computer notes - polymorphism

8/3/2019 Computer Notes - Polymorphism

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PolymorphismPolymorphism

http://ecomputernotes.com


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Class CompatibilityClass Compatibility

A class is behaviorally compatible with anotherA class is behaviorally compatible with another

if it supports all the operations of the other classif it supports all the operations of the other class

Such a class is called subtypeSuch a class is called subtype

A class can be replaced by its subtypeA class can be replaced by its subtype



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Class CompatibilityClass Compatibility

Derived class is usually a subtype of the baseDerived class is usually a subtype of the base

classclass

It can handle all the legal messagesIt can handle all the legal messages(operations) of the base class(operations) of the base class

Therefore, base class can always be replaced byTherefore, base class can always be replaced by

the derived classthe derived class



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ExampleExample Class CompatibilityClass CompatibilityShape

colorvertices

move

setColor

draw

Circleradiusdraw

computeArea

Linelength

draw

getLength

Triangle

angledraw

computeArea


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ExampleExample Class CompatibilityClass CompatibilityFile

size

open

print

ASCII Fileprint

PDF File

print

PS File

print


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PolymorphismPolymorphism

In general, polymorphism refers to existence ofIn general, polymorphism refers to existence of

different forms of a single entitydifferent forms of a single entity

For example, both Diamond and Coal areFor example, both Diamond and Coal are

different forms of Carbondifferent forms of Carbon



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Polymorphism in OO ModelPolymorphism in OO Model

In OO model, polymorphism means thatIn OO model, polymorphism means that

different objects can behave in different waysdifferent objects can behave in different ways

for the same message (stimulus)for the same message (stimulus)

Consequently, sender of a message does notConsequently, sender of a message does not

need to know exact class of the receiverneed to know exact class of the receiver



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ExampleExample PolymorphismPolymorphism

Shape

Line Circle Triangle

draw

draw

draw draw

drawView



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ExampleExample PolymorphismPolymorphism

File

ASCII File PDF File PS File

print

print

print print

printEditor



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PolymorphismPolymorphismAdvantagesAdvantages

Messages can be interpreted in different waysMessages can be interpreted in different ways

depending upon the receiver classdepending upon the receiver class

Shape

Line Circle Triangledraw

draw

draw draw

drawView



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New classes can be added without changing theNew classes can be added without changing the

existing modelexisting model

Squaredraw

Shape

Line Circle Triangledraw

draw

draw draw

drawView



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In general, polymorphism is a powerful tool toIn general, polymorphism is a powerful tool to

develop flexible and reusable systemsdevelop flexible and reusable systems



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ObjectObject--Oriented ModelingOriented Modeling

An ExampleAn Example



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Problem StatementProblem Statement

Develop a graphic editor that can draw differentDevelop a graphic editor that can draw different

geometric shapes such as line, circle andgeometric shapes such as line, circle andtriangle. User can select, move or rotate atriangle. User can select, move or rotate a

shape. To do so, editor provides user with ashape. To do so, editor provides user with a

menu listing different commands. Individualmenu listing different commands. Individual

shapes can be grouped together and canshapes can be grouped together and can

behave as a single shape.behave as a single shape.



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Identify ClassesIdentify Classes Extract nouns in the problem statementExtract nouns in the problem statement

Develop a graphicDevelop a graphic editoreditor that can draw differentthat can draw differentgeometricgeometric shapesshapes such assuch as lineline,, circlecircle andandtriangletriangle.. UserUser can select, move or rotate acan select, move or rotate ashapeshape. To do so,. To do so, editoreditor providesprovides useruser with awith amenumenu listing differentlisting different commandscommands. Individual. Individual

shapesshapes can be grouped together and cancan be grouped together and canbehave as a singlebehave as a single shapeshape..



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Identify ClassesIdentify Classes Eliminate irrelevant classesEliminate irrelevant classes

EditorEditorVery broad scopeVery broad scope

UserUser Out of system boundaryOut of system boundary



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Identify ClassesIdentify ClassesAdd classes by analyzing requirementsAdd classes by analyzing requirements

GroupGroup required to behave as a shaperequired to behave as a shape

Individual shapes can be grouped together and canIndividual shapes can be grouped together and canbehave as a single shapebehave as a single shape

ViewView editor must have a display areaeditor must have a display area



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Identify ClassesIdentify Classes

ShapeShape

LineLineCircleCircle

TriangleTriangle

MenuMenu

Group

View

Following classes have been identified:



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Object ModelObject Model Graphic EditorGraphic Editor

Line

Circle

Triangle

GroupShape

View

Menu



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Identify AssociationsIdentify Associations Extract verbs connecting objectsExtract verbs connecting objects

Individual shapes can be grouped togetherIndividual shapes can be grouped together

Group consists of lines, circles, trianglesGroup consists of lines, circles, triangles Group can also consists of other groupsGroup can also consists of other groups

