Testlet

1
Testlet


Student
Tutor
teach

• Which of the following are correct ways to read
  the association?
• Many Tutors teach many Students
• A Tutor Teaches many Students
• A Student is taught by many Tutors
• Many tutors teach a Student
• One Tutor teaches One student
• A Student Teaches many Tutors

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Use Case Modelling
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Session Aims

• Understand the purpose of Use Case diagrams and
  when they are used.
• Differentiate between scenarios and Use-Case
  diagrams
• Draw use Case Diagrams and document them

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Use Case Model

• Shows the Tasks in a system
• Shows the Actors in the System
• initiators of a task and those who benefit from
  the task
• Shows the System boundary

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Example Use Case Model
Watch
SetTime
ChangeBattery
WatchRepairPerson
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Use Case components

• Actors
• Use Cases
• Supporting Description
• blah blah blah blah blah blah blah blah blah

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Exercise 5.1

• What are the tasks (functions) of an ATM (Cash
  dispenser machine)?
• Show balance
• Dispense cash
• Print mini-statement
• Print receipt
• Read card details
• Who might be the Actors?

imperatives
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Example Use Case Model
ATM
Dispense Cash
Load Money
Bank Employee
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Exercise 5.2

• Which of the following are valid use case names

?
?
?
?
?
??
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Identifying Use Cases

• 1. Identify Actors
• 2. Identify Scenarios
• 3. Identify Use Cases
• 4. Identify relationships among Use Cases

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Actors

• Actors represent external entities that interact
  with the system.
• Actors may be human or an external system (but
  still shown as a stick person).
• A human actor is represented by a role (eg
  customer)
• A use case (task is always initiated by an Actor)

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Exercise Identify Actors
Chris has an ancient Nokia 3310 mobile phone, on
which he can make phone calls, store numbers,
send and receive sms text messages. (it doesnt
have a camera, or colour, or WAP capabilities,
but he is still happy with it since he comes from
Yorkshire and it is CHEAP to run)

• Possible Actors are
• PhoneUser (NOT chris, who is an instance of
  phoneUser)
• PhoneNetwork (the phone talks to the Fresh phone
  network)

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Identifying Scenarios

• A Scenario is a concrete, focussed informal
  description of a single feature of a system from
  the viewpoint of a single actor.

Example Some scenarios for borrowing a book from
a Library
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Mark tries to borrow the librarys 1st copy of
The Jelly-baby diet but is refused because he
has exceeded his book allowance
Sandra borrows the library's 3rd copy of
Jerboas make good pets, when she has no other
book out on loan. The system is updated
accordingly
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Use Cases
A Task-Coherent unit of functionality

• A scenario is an instance of a Use Case
• A use case is initiated by an Actor
• A Use case refers to a particular task (e.g.
  Borrow book, Admit patient)

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Exercise 5.5 Which Use case are these scenarios
of?

• James tries to enrol on the BA course but his
  A-level grade in English is too low so he is
  rejected
• Asif has had enough of Computer Networks and so
  transfers successfully into Software Engineering
  instead.
• Eileen has passed the first year and so can
  progress into the second year so she registers
  for the OO Software Development module.
• Janet tries to leave the Homer module to take the
  Aristotle module but is too late.

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Documenting a Use-Case
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Documenting a Use-Case
See example on page 85 Exercise 5.6
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Exercise 5.6 (see p 85)
Which of these use cases are handled correctly by
the description on page 85?

• The account clerk prints the bill for John Reeves
  with ID number 1236540. Mr Reeves is a basic
  customer.
• The account clerk needs to print the bill for
  Joseph Johnston but does not know his account
  number.

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Exercise 5.6 (see p 85)
Which of these use cases are handled correctly by
the description on page 85?

• The account clerk prints the bill for Jim
  Longstreet who is a bonus customer whose first
  100 units are at the special bonus price.
• The account clerk doesn't print the bill for
  Amelie Harris when he sees that there is no
  charge since she has not used any units. He
  prints an advisory note instead.

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Connections with Classes

• How do Use cases link with Class Diagrams?
• They show a complementary view of the system
• The system may record data about Actors, which
  become Classes

• Not all Actors become Classes!!

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When use Use-Cases?
Wonderful!

• Requirements analysis -
• Finding Actors, functionality
• Planning
• Helps in estimating effort / prioritisation
• Design
• Refining Class Diagram which objects need to
  collaborate to implement a Use-case
• Verification
• Verify system performs as required.

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Possible Problems

• Focus on Functions rather than objects
• May not be Object Oriented solution!
• Possibility of missing requirements.
• Not all requirements can be obtained this way.

Oh No!
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Summary

• Technique for finding functionality / Actors
  (Beneficiaries)
• Model of functionality
• Covers all scenarios for a task
• Useful throughout development
• Documentation