Download - Sept 200491.3913 Ron McFadyen1 Use Cases Introduced by Ivar Jacobson in 1986 literal translation from Swedish ”usage case” - may be of.

Sept 2004 91.3913 Ron McFadyen 1

Use Cases

Introduced by Ivar Jacobson in 1986

•literal translation from Swedish ”usage case”

www.usecases.org - may be of use to you in the future

“blackbox” style is recommended - specify what the system must do, and not how it must do it.

A project may begin with the definition of many “brief” or “casual” use case definitions. Later on, these can be become “fully dressed”


Use Cases

Widely used.

Not just an OO technique.

Each Use Case will meet one or more user goals

Collectively, Use Cases represent the functionality required by a system.

Stories of using a system to meet goals


Use Cases

Ch 6. Use Case example is very lengthy and fairly complete

must read: pages 45-61, and sections 6.12, 6.13, 6.15

Ch 25 shows ways of managing Use Cases

Ch 25. The Use Case has been broken down into multiple Use Cases that are related via <<extend>> and <<include>>

must read: sections 25.1, 25.2, 25.3, 25.5

Ch 13. Contracts

Ch 9. System Sequence Diagrams

SSDs show the events that actors generate. The system is a black box and so the diagram emphasizes the events the system must respond to.


Use Cases in the UML

•Functionality under consideration is represented by use cases (named ellipses) enclosed in a box

•Actors are shown outside the box

•Actors are connected to use cases they use by associations

Sept 21, 2003


• Actor

• Use Case

• Associations

– Between Actors and Use Cases

– Between Use Cases

– Between Actors

Use Cases in the UML


Actors

• Actors represent the people or other systems that interact with the system.

• Each actor is drawn as a stick person with the name written underneath

• An actor represents a set of roles that users of a system may assume

• In a university registration system, we might have:

Student Registration Manager

Registration Clerk

Department Chair


Use Case Behaviour

• A Use Case describes a sequence of actions that a system performs to achieve an observable result of value to an actor (something meaningful to a user)

• A use case is drawn as an ellipse, with its name within or below the ellipse

• A Use Case Model comprises the individual use cases that describe the system

• A use case describe what a system does, not how it does it

• A use case must be written as a behaviour specification separately from the diagram (the written use case)

• We will see that other UML diagrams (e.g.statechart, sequence) can illustrate behavioural information about a use case

• A use case will be implemented as a computer program and the actors initiate execution (or instantiation) of a use case


Use Case Example

• We might begin describing a registration system as:

Register

Manage coursesStudent

Department Chair

Generate report

Registration Clerk

Registration

Between actors and use cases we draw associations (the lines) to indicate the use cases each actor can instantiate


Figure 6.2 – a portion of

Handle ReturnsCashierPayment

Authorization Service

Process Rental

NextGen

…

Process Sale

Page 50-53 gives a written description of the Process Sale use case


Process Sale Use Case P. 50+

Primary Actor:

Preconditions: describes a state the system must be in prior to the execution of the use case

Postconditions: describes a state the system must be in after execution of the use case

Main Success Scenario:

Step by step description of the interaction between the actor and the executing use case

Extensions or Alternate Flows

Alternate ways steps could terminate

At this point in time (Inception), processing a sale is described in one use case that contains a main success scenario and all possible alternative scenarios.

Some components of the written use case:



• Use Case UC1: Process Sale• Primary Actor: Cashier• …• Preconditions: Cashier is identified and authenticated• Postconditions: Sale is saved…Commissions recorded…Payment

authorization approvals recorded• Main Success Scenario:1. Customer arrives at POS checkout with goods and/or services to

purchase2. Cashier starts a new sale3. Cashier enters item identifier4. System records sale line item and presents item description, price, and

running total. Price calculated from a set of price rules.Cashier repeats steps 3-4 until indicates done.5. System presents total with taxes calculated…



Cashier repeats steps 3-4 until indicates done.

