3b Re Tech Softeng 0203

8/12/2019 3b Re Tech Softeng 0203

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Requirements Engineering

System modeling notations andtechniques

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1: knowledge

2: request more knowledge

3: requirements models

4: validation results

5: models to be validated

RE process framework

Problem

domain

Elicitation ValidationSpecification

Userrequirements

User feedback

Domainknowledge

Domainknowledge

1 3

4

reqs

spec

2

5

User


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Elicitation: Guidelines

Elicitation should identify

A representation of relevant information

A model of software behaviour

Elicitation should consider how the system

Will be structured (from the user perspective)

Will provide functionality

Complex systems must be partitionedhierarchically

Elicitation is then carried out separately ondifferent parts

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System Modeling Techniques

Object-oriented model: Use Cases, Activitydiagrams, Class diagrams, CRC cards.

Functional model: SADT (Structured Analysis andDesign Technique), dataflow diagrams

Data-oriented model: Entity-Relationship diagrams

(akin to class diagrams) Dynamic model: state transition diagrams, Petri-

nets


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Levels of formality

Informal specification

Based on natural language (ambiguous)

Formal specification

Based on mathematical language

Semi-formal specification

Made of both informal and formal parts

Use Case Diagram


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Use Case Diagram

Semi-formal notation

Specifies the interactions between the system andthe external world

Useful to

Oblige the analyst to state well-defined boundariesbetween system and external world

Organize system functions into elements (usecases) on which attention is focused

Supply a first basis for the specification of system

structure from the user perspective

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Elements of a Use Case Diagram Someone (user) or something (external system,

hardware) that

Exchanges information with the system

Supplies input to the system, or receivesoutput from the system

A functional unit (functionality) part of thesystem

Actor

Use Case


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Relations among Use Cases

Usage. Models that a selectedfunctionality is used in the context ofanother functionality (one is a phase ofthe other)

Generalization. Defines a functionality

as extension of another already

existing functionality (special case)

Interaction. Models:

Which actors participate in a use case Where execution starts

Actor

Use Case

Actor

Use Case

Use Case B

Use Case B

Use Case A

Use Case A

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Online Courses At the beginning of the semester, the students office has to

supply the students with the list of available coursesthrough a system of online registration.

Each student must select 4 available courses, and statetwo possible alternatives.

Each course must have from 3 to 20 students: courseswith less than 3 students are cancelled, and those withmore than 20 are split.

In the previous period, the professors access the systemonline to insert their courses in the list, while in the periodafter students selection they access to check current stateof enrolled students to their courses.

After the selection period, the system creates def. lists, bysolving all anomalies (courses with less than 3 or morethan 20 selections) and it sends them to administrativeoffice to state enrolment charges due by each student.

The students access the system for the list of their courses

All user interactions are authenticated


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UCD for Online Courses

Administrative Office

Students

Request Course List

Course Selection Request Students List

User Authentication

Professor

Insert Course

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Further example of relationsInclude/Generalize

Customer

Check availability

Hotel Booking

Acquire Customer data

Hotel Booking guaranteed by

Credit Card

Acquire Credit Card data


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Domestic Security System Once installed, the system monitors all connected sensors

(of movement, entrance, smoke), and interacts with the

user though a keyboard and a display inside a dedicatedcontrol panel.

During installation, the control panel is used to configurethe system. Each sensor is assigned with a unique numericcode and the phone number of the monitoring service tocall in case of events related with a sensor. It also defines apassword for activate/deactivate the system.

In case of an event related with some sensors, the systemdisplays a message and produces an acoustic alarm. Aftera period (defined during user configuration) the system

calls the phone number associated with the sensor. Shouldthe number be busy, the system repeats the call every 20seconds.

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UCD for Domestic Security System

Sensor

Monitoring Service

Acoustic Transmitter

Set password

Configuration

Configure sensor

Event management

User


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Data Flow Diagram

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Dataflow Diagram

Semi-formal notation

Popularized in the 70s and 80s by Yourdon andConstantine

Two step process: Structured Analysis (SA) andStructured Design (SD)

SA: dataflow diagrams SD: structure charts


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Entities in a data flow diagram

external entities

processes

data flows

data storesData label

Data label

Entity name

Function

name

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Context Diagram

Management

Front-Desk

Personnel

Guest

Room Service

Personnel

Catering

Personnel

reports

request for

information

reservationconfirmation

check-in/

check-out formorder

update billbill

order

update bill

Hotel

ReservationSystem


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Dataflow Diagram (Fragment)

Check Out

client reservation

log

bill

Check out decomposition:calculate bill, generate bill, generate log info

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Dataflow Diagrams

Dataflow diagrams can be further elaborated with

Data dictionaries: detailing data stores and flows

Mini-specs: detailing functional aspects


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Activity diagram

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Activity Diagram

Formal notation

In a conceptual model it represents the activities(sub-tasks) that needs to be done

Use cases and a hierarchical task decompositioncan be represented/visualized in activity diagrams

It can pinpoint the interactions between differentactivities (e.g. tasks and use cases)


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Activity Diagram (cont.)

