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COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
COMP1007 Intro to System Requirements
Lecture 2
Use-Cases I
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
Use Cases
Represented diagrammatically. The specification need not be purely
textual – may include state charts, activity
diagrams, collaboration diagrams and sequence diagrams.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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?
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
Use Case Specification
Rational Rose 2000
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
An Example Use Case Diagram
DepositMoney Withdraw
Money
Transfer BetweenAccounts
Bank Customer
An ATM application which offers three use cases is shown.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.”
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
Use Case Formats
Context of use Scope Level: Summary, User-goal Primary Actors Stakeholders & Interests
Cockburn, Alistair, Writing Effective Use Cases
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
Class Discussion: Use Cases
MakesSaleOf_StockItem
MakesSaleOf_StockItem
CheckOutOperator
CheckOutOperator
Customer
Which is right…
and why?
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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.
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
COMP1007 Intro to Systems Requirements
© Copyright De Montfort University 2002All Rights Reserved
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
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