5. System presents total with taxes calculated

6. Cashier tells customer the total, and asks for payment.

7. Customer pays and System handles payment

8. System logs completed sale and sends sale and payment information to the external Accounting System and Inventory System.

9. System presents receipt

10. Customer leaves with receipt and goods

• Extensions or Alternate Flows

*a. At any time, the System fails:

1. Cashier restarts System, logs in, and requests recovery of prior sale

2. System reconstructs prior state. …

…



• Extensions or Alternate Flows*a. At any time, the System fails:

1. Cashier restarts System, logs in, and requests recovery of prior sale2. System reconstructs prior state. …

3a. Invalid identifier:1. System signals error and rejects entry

3b. There are multiple of same item category and tracking unique item identity not important (e.g. 5 packages of veggie burgers)1. Cashier can enter item category identifier and the quantity

…4a. The system generated item price is not wanted …

1. Cashier enters override price.2. System presents new price.

… Identifies the step where this condition may arise.

“*” means any step; “4” means at step 4; a letter following identifies a different exception


Use Cases in the text

Some places where use cases are specifically addressed:

• Chapter 9 introduces sequence diagrams, system events and system operations

• Chapter 13 adds contracts

• Chapter 25 discusses Include, Extend, and Generalization

…


Chapter 25 Relating Use Cases

• Include relationship• If one use case (the base use case) is designed to use the

functionality of another use case (the addition use case), there is an “include” dependency

• Consider that the base use case is executing. When it reaches the inclusion point, the addition use case is executed and when it finishes the base use case continues.

• Include is a way to factor commonality out of use cases to make the design more modular

• Suppose the Process Sale use case is designed to use the functionality of Handle Credit Payment and the Handle Check Payment use cases. The written Process Sale use case has in its Extensions section two inclusions:


Include Relationship

• UC1:Process Sale

…

• Main Success Scenario

…

7. Customer pays and System handles payment.

…

• Extensions or alternate flows:

7a. Paying by cash …

7b. Paying by credit: include Handle Credit Payment

7c. Paying by check: include Handle Check Payment



• In the UML diagram

Process Sale

Handle Check Payment

Handle Credit Payment

<<include>>

<<include>>

Process Sale has a dependency on Handle Check Payment, and another dependency on Handle Credit Payment

The dashed lines and the stick arrowhead are the correct way to depict dependencies



• Suppose a Purchase order system has two use cases Place Order and Track Order and both contain customer validation.

• Customer validation could be factored out, resulting in:

Validate Customer

<<include>>

<<include>>

Track Order

Place Order

Customer


More terminology

• Place Order and Track Order are called concrete use cases because they are initiated directly by an actor.

• Validate Customer is an abstract use case because it is never instantiated by itself; Validate Customer is only instantiated as part of another use case.



• Suppose ATM is a base use case and contains:

1. Show advertisement of the day

2. Include identify customer

3. Include validate account

4. …

Validate Account

<<include>><<include>>

Identify customerCustomer

ATM session


Generalization/Specialization Relationship

• Suppose there is more than one version of a use case, and that the different versions have some things in common and other things that differentiate them

• The general notation is

Specialized use case name

Generalized use case name



• Example. Suppose we have a university registration system where people must be admitted to become students. Some potential students, who attended high school in the city, are already known to the university because they went through some prior screening processes.

Admit new student

Admit Student

Admit HS student

Admissions Clerk



• Example. Suppose we have two ways to check someone’s identity: via a password or via a retinal scan

Check Password

Verify Identity

Retinal Scan


Generalization/Specialization of Actors

• Generalization can be applied to Actors.

• Example. Suppose for the department chair, who is a member of the faculty, there are some special duties

Faculty

Chair

Assign Grades

Manage Sections

Note that the Chair can do everything a faculty member can do, but also the Chair manages the sections of courses the department offers.


Use Case Associations

• Between use case and actor

– uses (unlabeled)

• Between use cases

– include (dependency)

– extend (dependency, condition)

– generalization

• Between actors

– generalization