(Composition)(Composition)



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Identify AssociationsIdentify AssociationsVerify access pathsVerify access paths

View contains shapesView contains shapes

View contains linesView contains linesView contains circlesView contains circles

View contains trianglesView contains triangles

View contains groupsView contains groups

(Aggregation)(Aggregation)



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Identify AssociationsIdentify AssociationsVerify access pathsVerify access paths

Menu sends message to ViewMenu sends message to View

(Simple One(Simple One--Way Association)Way Association)



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TriangleCircleLine

ShapeView

nnnnnn

nnMenu

Group

nn

nn

nn

nn

nn



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Identify AttributesIdentify Attributes Extract properties of the objectExtract properties of the object

From the problem statementFrom the problem statement

Properties are not mentionedProperties are not mentioned



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From the domain knowledgeFrom the domain knowledge

Line Color

Vertices

Length

Circle Color Vertices

Radius

Triangle Color

Vertices

Angle

Shape

Color

Vertices



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From the domain knowledgeFrom the domain knowledge

Group noOfObjects

View noOfObjects

selected

Menu Name

isOpen



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Menu

nameisOpen

View

noOfObjectsselected

Shape

colorvertices

Line

length

Circle

radius

Group

noOfObjects

Triangle

angle

nn

n

nn

n

nn

n

n

nn

nn

nn

n



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Identify OperationsIdentify Operations Extract verbs connected with an objectExtract verbs connected with an object

Develop a graphic editor that can draw

different geometric shapes such as line, circle

and triangle. User can select, move orrotate ashape. To do so, editorprovides user with a

menu listing different commands. Individual

shapes can be grouped together and canbehave as a single shape.



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Identify OperationsIdentify Operations Eliminate irrelevant operationsEliminate irrelevant operations

DevelopDevelop out of system boundaryout of system boundary

BehaveBehave have broad semanticshave broad semantics



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Identify OperationsIdentify Operations Following are selected operations:Following are selected operations:

Line Draw

Select

Move

Rotate

Circle Draw

Select

Move

Rotate



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Triangle Draw

Select

Move

Rotate

Shape Draw

Select

Move

Rotate



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Group

Draw

Select Move

Rotate

Menu

Open

Select Move

Rotate



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Identify OperationsIdentify Operations Extract operations using domain knowledgeExtract operations using domain knowledge

View

Add Remove

Group

Show

Select

Move

Rotate



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Group

noOfObjects

draw()

Triangle

angle

draw() nn

Circle

radius

draw()nn

Line

length

draw()

nn

Shape

colorvertices

draw()select()move()rotate()

nn

ViewnoOfObjectsselected

add()

remove()group()show()select()move()

rotate()nn

nnnn

nn

Menu

nameisOpen

open()select()move()rotate()

n



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Identify InheritanceIdentify Inheritance

SearchSearchis a kind ofis a kind ofby looking at keywords likeby looking at keywords likesuch assuch as,,for examplefor example, etc, etc

shapes such as line, circle and triangleshapes such as line, circle and triangle Line, Circle and Triangle inherits from ShapeLine, Circle and Triangle inherits from Shape



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Identify InheritanceIdentify Inheritance

By analyzing requirementsBy analyzing requirements

Individual shapes can be grouped together andIndividual shapes can be grouped together andcan behave as a single shapecan behave as a single shape

Group inherits from ShapeGroup inherits from Shape



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Refining the Object ModelRefining the Object ModelApplication of inheritance demands an iterationApplication of inheritance demands an iteration

over the whole object modelover the whole object model

In the inheritance hierarchy,In the inheritance hierarchy,All attributes are sharedAll attributes are shared

All associations are sharedAll associations are shared

Some operations are sharedSome operations are shared Others are overriddenOthers are overridden



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Refining the Object ModelRefining the Object Model

Share associationsShare associations

View contains all kind of shapesView contains all kind of shapes

Group consists of all kind of shapesGroup consists of all kind of shapes



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Share attributesShare attributes

ShapeShape Line, Circle, Triangle and GroupLine, Circle, Triangle and Group Color, verticesColor, vertices



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Share operationsShare operations

ShapeShape Line, Circle, Triangle and GroupLine, Circle, Triangle and Group SelectSelect

MoveMove

RotateRotate



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Share the interface and override implementationShare the interface and override implementation

ShapeShape Line, Circle, Triangle and GroupLine, Circle, Triangle and Group DrawDraw



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Linelength

draw()

Circleradius

draw()

Triangleangle

draw()

GroupnoOfObjects

draw()

Shape

colorvertices


n


add()


rotate()

nn

Menu

nameisOpen


n



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Group

noOfObjects

draw()

Triangle

angle

draw() nn

Circle

radius

draw()nn

Line

length

draw()

nn

Shape

colorvertices


nn


add()


rotate()nn

nnnn

nn

Menu

nameisOpen


n


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