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Use Case Modelling

Use Case Diagram Example

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Use Case Modelling

• A use case is a modelling technique used to describe what a new system should do or what an existing system already does.

• System developers and customers/end-users discuss a use case model. In an iterative process, this lead to a requirement specification on which all agree.

• A use case diagram describes the interaction between a set of use cases and the actors involved in these use cases.

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Use Case definition

• Fowler:  A use case is a typical interaction that a user has with a system in order to achieve some goals.

 A use case is a description of a set of sequence of actions, including variants, that a system performs to yield an observable result of value to an actor.

• Cockburn:  A use case describes a system’s behavior.

Actor

• An actor is someone or something that interacts with the system. It is who or what uses the system. • An actor communicates with the system by sending and receiving messages. • An actor is a role that a user plays with respect to the system.

• Actors – what exists outside the system (Rumbaugh) [external “participants”/”roles”] 7M701 5

Use cases

• A use-case is a set of sequences of actions a system performs that yield an observable result of value to a particular actor.

• A use-case describes a requirement for the system, that is, what it should do, but not how it should do it.

• A use-case is a set of scenarios tied together by a common user goal.

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Use Case Diagram Example

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Use Case Diagram Example

Videoshop

Hire a videotape Supply new video tapes Client Film importer Retrun the video tape 7M701 8

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Scenario

A scenario is a sequence of steps describing an interaction between a user and a system.

• • A scenario is an instance of a use-case.

A scenario describes a possible interaction with the system.

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Scenario Example

• Consider a Web-based on-line store, we might have a ‘Buy a Product’ scenario that would say this : The customer browses the catalogue and adds desired items to the shopping basket. When the customer describes the shipping and credit card information and confirms the sale. The system checks the authorization on the credit card and confirms he sale both immediately and with a follow-up mail.

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Example of a Use Case Text

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Template of an Use Case Text

Name Actors

Name used to refer to a use case. All actors involved.

Preconditions Description Exceptions Result

Condition of the system at the start of the use case. A complete stepwise description of the interaction between system and actor(s). Special and/or unexpected exceptional cases. Condition of the system at the end of the use case. 7M701 13

Steps

1. Define the system boundaries 2. Define actors 3. Define use cases 4.

Define scenario’s 5. Describe each use case 6. Identify communal sub-cases 7M701 14

Use Case relationships

• Generalization • Include relation • Extend relation 7M701 15

Generalization

• Generalization is used when there is one use case similar to another.

• Inheriting parent behaviour, adding and overriding with the child’s behaviour.

• Sub use case inherits behaviour and semantics from super use cases.

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Use Case Diagram Example

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Include / Uses

• Uses / Include : this realtionship is used when there is a common chunck of behaviour across more than one use case.

• Base use case includes the functionality of included use case.

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Use Case Diagram Example

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Extend

• Extend : is similar to genralization but is used to add behaviour to the base use case at certain extension points.

• A use case is optionally extended by functionality of another use case.

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Extend and Uses relationships

Signing Insurance Policy «uses» Signing Insurance Policy «uses» «extends» Signing Car Insurance Signing Life Insurance Signing Car Purcha se Contract 7M701 21

Relationships between use cases

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Short Summary

What is Use Case modeling?

• Use Case model: a view of a system that emphasizes the behavior as it appears to outside users. A use case model partitions system functionality into transactions(“use cases’) that are meaningful to users (‘actors’) • A Use Case Diagram visualizes a use case model.

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Core Elements

Construct Description use case actor

A sequence of actions, inclu ding variants, that a system (or other entity) can perform, interacting with actors of the system. A coherent set of roles that users of use cases play when interacting with these use cases.

Syntax UseCaseName system boundary

Represents the boundary between the physical system and the actors who interact with the physical system.

ActorName

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Core Relationships

Construct Description Syntax association extend generalization

The participation of an actor in a use case. I.e., instance of an actor and instances of a use case communicate with each other. A relationship from an

extension

use case to a

base

use case, specifying how the behavior for the extension use case can be inserted into the behavior defined for the base use case.

A taxonomic relationship between a more general use case and a more specific use case. <> 7M701 26

Core Relationships

Construct Description Include (uses)

A relationship from a

base

use case to an

inclusion

use case, specifying how the behaviour for the inclusion use case is inserted into the behaviour defined for the base use case.

Syntax

<> <>) 7M701 27

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Use Case Diagram Use Cases Use Case text : Example

NS Ticket service

Destination • Define a use case diagram of ‘NS Ticket service’ • Describe an use case.

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Use Case diagram ‘NS Ticket service’

Traveller Buy OV ticket «extends» «uses» Buy NS ticket Pay ticket 7M701 30

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Use Case Actors Preconditions Description Extension Exceptions Result

Buy OV Ticket Traveller Traveller has a valid pass 1.

2.

3.

4.

5.

6.

7.

Ticket device expects destination code Traveller enters destination code Extension point: NS ticket Ticket device checks code and calculates the charge. Shows destination code & fare. Activates ticket machine for paying Traveller pays (use case: Pay ticket) Ticket device print and supplies ticket Traveller takes ticket Destination code = NS station.

3a. Ticket device expects ticket type 3b. Traveller enters Single/Return, Discount Y/N, Class Traveller interrupt the interaction or walk away Traveller enters an incorrect destination code Payment is not finished off successful Traveller has ticket. (NS can look forward to the payment)