11

CSc 131CSc 131

Computer Software EngineeringComputer Software Engineering

Fall 2012Fall 2012Lecture # 7Lecture # 7

Object-Oriented Design Object-Oriented Design & UML Class Models& UML Class Models

22

The OO MindsetThe OO Mindset

problem domainproblem domain

objectsobjects

33

ClassesClasses• Object-oriented thinking begins with the definition Object-oriented thinking begins with the definition

of a class often defined as:of a class often defined as:

– templatetemplate

– generalized descriptiongeneralized description

– “ “blueprint” ... describing a collection of similar blueprint” ... describing a collection of similar itemsitems

• A metaclass (also called a superclass) is a collection A metaclass (also called a superclass) is a collection of classes.of classes.

44

What Is a Class?What Is a Class? A class is a description of a set of objects that share the A class is a description of a set of objects that share the

same same attributesattributes, , operationsoperations, , relationshipsrelationships, and , and semantics. semantics.

– An object is an instance of a class.An object is an instance of a class.

A class is an abstraction in that itA class is an abstraction in that it– Emphasizes relevant characteristics.Emphasizes relevant characteristics.– Suppresses other characteristics.Suppresses other characteristics.

55

What is a Class?What is a Class?

external entities

things

occurrences roles

organizational units

places

structures

class name

attributes:

operations:

66

What is an Object? What is an Object? Objects are key to understanding Objects are key to understanding object-oriented

technology. technology.

Many examples of real-world objects: your desk, Many examples of real-world objects: your desk, your computer, your bicycle. your computer, your bicycle.

These real-world objects share two characteristics: These real-world objects share two characteristics: statestate and and behaviorbehavior..

77

ObjectsObjectsSoftware objects are modeled after real-world objects Software objects are modeled after real-world objects

in that they too have in that they too have state and behavior. state and behavior.

A software object maintains its state in one or more A software object maintains its state in one or more variables . .

A software object implements its behavior with A software object implements its behavior with methods.

A method is a function (subroutine) associated with A method is a function (subroutine) associated with an object. an object.

88

Object TypesObject TypesExternal entities: sensors, actuators, control panel, External entities: sensors, actuators, control panel,

devicesdevices

Information items : displays, commands, etc.Information items : displays, commands, etc.

Entities which establishes the context of the Entities which establishes the context of the problem : controller, monitors, schedulersproblem : controller, monitors, schedulers

99

Identifying ClassesIdentifying ClassesAn object may appear as a noun (ex. An object may appear as a noun (ex.

Measurement) or disguised in a verb (to measure)Measurement) or disguised in a verb (to measure)

A method might appear as a verb (ex. Investigate) A method might appear as a verb (ex. Investigate) or disguised in a noun (investigation)or disguised in a noun (investigation)

Attributes describe some kind of characteristics Attributes describe some kind of characteristics for the object (adjectives). for the object (adjectives).

1010

Criteria for Evaluating Candidate ClassesCriteria for Evaluating Candidate ClassesOne or more attributesOne or more attributes

Needed functionality / Services (one or more Needed functionality / Services (one or more methods)methods)

Common attributes (apply to all objects of a Common attributes (apply to all objects of a specific class)specific class)

Common functionality/operations (apply to all Common functionality/operations (apply to all objects of a specific class)objects of a specific class)

1111

Building a ClassBuilding a Class

class name

attributes:

operations:

attributes:

operations

1212

Encapsulation/HidingEncapsulation/HidingThe object encapsulatesboth data and the logicalprocedures required tomanipulate the data

Achieves “information hiding”

method # 1

data

method # 2

method # 4

method # 5

method # 6

1313

Class HierarchyClass Hierarchy

chairtable desk

instances of chair

furniture (superclass)

subclasses of thefurniture superclass

1414

MethodsMethods(a.k.a. Operations, Services)(a.k.a. Operations, Services)

An executable procedure that is encapsulated in a class and is designed to operate on one or more data attributes that are defined as part of the class.

A method is invoked via message passing.

1515

Encapsulation and InheritanceEncapsulation and Inheritance A class A class encapsulatesencapsulates data and operations that manipulates the data and operations that manipulates the

data in a single packagedata in a single package

Benefits of encapsulation:Benefits of encapsulation:

– implementation details are hidden (information implementation details are hidden (information hiding)- reduces the propagation of side effects hiding)- reduces the propagation of side effects when changes occur.when changes occur.

– Data structure and operations are in a single entity Data structure and operations are in a single entity (class) – component reuse(class) – component reuse

– Object interfaces are simplified – by message Object interfaces are simplified – by message passing and objects are not concerned with the passing and objects are not concerned with the details of internal data structures.details of internal data structures.

