USE CASES

Ch 6 in Textbook(Applying UML & Patterns)(Applying UML & Patterns)

Chapter objectives

Describe the meaning of Use caseIllustrate Use Case diagram, and the relationship types inside use case

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relationship types inside use case diagram.Present different methods to write use case descriptions such as high level format and Expanded format

It describes the main system functions from the standpoint of an external observer

Used during requirements elicitation to represent external behavior

It emphasize on what the system does rather than how it

Capturing functional requirements with use case diagram

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It emphasize on what the system does rather than how it does it

It is created during the early stages of a project - during the analysis phase rather than during the design ph ase

It provides a high-level view of what the system do es and who uses it

It provides the basis for determining the user inte rfaces (UIs) to the system

ATM

Example – ATM banking systemUse case Diagram

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Use case diagramUse cases are drawn as ellipses with the name of the use case written inside the ellipseUse case name typically consists of an active verb and one or more nouns that concisely describe the system function modelled

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System name

Actor 1

Use case 2

Use case 1

Actor 3

Actor 2

Use case diagramActors are drawn as stick persons with the role of the

actor written below

Actor names typically represent the role that an ac tor plays with respect to the system

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ATM system example: all people interacting

with the ATM system’s user interface are

represented by a single actor, with the role

name ‘customer’, even if each person has

a different real name, a different job,…

use case diagram

Use case diagrams contain:– Actors – Use cases– Associations/ relationships among actors and use c ases

1- An actor:Is someone (e.g. human beings) or something (e.g.

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Is someone (e.g. human beings) or something (e.g. other objects or systems) that interact with the system. An actor has a unique name.

Is entity external to the system who participates i n the story of the use case (receive or input), such as a person, computer system, or organization.

�e.g. a library clerk, cashier, customer, Passenger, GPS satellite, bank, customer department.

2- Use cases : a collection of related scenarios that describe actors using a system to support a goal.

One use case:Is a summary of scenarios.A scenario is a possible example of what happens

use case

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A scenario is a possible example of what happens when an actor interacts with the systemRepresents a unit of functionality that the system providesScenario : contains a specific sequence of actions and interactions between actors and the system under discussion.� e.g. the scenario of entering book information into the

catalog, or buying items at a store, or using an AT M.

Use case diagramActors are connected to the use case/s with which they

interact by a line , which represents the relationshipbetween the actor(s) and the use case(s)

One actor may be associated with one or more use ca ses

One use case may be associated with one or more act ors

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One use case may be associated with one or more act ors

Customer

Bank System

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Use Case Diagrams ExampleLibrary Information System

Borrow

Resources

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Librarian Patron

Return

Resources

Add

Resources

Class Exercise

Ask students to draw the Use Case diagram of a system (e.g. Airline reservation system)

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Use case behaviour description

Each use case in a use case diagram can have an associated behavior specification

A behavior specification describes the sequence of actions making up a use case scenario

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Types of use case behaviour description format

High level

use case

Expanded

use case

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use case use case

HL use case describes a process very briefly, usually in 2 or 3 sentences.

Expanded use case describes a process in details. It has an additional section not present in HL, Typical course of events

High level use case format

It has the following format:Use Case: Use case nameActors: List of actors (external agents),

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Actors: List of actors (external agents), indicating who initiates the use caseDescription (Success scenario): Narrative description of the process

Example: High Level Use case Buy Items

Use Case: Buy ItemsActor: Customer(initiator) , CashierDescription (Success scenario):

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Description (Success scenario):� A customer arrives at a checkout with

items to purchase. The Cashier records the purchase items and collects payment. On completion, the Customer leaves with the items

Sample High-Level Primary Use Cases

Name: Add Resources.Actors: Librarian.Description: The use case begins when the Librarian receives new resources (books and

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Librarian receives new resources (books and videos) to add to the catalog. The title, call number, and other information are recorded. Then the resources are placed on a shelf organized by resource type and call numbers.

Sample High-Level Primary Use CasesOther possible use cases.

Return a Resource.

Delete a Resource.

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Notify Overdue Patrons.

Collect Fines.

Expanded use case format

Use Case: Name of use caseActors: List of actors (external agents), indicating who initiates the use casePurpose: Intention of the use caseOverview (Success scenario):

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Overview (Success scenario):� Repetition of HL use case, or some similar summary

Type: 1-primary, secondary or optional2- essential or real

Cross References: Related use cases and system functions.Typical course of actions: describes in detail the conservation of interaction between the actors and the system.

Importance Levels of Use Cases

PRIMARY SECONDARY

Describes a major, common Describes a rare, unusual, or

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Describes a major, commonprocess.

Describes a rare, unusual, or exceptional process.

Primary, Secondary and Optional Use cases

Type of Use case can be one of the following:1- Primary: represents major common

processes, such as Buy Items2- Secondary: represents minor or rare

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2- Secondary: represents minor or rare processes, such as Request for Stocking New Product.

3- Optional: represents processes that may not be tackled.

