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BBM102– IntroductiontoProgrammingIISpring 2017

Instructors:AyçaTarhan,FuatAkal,GönençErcan,Vahid Garousi

AbstractClassesandInterfaces

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Today

¢ AbstractClasses§ Abstractmethods§ Polymorphismwithabstractclasses§ Exampleproject:PayrollSystem

¢ Interfaces§ WhatisanInterface?§ DefininganInterface§ ImplementinganInterface§ ImplementingMultipleInterfaces§ ExtendingaClassandImplementingInterface(s)§ ExtendinganInterface§ InterfacesasTypes

¢ InterfacesvsAbstractClasses

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AbstractClasses

¢ Anabstractclass isaclassthatisdeclaredabstract¢ Anabstractclassmayormaynotincludeabstractmethods.¢ Abstractclassescannotbeinstantiated,buttheycanbesubclassed.

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Abstract Classes:Revisiting the Shapes

Shape

Circle Quadrilateral RightTriangle

Square Rectangle

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Abstract Classes¢ Shapesallhavecertainstates(forexample:position,orientation,linecolor,fillcolor)andbehaviors(forexample:moveTo,rotate,resize,draw)incommon.

¢ Someofthesestatesandbehaviorsarethesameforallshapes(forexample:position,fillcolor,andmoveTo).

¢ Othersrequiredifferentimplementations(forexample,resizeordraw).

¢ AllShapesmustbeabletodraworresizethemselves;theyjustdifferinhowtheydoit.

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AbstractClasses

publicclassShape{privateStringname;

publicShape(Stringname){this.name=name;

}

publicStringgetName(){returnname;

}

publicvoiddraw(){//whatistheshape?// Code...?!Nothing!

}}

publicabstractclassShape{privateStringname;

publicShape(Stringname){this.name=name;

}

publicStringgetName(){returnname;

}

publicabstractvoiddraw();}

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AbstractMethods

¢ Anabstractmethod isamethodthatisdeclaredwithoutanimplementation§ withoutbraces,andfollowedbyasemicolon,likethis:

public abstract void draw();

¢ When anabstract class issubclassed,the subclass usuallyprovides implementations for all ofthe abstract methods initsparent class.§ However,if itdoes not,then the subclass must also bedeclared abstract.

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AbstractClassespublicclassRightTriangle extendsShape{

privateinta;

publicRightTriangle(Stringname,int a){super(name);this.a=a;

}

publicintgetA(){returna;

}//overrideabstractmethodpublicvoiddraw(){

for(int line=1;line<=a;line++){for(inti=0;i<line;i++){

System.out.print("*");}System.out.println();

}}

}

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AbstractClasses

publicabstractclassQuadrilateralextendsShape{

publicQuadrilateral(Stringname){super(name);

}

//stillnothingto draw!publicabstractvoiddraw();

}

publicclassSquareextendsQuadrilateral{privateinta;

publicSquare(Stringname,int a){super(name);this.a=a;

}publicintgetA(){

returna;}

//overrideabstractmethodpublicvoiddraw(){

for(int line=0;line<a;line++){for(intcol=0;col<a;col++){

System.out.print("*");}System.out.println();

}}

}

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AbstractClasses

publicclassProgram{

publicstaticvoidmain(String[]args){//compilationerror!:"CannotinstantiatethetypeShape"Shapeshape =newShape("Shape");

//compilationerror!:"CannotinstantiatethetypeQuadrilateral"Quadrilateralquadrilateral=newQuadrilateral("Quadrilateral");

Squares=newSquare("Square",4);s.draw();

Rectangler=newRectangle("Rectangle",3,7);r.draw();

RightTriangle t=newRightTriangle("RightTriangle",5);t.draw();

}}

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AbstractClasses

¢ ArepartoftheinheritancehierarchyCircleextendsShapeSquareextends Quadrilateral

¢ Canhaveconstructor(s),butnoobjects oftheseclassescanbecreated

Shapeshape =newShape("Shape");//compilationerror!:"CannotinstantiatethetypeShape“

¢ Classesthatcanbeusedtoinstantiateobjectsarecalledconcreteclasses.

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Example-1

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Example-2

¢ Imaginethereareseveralinstruments,eitherstringedorwind.¢ Designaclasshierarchyforonlytwotypesofinstruments,guitarsandflutes.

¢ Youhavetodesignyourmodelinawaythatnewinstrumentscanbeadded inthehierarchylateron.

¢ Imaginethereisonlyonefeatureforeachinstrumentatthemoment,whichistheplay feature.