1616

InheritanceInheritance It is a key concept in OO paradigmIt is a key concept in OO paradigm

A subclass inherits all of the attributes and A subclass inherits all of the attributes and operations associated with its superclass.operations associated with its superclass.

It differentiates between conventional and OO It differentiates between conventional and OO development.development.

This implies that all data structures and This implies that all data structures and algorithms that implemented for superclass are algorithms that implemented for superclass are available for the subclass- available for the subclass- Reuse is Reuse is accomplished.accomplished.

1717

Representing Classes in UMLRepresenting Classes in UML

A class is represented using a rectangle A class is represented using a rectangle with compartments.with compartments.

Professor

- name- employeeID : UniqueId- hireDate- status- discipline- maxLoad

+ submitFinalGrade()+ acceptCourseOffering()+ setMaxLoad()+ takeSabbatical()

1818

What Should You Name Your Classes? Name classes to reflect what they represent.Name classes to reflect what they represent. The name should be a noun and not have a prefix or suffix.The name should be a noun and not have a prefix or suffix. Typically, capitalize the first letter in every word in the class Typically, capitalize the first letter in every word in the class

name.name.

Schedule

CourseOffering

Simple Names

1919

What Is Class Responsibility? A responsibility is a A responsibility is a

contract or obligation of the contract or obligation of the class.class.

The class responsibilities The class responsibilities are carried out by the are carried out by the corresponding attributes corresponding attributes and operations.and operations.

Responsibilities translate Responsibilities translate into operations and into operations and attributes as models are attributes as models are refined.refined.

2020

Classes/Objects Need to Collaborate Objects are useless unless they can Objects are useless unless they can

collaborate together to solve a problem.collaborate together to solve a problem.

– Each object is responsible for its own behavior Each object is responsible for its own behavior and status.and status.

– No one object can carry out every responsibility No one object can carry out every responsibility on its own.on its own.

How do objects interact with each other?How do objects interact with each other?– They interact through messages.They interact through messages.

2121

Where Do You Find Classes?

Problem StatementProblem Statement Use-case documentationUse-case documentation Use-case modelsUse-case models GlossaryGlossary Stakeholder RequestsStakeholder Requests Supplementary SpecificationSupplementary Specification Vision documentVision document Any other project documentationAny other project documentation

2222

How Do You Filter Nouns? Traditional filtering nouns approachTraditional filtering nouns approach

– Underline noun clauses in the use-case flow of Underline noun clauses in the use-case flow of eventsevents

– Remove: Remove:

Redundant candidatesRedundant candidates Vague candidatesVague candidates Actors (out of scope)Actors (out of scope) Implementation constructsImplementation constructs OperationsOperations

2323

Pitfalls When Filtering Nouns When identifying nouns, be aware thatWhen identifying nouns, be aware that

– Several terms may refer to the same object.Several terms may refer to the same object.– One term may refer to more than one object.One term may refer to more than one object.– Natural language is very ambiguous. Natural language is very ambiguous. – Any noun can be disguised as a verb and any verb can be Any noun can be disguised as a verb and any verb can be

disguised as a noun.disguised as a noun. Results are dependent on the author’s writing skills.Results are dependent on the author’s writing skills.

Filtering nouns can identify many unimportant objects.Filtering nouns can identify many unimportant objects.

2424

Find Classes from Use-Case Behavior

Identify a candidate set of model elements Identify a candidate set of model elements (classes) which are capable of performing (classes) which are capable of performing the behavior described in use cases.the behavior described in use cases.

The complete behavior of a use case has to The complete behavior of a use case has to be distributed to classes.be distributed to classes.

2525

Use Case Use-Case Realization

Sequence Diagrams Collaboration Diagrams

Distribute Use-Case Behavior to Classes

For each use-case flow of events: For each use-case flow of events: – Identify classes Identify classes – Allocate use-case responsibilities to classesAllocate use-case responsibilities to classes– Model class interactions in interaction diagramsModel class interactions in interaction diagrams

2626

Distribute Use-Case Behavior to Classes Purpose of this step:Purpose of this step:

– To express the use-case behavior in terms of To express the use-case behavior in terms of collaborating classes.collaborating classes.

– To determine the responsibilities of classes.To determine the responsibilities of classes.

2727

What Is an Association Name?

To clarify its meaning, an association may be named.To clarify its meaning, an association may be named. The name is represented as a label placed midway The name is represented as a label placed midway

along the association line.along the association line. An association name is usually a verb or verb phrase.An association name is usually a verb or verb phrase.