Abstraction Levels of Use Cases

ESSENTIAL REAL

The essence of the process.Analysis-oriented.

Concrete, design-oriented.

Expresses process Expressed in terms of the

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Expresses process(relatively) independent ofa hardware/software solution.

Expressed in terms of the solution—screen shots of windows, entry into input fields, and so forth.

Essential versus Real Use Cases

Essential Real

The Librarian records the callnumber.

The Librarian uses the laserwand to scan the bar code forthe call number, which istransmitted to the computer.

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The AccountHolder identifieshimself to the ATM.

The AccountHolder insertsthe card into the ATM cardreader. He is prompted toenter his PIN (see screenshot 4), which he inputs with anumeric keypad.

Essential vs. Real Use Cases

EssentialVery abstract

RealVery concrete

Use Case Degree of Design Commitment

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Essential use cases are expanded use cases that are expressed in an ideal form free of technology and implementation details. HL are always essential

Real use cases concretely describes the process in terms of its real current design, committed to specific input and output technology

Example 1: Essential vs. Real Use CasesATM withdraw cash

Actor Action1. This Customer identifies

themselves

System Response2. Presents options

Essential Use Case

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Actor Action1. This Customer inserts his

card3. Enter PIN on keypad

System Response2. Prompts for PIN

4. Display options menu

Essential Use Case

Real Use Case

Example 2: Essential vs. Real Use CasesBuy Items

Actor Action1. This Cashier records the

identifier for each item

System Response2. Determines the item price &

adds the item information to the running sales transaction

The description & price of the Essential Use Case

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Actor Action1. For each item, the Cashier

types in the UPC in the UPC field of Window1. They then press “Enter Item” button with the mouse or Enter key

The description & price of the item are presented

System Response2. Display the item price &

adds the item information to the running sales transaction

The description & price of the current item are displayed in Textbox 2 of Window 1

Essential Use Case

Real Use Case

Example: Expanded Use case Buy Items with Cash

Use Case: Buy Items with CashActor: Customer (initiator), cashierPurpose: Capture a sale & its cash paymentOverview (Success scenario):

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Overview (Success scenario):� A customer arrives at a checkout with items to

purchase. The Cashier records the purchase items and collects a cash payment. On completion, the Customer leaves with the items

Type: primaryCross References: Functions R1.2,…

Typical Course of EventsActor Action

1. This use case begins when a Customer arrives at the POST checkout with items to purchase

2. The Cashier records the identifier from each item

System Response

3. Determines the item price and adds the item information to

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identifier from each item

If there is more than one of the same item, the Cashier can enter the quantity as well

4. On completion of item entry, the Cashier indicates to the POST that item entry is complete

6. The Cashier tells the Customer the total

adds the item information to the running sales transaction

The description and price of the current item are presented

5. Calculate and presents the sale total

Typical Course of Events

Actor Action7. The Customer gives a cash

payment possibly greater than the sale total

8. The Cashier records the cash received amount

System Response

9. Shows the balance due back to the Customer & generate a receipt.

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10. The Cashier deposits the cash received & extracts the balance owingThe Cashier gives the balance owing, & the printed receipt to the Customer

12. The Customer leaves with the items purchased

11. Logs the completed sale

Alternatives:Line 2. Invalid identifier entered. Indicate errorsLine 7. Customer didn’t have enough cash. Cancel sales tran saction

Identifying Use Cases

Type 1: Actor-based1. Identify the actors related to an organization2. For each actor, identify the processes they

initiate or participate in

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initiate or participate in

Type 2: Event-based1. Identify the external events that a system must

respond to2. Relate the events to actors and use cases

Example: POST

Cashier Log inCash out

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Customer Buy ItemsRefund Items

Examples of processes

A use case describes a process, such as business process.A process describes, from start to finish, a sequence of events, actions and transactions required to produce or complete something of

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required to produce or complete something of value.

Examples of processes:Withdraw cash from an ATMOrder a productRegister for courses at a schoolCheck the spellingHandle a call

Decision Point & BranchingBuy Items

A Use case may contain decision points such as in Buy Items , the customer may choose to pay via cash, credit or check .If one of them is the typical case (usual), then the typical case is the one written in Typical course of events, and the other alternatives should be written in

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events, and the other alternatives should be written in the Alternatives section.If all the alternatives are equal in their likelihood, (like the payment types), write in the main section of Typical course of events a branch event , that indicates that the possible branches are written in subsections.Then write a subsection for each branch, again

using Typical course of eventsIf subsections have alternatives, write them in an Alternatives section.