DoSomething

[condition]

Start Point

End Point

Synchronization Bar

Activity

Decision Activity

Guard

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Construct an activity diagram for order processing.

When an order arrives, payment must be authorizedand all ordered items must be in stock (otherwise theorder is suspended and items are reordered). Whenall items are present and payment took place, theorder is dispatched to the customer.


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DispatchOrder

ReorderItem

Receive order

Cancel orderAuthorizePayment

CheckLine Item

Assign toOrder

[failed]

[need toreorder]

[for each lineitem in stock]

[in stock]

[stock assigned to allline items and

payment authorized]

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What is not modeled with activity diagrams

Links between actions/responsibilities and objects(use CRC Cards)

Collaborations between objects (use interactiondiagrams)

Object life cycle (use state diagrams)


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Class diagram

Type level

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Classes and Class Instances

How to model a system able to open new Faculties(having a name and an athenaeum) and managingenrolment of new students (having a name and ID)?


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Classes and Class Instances

association instances

(or links)

Student

ID

Name

matriculate()

Faculty

Name

Athenaeum

open()

11..n1..n 1

enrol

Class level

Instance level

association

Engineering

Architecture

Computer Science

Economics

X

X

Arts

X

Galileo Galilei

Albert Einstein

Martin L. King

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What is a Class DiagramA Class Diagram describes

The types of objects in the system

The types of static relationships among them

We draw Class Diagrams under three perspectives

Conceptual Software independent

Language independent

Specification

Focus on the interfaces of the software

Implementation

Focus on the implementation of the software


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Class

Window

- visible

- position

- size

+ resize()

+ display()

+ hide()

+ move()

Class Name

Attributes

Operations

private

public

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Class Extensibility mechanisms to add new elements to UML

Stereotypes

A domain-specific or user-defined class level element

For classes

And associations

Tagged values

A pair describing a property of a model element


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ClassName

publicState : UserDef = tested

privateAuthor : UserDef = pam

ProtectedOp(argname) : return

ClassInterface A

ClassInterface B

Math

Class ...

Attribute syntaxattrName : attrType = initialValue

Operation syntaxVisibility opName (argName:argType=DefaultValue,

):returnType

Interfaces

Utilities: no instances!

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Associations

Structural relationships between classes ofobjects

Association multiplicity

Number of potential participating objects

1, 0..1, M..N, *, 0..*, 1..*

Class multiplicity

number of classes involved in the association

binary, ternary, N-ary


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Association Multiplicity indicators

n Exactly n

*Zero o more

0..1Zero or one (optional)

m..nBetween m and n (included)

m..*From m above

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Class Multiplicity: Binary Associations

Exam

-date

-subject

-studentsList

+print()

+enrolStudent()

+print()

-name

-surname

-code

ExamDate

+print()

-day

-month

-year

-time

Student Lesson

+print()

-subject

-teacherattends3..10 *

Name of

relationship

Takes-place-on* 1

Multiplicity

+attendee +attended

Roles


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In other words Use

Two classes are in USE association (B uses A) ifinstances of B can send messages to instances of

A An object uses another object if able to send

messages to it

Company Personhiring

+Candidate+Employer

+apply() +interview()0..1 1..*

+seekJob()-planInterviews()

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Associations (cont.)

Association naming and direction

To facilitate model understanding

Role naming

End of an association

When

Many associations connect two classes

One association connects the same class morethan once


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Association Naming

Subscription

User Profile

Terminal

-Address+Type

Service Profile

+userPreferences

Service

+startService()1..*1..*

supportsdefault

1

0..*

1

0..*

constraints

User

1..*

0..*

1..*

registration

0..*

1

0..*

1

user service information

1

1

1

1

user

Information

0..*

0..*

0..*

0..*

subscription

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Association Direction and RoleNaming

PersonPlanePassengers

Pilot

?

?

?

?


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Associations (cont.)

Constraints on associations

Association classes

Grade

Student AssignmentDelivers

{time


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Aggregation Composition as physical

containment

EngineCar

1 1

aggregate class

ComponentAggregate

0..1 *

Example

+start() #start()

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Aggregation

Model the system allowing students to enroll inexams

Each exam date includes a date made of day,month. Year and time; it is associated with asubject.

On request, a student can enroll in a selectedexam.

The system further allows to

Move the date of an exam

Print out information about each object


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Aggregation

Exam

+print()

+manageEnrollment()

+move()

+print()

+enroll()

-name

-surname

-code

ExamDate

+print()

+changeState()

-day-month

-year

-time

Student

Lesson

+print()

-subject

-teacher

* 1

*

*

1

*

- studentsList

- room

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Generalization

Classification relationship between a generalelement and a more specific element

Inheritance in OO programming

Construct a class from another

Sharing attributes, operations and constraints

Visibility between superclass and subclass

Single (Java) or multiple (C++)

Inheritance and Delegation


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Pure Specialization (Extension)

+refresh()

-image

Image Window

+resize()

+move()

+close()

-title-position

-size

Window

+replaceText()

+deleteText()

Text Window

Generalization

association

Base class(superclass)

