 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

May involve several processes Represents general description of

what happens 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 Represent the functionality of system

or classifier such as subsystem or class.

A classifer is – A mechanism that describes behavioural

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.



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

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

Context of use Scope Level: Summary, User-goal 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

Manager

Staffmember

ClockOn

ClockOff

RecordSick

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 Consider the use of generalisation. Identify some of the problems with

use cases



References Maciaszek, Requirements Analysis & Systems design,

Chp 2 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

COMP1007 Intro to Systems Requirements © Copyright De Montfort University 2002 All Rights Reserved COMP1007 Intro to System Requirements Lecture 2 Use-Cases.

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