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Example-2publicabstractclassInstrument{

protectedStringname;abstractpublicvoidplay();

}

abstractclassStringedInstrument extendsInstrument{protectedint numberOfStrings;

}

Stillabstract

Abstractclass

publicclassGuitarextendsStringedInstrument{

publicvoidplay(){System.out.println(“Guitarisrocking!”);

}}

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Example-2

abstractclassWindInstrument extendsInstrument{//features

}

publicclassFluteextendsWindInstrument{

publicvoidplay(){System.out.println(“Fluteisrocking!”);

}}

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ExampleProject:PayrollSystem

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Overviewoftheclasses

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Employee.java(1)

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Employee.java(2)

Earningswillbecalculatedinsubclasses

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SalariedEmployee.java

Overriddenmethods

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HourlyEmployee.java(1)

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HourlyEmployee.java(2)

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CommissionEmployee.java(1)

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CommissionEmployee.java(2)

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BasePlusCommissionEmployee.java

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PayrollSystemTest.java(1)

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PayrollSystemTest.java(2)

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Interfaces

GUI

Laptop

LCD/LEDTV

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ConceptofInterface

¢ Aninterfaceisacontract.Itguaranteesthatthesystemwillhavecertainfunctionalities.

¢ Aninterfaceisanintegrationpointbetweentwosystems.

¢ Asystemcanhavemanyinterfaces,soitcanbeintegratedtomanyothersystems.

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publicinterface Shape{publicabstractvoiddraw();

publicstaticfinaldoublePI=3.14;}

DefininganInterface

¢ Keywordinterface isusedtodefineaninterface

¢ Methodsinaninterfacemustbepublic andabstract,thesekeywordsarecommonly omitted

¢ Interfacescanincludepublic static final variables(constants),thesekeywordsarecommonlyomitted

Noneedtowrite

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ImplementinganInterface

¢ Aninterfaceisimplementedbythekeywordimplements

¢ Anyclassimplementinganinterfacemusteitherimplementallmethodsofit,orbedeclaredabstract

publicclassRightTriangle implements Shape {//.....publicvoiddraw(){

for(int line=1;line<=a;line++){for(inti=0;i<line;i++){

System.out.print("*");}System.out.println();

}}

}

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ImplementingMultipleInterfaces

¢ Morethanoneinterface canbeimplementedbyaclass.¢ Namesofinterfacesare separated bycomma

Question:Whatifatleasttwointerfacesincludethesamemethoddefinition?

publicclassLedTvimplements Usb,Hdmi,Scart,Vga {

//.....

}

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ExtendingaClassandImplementingInterface(s)

publicclassCarextendsVehicleimplementsShape {

publicvoiddraw(){//....

}}

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ExtendinganInterface

¢ Itispossibleforaninterfacetoextendanotherinterface

publicinterfaceI1{voidm1();

}

publicclassC2implements I2 {publicvoidm1(){

//...}publicvoidm2(){

//...}

}

publicinterfaceI2extendsI1{voidm2();

}

publicclassC1 implementsI1 {

publicvoidm1(){//...

}}

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InterfacesasTypes

¢ Whenyoudefineanewinterface,youaredefininganewreferencedatatype.

¢ Youcanuseinterfacenamesanywhereyoucanuseanyotherdatatypename.

¢ Ifyoudefineareferencevariablewhosetypeisaninterface,anyobjectyouassigntoitmustbeaninstanceofaclassthatimplementstheinterface.

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InterfacesasTypes

publicclassProgram{publicstaticvoidmain(String[]args){

Shapeshape;

shape=newSquare(4);shape.draw();

shape=newRectangle(3,7);shape.draw();

shape=newRightTriangle(5);shape.draw();

}}

publicclassProgram{publicstaticvoidmain(String[]args){

Shape[]shapes=newShape[3];shapes[0]=newSquare(5);shapes[1]=newRectangle(2,8);shapes[2]=newRightTriangle(3);for(Shapes:shapes){

drawIt(s);}

}

publicstaticvoiddrawIt(Shapes){s.draw();

}}

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ExampleProject:PayrollSystemRevisited

Interfaceimplementation

symbol

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Payable.java

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Invoice.java(1)

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Invoice.java(2)

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Employee.java

/*Restoftheclassissameasthepreviousexampleexceptthereisnoearnings() method!*/

Payable interfaceincludesgetPaymentAmount() method,butEmployee classdoesnotimplementit!

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SalariedEmployee.java

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PayableInterfaceTest.java

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Interfaces vs Abstract Classes

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Summary

¢ Abstractclassisdefinedwiththekeywordabstract¢ Ifaclassincludesanabstractmethod,itmustbedeclaredasabstract

¢Objectsofabstractclassescannotbecreated¢ Interfaceisdefinedwiththekeywordinterface¢ Aclasscanimplement aninterface,aninterfacecanextend aninterface

¢ Aclasscanimplementmanyinterfaces¢Objectsofinterfacescannotbecreated

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Acknowledgements

¢ Thecoursematerialusedtopreparethispresentationismostlytaken/adoptedfromthelistbelow:¢ Java- HowtoProgram,PaulDeitel andHarveyDeitel,PrenticeHall,2012

¢ http://www.javatpoint.com/difference-between-abstract-class-and-interface