Derived classes

(subclasses)

-text

inherits from

derives from

is a

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Class HierarchyBuild the inheritance associations existing among thefollowing objects:

Living Being

Human Being

Flower

Animal

Vegetable

Merchant

Floweriest

Client

Suggestion

Group elements in classification sets, and then build thegeneralization hierarchy


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Class Hierarchy

Animal

Merchant

Living Being

Vegetable

Flower

Human Being

Floweriest

Client

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Impure Specialization

Sometimes we want to define exceptions to thespecialization mechanism

Overriding

To modify inherited properties

Usually for operation implementations (methods)

Never for interfaces

A subclass implements the interface of all itssuperclasses


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Impure Specialization (Overriding)

+print()

-ID

Student

+print()

-name-surname

Person

+print()

-qualification

-salary

Employee

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Multiple Inheritance

A class is derived from many superclasses

For its support, the language has to solve possibleconflicts

What happens if both superclasses support thesame message?


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Properties of Inheritance

Transitivity

If B derives from A, then B derives from allsuperclasses of A

When multiple inheritance is not supported

The inheritance hierarchy becomes a tree

We can always transform inheritance in delegationassociations

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Eliminating Multiple Inheritance

Displayable

Object

color

coordinates

filled

perimeter()

Polygon

vertices

translate()

rotate()

perimeter()

Displayable

Polygon

Displayable Object

color

coordinates

filled

perimeter()

Displayable Polygon

Polygon

vertices

translate()

rotate()

perimeter()

delegation


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Abstract classes

Base for extensible systems

The set of general mechanisms is describedaccording to the specifications of the abstractclasses, without considering specific featuresgathered within concrete classes

Abstract property

Applicable to both classes and operations (themethod to be defined in subclasses)

An abstract class cannot be instantiated directly

It is allowed for types, packages and stereotypes

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Abstract classes

Ast ratto

properties

operationOnly()

Abstract Class Name

(in italic)

No methods allowed!


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When to use Class Diagrams

Do

Use it always!

Base for all OO methods

Base for tool-implemented code generation

Fit the perspective you are in

Conceptual Model during requirementsengineering

Specification models during design

Implementation models during language-specificphases

Showing a particular implementation technique

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When to use Class Diagrams (cont.)

Do Not

Use all constructs (start from basics and useadvanced ones only if needed)

Draw models for everything

Focus on key aspects in few diagrams

Go down to implementation details too early


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How to derive a Class Diagram

A noun could be a class.

Example: When we receive an order, we check each line itemon the order to see if we have the goods in stock. If we do,we assign the goods to the order. If this assignment sendsthe quantity of those goods in stock below the reorder level,we reorder goods.

What other classes can we identify?

Finding the classes: examine the use cases

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How to derive a Class diagram(cont.)

Finding the classes: candidate objects

Order

Supply

Line Item

Reorder Level (attribute)

Product, Good (= Product) (individual instance) Product Description (meta level object describing

instances)

Stock (do we need this one?)

Company (not in use cases)

Customer (not in use cases, but needed to check thecreditworthiness)

Invoice (not in use cases)


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Finding the classes: carefully consider the candidate list

Equate synonymous terms

Product and Good Product Eliminate operation names, if possible

E.g. Reorder

Be careful in what you really mean

Is it Product rather than Product Description?

Eliminate individual objects (as opposed to classes)

Reorder Level attribute of Product Description

Eliminate implementation constructs

E.g. software

Replace or eliminate vague terms

system computer

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ProductDescProduct

P1P2

PD1PD2

Order

Line Item

TradingCompany

*

1

1

1

1

1

*

*

***

*

1

Customer

1*

1


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Assigning Responsibilities to Classes

At this point, we have derived activity and classdiagrams from performing task analysis and

developing use cases. The next step is to assign responsibilities to

classes.

CRC Cards can be used for this purpose.

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CRC Card

A CRC (Class-Responsibility-Collaboration) cardis an index card with three elements

Class: element under analysis (a class as usuallyused for object-oriented analysis)

Responsibility: a high level description of the

purpose of the class Collaboration: the classes involved to fulfill a

responsibility

Very simple yet very useful


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CRC Card (cont.)

Class

Responsibility

Collaborators

Check if item in stock

Determine price

Check for valid payment

Dispatch to delivery address

Order OrderLine

Customer

But, how are responsibilities carried out? Who is collaborating

with whom?

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CRC Card (cont.)

Elaborate the CRC Cards for the system allowingstudents to enroll in exams.


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Prototyping the User Interface

Use rapid prototyping tools (e.g., Visual Basic,Java Beans compliant)

Use cases are the starting point

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Assessment of Risks

A risk is a possible future negative event that mayaffect the success of an effort.

Examples are

Requirements

Technological

Skills

Political


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RE Techniques Interactions

From a task model (conceptual model, focus onthe user) to use cases (system model, focus on

user using a system). Tasks act on things (e.g., entities, objects)

class diagrams

The relation between actions and objects can bedefined with CRC cards, which can be refined ininteraction diagrams

3b Re Tech Softeng 0203

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