Use Case Diagrams

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Use Case Diagrams
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Lecture Objectives

• Explain use cases and use case modelling,
  including function decomposition
• Explain use scripts

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Use Case DiagramsWhat are they?

• One of the five diagramming techniques used in
  the UML to describe the dynamic properties of an
  IS.
• Use cases are used to describe the outwardly
  visible requirements of a system. Schneider
  Winters (2001, p. 1)
• A use case achieves a discrete goal for the
  user. Fowler with Scott (1997, p. 43)?
• Always involves a process so description must
  contain a verb.

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Use Case DiagramsOverview
Tuneswarehouse
Title
Sales Order Processing
Outbound logistics
Customer
Actor
Boundary
Stock Control
Inbound logistics
Use Case
Relationship
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Use Case DiagramsOverview
Tuneswarehouse
Title
Sales Order Processing
Outbound logistics
Customer
Actor
Boundary
Stock Control
Inbound logistics
Use Case
Relationship
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Use Case DiagramsUses
Sales Order Processing
Create Order
Delete Order
Uses
Update Order
Find Order
Uses
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Use Case DiagramsExtends
Hi-Fi Sales
Sell Hi-Fi
Extends
Sell Warranty
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Use Case Scripts
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Use Case DiagramsApproaches to Finding Use Cases

• Actor-based approach Use cases serve the actors.
  First identify the systems actors e.g.,
  Customer, Member, Finance Manager, then
  define the processes that are required to meet
  their needs e.g., Customer orders tracks and
  Finance Manager pays royalties. In the case of
  human actors, the processes required are usually
  identified using techniques such as interviewing,
  questionnaires, user profiling, task observation
  and document inspection. Likewise, producing and
  testing screen mock-ups and prototypes of the
  system can also be very useful here. Essentially
  an outside-in approach.
• Event-driven approach Identify what events the
  system must generate e.g., download track or
  create member, then work out which actors are
  able to initiate these events. Essentially an
  inside-out approach.
• Use case patterns Standard use case patterns
  that tend to occur frequently in systems design.
  Available in text books and on the web. Can save
  us a great deal of time when modelling a system.
  However, the correct application of comes with
  experience and those new to modelling should be
  careful when using this approach. Also, because
  these patterns are standard, they cannot be used
  in isolation since all systems are unique.

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Use Case ScriptsExample

• Title Search for Hi-Fi by price
• Actors Customer
• Use case does Allows the use to select a H-Fi up
  to a maximum price.
• Use case begins When the customer clicks the
  search by price button on the main search page.
• Use case ends When the system displays the Hi-Fi
  up to the select price.
• Actors System
• 1. Selects the maximum price for Hi-Fi.
• 2. Clicks the search button. 3. Displays the
  available Hi-Fis in price range.
• Alternative Actions
• 2. Customer may select a brand.
• 2. Customer may go back to main search Page.

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Use Case Scripts?

• Textual description of a use case
• More detailed and structured than a use case (on
  a diagram)?
• Allows modelling of alternate actions.
• Conversational style between actor(s) and system
• Often used to help define/find classes (see
  later)?
• Describe only common behaviours e.g., do not go
  into rare events such as system crashes - Use
  Case Scripts should make sense to users.
• Script only the lowest level (atomic) uses case.

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Lecture Objectives

• Explain use cases and use case modelling,
  including function decomposition
• Explain use scripts

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Lecture Objectives

• Explain use cases and use case modelling,
  including function decomposition
• Explain use scripts

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Lecture Exercises

• Produce a pack of use cases for a drinks vending
  machine.
• Produce a use case script for one of the uses
  cases you produced above.