Example: Decision Point & BranchingBuy Items

Actor Action1. This use case begins when a

Customer arrives at POST checkout with items to purchase

2. ..3. Customer chooses payment type:

a. If cash payment, see section

System Response

Section: Main

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a. If cash payment, see section pay by Cash

b. If credit payment, see section pay by Credit

d. If check payment, see section pay by Check

6. The cashier gives the receipt to the customer

7. The customer leaves with the items

4. Logs the completed sale5. Prints a receipt

Section: Pay by Cash

Actor Action1. The customer gives a

cash payment- possible greater than the sale total

2. The Cashier records the cash tendered

System Response

3. Shows the balance due back to the Customer

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4. The Cashier deposits the cash received and extracts the balance owingThe Cashier gives the balance owing to the Customer

to the Customer

Alternative Courses:Line 4: Insufficient cash in drawer to pay balance. Ask for cash from supervisor, or ask Customer for a payment clos er to sale total

Plan & Elaborate Phase Steps

Define system functionDefine system boundary, actors & use casesHL use cases

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Draw use case diagramExpand critical use cases (essential / Analysis)Real use case (Design)Rank use case (not discussed)

Cashier Log inCash out

Customer Buy ItemsRefund Items

Manager Start UpShut Down

System Administrator

Add New Users

Use cases in high level formatUse Case: Buy Items

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Use Case: Buy ItemsActors: Customer (initiator), CashierType: PrimaryDescription: A Customer arrives at a checkout with items to purc hase. The Cashier records the purchase items and collects a payment. On completion, the Customer leaves the store with the items.

Use Case: Start UpActors: ManagerType: PrimaryDescription: A Manager powers on a POST in order to prepare it f or use by Cashiers. The Manager makes sure the date an d time are correct, after which the system is ready for Cashie rs’ use.

POST

Refund Purchased Items

Log In

Buy Items

Customer

Cashier

<<actor>>

CAS

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Start Up

Items

System administrator

Manage Users

Etc.

Manager

Cashier

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Alternatives?

Example Use case: Make Book Entry

Use Case: UC1 - Make Book EntryActor: Library clerkSuccess scenario:

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� A library clerk accesses a terminal in order to enter one or possibly many book entries in a database. At the end of every entry, the system displays a confirmation message. At the end of the session the system would display an informative message.

UML use case diagramLibrary Information System

Library clerk

Make Book Entry

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Library clerk

Browse Catalog

Handle Returns

Make Booking

Library member

The <<extends>> Relationship

<<extends>> relationships represent exceptional or seldom invoked cases.A reusable use case (component) that conditionally interrupts (is invoked optionally -- like a menu selection in an application) the execution of another use case to augment its functionality.

� The functionality in the original problem statement needs to be extended.

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extended.The exceptional event flows are factored out of the main event flow for clarity.The base use case can be executed without the use case extension in extend associations. The responsibility for deciding when the extending use case should be used lies with the extending use case. Arrow points to use case being extended.

Extendeduse case

Extendinguse case

The <<extends>> Relationship

Use cases representing exceptional flows can extend more than one use case.The direction of an <<extends>> relationship is to the extended use caseFor example: the use case “ReportEmergency” is complete by itself , but can be extended by the use case “Help” for a specific scenario in which the user requires help

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by itself , but can be extended by the use case “Help” for a specific scenario in which the user requires help

ReportEmergency

FieldOfficerfHelp

<<extend>>

When to Use <<extends>> Relationship

Major variation: If you have a major alternative path in the use case, and it’s complex enough to have its own alternative paths, then placing it on your diagram will honestly expose the complexity—which is helpful in costing, assignment, and scheduling.Optional subgoal: If you have parts of the use case that would be optional to implement (or even optional to execute) to meet the actor’s goals, put those parts into their own use case. Doing so clarifies the relationships between actors and their goals. It also emphasizes that you may deliver these optional goals in later releases.

46http://dotnet.org.za/hannes/archive/2006/01/31/use-cases-when-to-use-includes-generalization-and-extending.aspx

Passenger

PurchaseTicket

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TimeOut

<<extends>>

NoChange

<<extends>>OutOfOrder

<<extends>>

Cancel

<<extends>>

The <<extends>> Relationship

ManageIncident

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CreateIncidentHandleIncident CloseIncident

<<Extend>>

<<Extend>>

<<Extend>>

The <<extends>> Relationship

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The <<extends>> Relationship

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Bank System

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The <<includes>> Relationship

<<includes>> relationship represents behavior that is factored out of the use case. A use case uses anoth er use case (“functional decomposition”) Used to indicate that one use case includes the functionality of another use case .A function in the original problem statement is too complex to be solvable immediately

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complex to be solvable immediately� Describe the function as the aggregation of a set of

simpler functions. The associated use case is decomposed into smaller use cases

� A reusable use case (component) that is uncondition ally called into the execution of another use case (alwa ys included in the process – like running BIOS in a sys tem boot).

Included use case

Including use case

Passenger

�Responsibility for the decision about when to use i t lies with the calling use case. � Arrow points to the included use case. The direction of a <<includes>> relationship is to the using use case (unlike <<extends>> relationships).

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PurchaseSingleTicket

PurchaseMultiTicket

NoChange

<<extends>>

Cancel

<<extends>>

<<includes>>

CollectMoney

<<includes>>

The <<includes>> Relationship

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Actor Generalization

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University Registration System

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