SchedulemanagesRegistrationController

2828

Good and Bad Examples of Association Good and Bad Examples of Association NamesNames

Poor examplesPoor examples– A student A student has ahas a schedule schedule– A department A department contains a contains a professorprofessor

Good examplesGood examples– A student A student creates creates a schedulea schedule– A department A department employsemploys a professor a professor

2929

Example: Classes for University Registration System

Student

Professor

CourseOffering

Schedule

Course

CourseCatalogSystem

BillingSystem

GraduationGown

Dormitory Fraternity

3030

What Is Multiplicity?What Is Multiplicity? Multiplicity is the number of instances of one class Multiplicity is the number of instances of one class

relates to one instance of another class.relates to one instance of another class.

For each association, there are two multiplicity For each association, there are two multiplicity decisions to make, one for each end of the association.decisions to make, one for each end of the association.– For each instance of Professor, many Course Offerings may For each instance of Professor, many Course Offerings may

be taught.be taught.– For each instance of Course Offering, there may be either For each instance of Course Offering, there may be either

one or zero Professor as the instructor.one or zero Professor as the instructor.

Professor<<entity>>

CourseOffering<<entity>>

0..1 0..*0..1 0..*

instructor

3131

Multiplicity IndicatorsMultiplicity Indicators

2..4

0..1

1..*

0..*

1

*

Exactly oneExactly one Zero or more (many, Zero or more (many,

unlimited)unlimited) ManyMany One or moreOne or more Zero or one (optional Zero or one (optional

scalar role)scalar role) Specified rangeSpecified range Multiple, disjoint rangesMultiple, disjoint ranges

2, 4..6

3232

What Is Aggregation?What Is Aggregation? An aggregation is a special form of association An aggregation is a special form of association

that models a whole-part relationship between that models a whole-part relationship between an aggregate (the whole) and its parts.an aggregate (the whole) and its parts.– An aggregation “Is a part-of” relationship.An aggregation “Is a part-of” relationship.

Multiplicity is represented like other associations.Multiplicity is represented like other associations.

PartWhole

1..211

3333

AggregationAggregation Aggregation is also a familiar concept Aggregation is also a familiar concept

from real life:from real life:

– a forest is an aggregate of treesa forest is an aggregate of trees

– a flock is an aggregate of sheepa flock is an aggregate of sheep

Aggregation is a group/member Aggregation is a group/member associationassociation

– The aggregate object may potentially The aggregate object may potentially exist without its constituent objects.exist without its constituent objects.

3434

CompositionComposition Composition is a common structure in software Composition is a common structure in software

systems, because many composite objects appear systems, because many composite objects appear in real life:in real life:

– a dog is a composite of a head, a body, a tail a dog is a composite of a head, a body, a tail and 4 legsand 4 legs

– an email is composed of a header and a text an email is composed of a header and a text message; the header is composed of a From: message; the header is composed of a From: line a To: line, etc.line a To: line, etc.

– The composite object does not exists without its The composite object does not exists without its componentscomponents

– At any time, each component may be part of At any time, each component may be part of only one composite.only one composite.

3535

CompositionComposition A solid diamond denotes A solid diamond denotes compositioncomposition, a , a

strong form of aggregation, where strong form of aggregation, where components cannot exist without the components cannot exist without the aggregate. aggregate.

Window

scrollbar title body

Header Panel

2 1 1

Slider

111

3636

What Is Generalization?What Is Generalization? A relationship among classes where one A relationship among classes where one

class shares the structure and/or behavior class shares the structure and/or behavior of one or more classesof one or more classes

Defines a hierarchy of abstractions in which Defines a hierarchy of abstractions in which a subclass inherits from one or more a subclass inherits from one or more superclassessuperclasses– Single inheritanceSingle inheritance– Multiple inheritanceMultiple inheritance

Is an “is a kind of” relationshipIs an “is a kind of” relationship

3737

Example: Single InheritanceExample: Single Inheritance

One class inherits from anotherOne class inherits from another

CheckingSavings

Superclass (parent)

Subclasses

Generalization Relationship

Account

- balance- name- number

+ withdraw()+ createStatement()

3838

– All classes have meaningful, domain-All classes have meaningful, domain-specific names.specific names.

– Each class has a small set of collaborators.Each class has a small set of collaborators.

– There are no “indispensable” classes. There are no “indispensable” classes. – A class that collaborates with everyone A class that collaborates with everyone

needs to be redefined.needs to be redefined.

– The classes can handle a change in The classes can handle a change in requirements.requirements.

Things Are Going Well If:

3939

–A number of classes have no A number of classes have no responsibilities.responsibilities.

–A single responsibility gets assigned to A single responsibility gets assigned to several classes.several classes.

–All classes collaborate with All classes collaborate with all other classes.all other classes.

Things Are Not Going Well If:

4040

QuestionsQuestions

What is Next : More on Sequence diagrams & coupling and More on Sequence diagrams & coupling and

cohesioncohesion Continue with Object Oriented Design & UML Continue with Object Oriented Design & UML

Class ModelsClass Models

Continue with “Design and implementation..”Continue with “Design and implementation..”