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(remedial)Java

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1 ABezerianos - Remedial Java - Session-2.key - 6 September 2019

ObjectOrientedProgramming



▪ Alllanguagesmakeabstractions

▪ OOPattemptstorepresenttherealworldascloseaspossible,i.e.closertotheproblemspace

▪ Useofthenotionofobjects



▪ Representtherealworld

Car



▪ Representtherealworld

CarMakeColor

HorsepowerMPG

ManualKilometers



▪ Objectsgrouptogethercharacteristics:▪ Primitives(int,double,char,etc..)▪ Objects(String,etc...)

CarString MakeString Color

int Horsepowerdouble MPG

boolean Manualint Kilometers


Whyuseclasses?

▪ Wecouldusejustprimitives…// my carString myCarMake;String myCarColor;int myCarHorsepower;double myCarMPG;boolean myCarManual;int myCarKilometers;


Whyuseclasses?

▪ Wecouldusejustprimitives…

▪ WhatifIhad800cars…

// my carString myCarMake;String myCarColor;int myCarHorsepower;double myCarMPG;boolean myCarManual;int myCarKilometers;

// my father’s carString dadsCarMake;String dadsCarColor;int dadsCarHorsepower;double dadsCarMPG;boolean dadsCarManual;int dadsCarKilometers;


Whyuseclasses?

MakeColor

HorsepowerMPG

ManualKilometers

myCar

Make=…Color=…

Horsepower=…MPG=…

Manual=…Kilometers=…

Car


Whyuseclasses?

797morecars…myCar

MakeColor

HorsepowerMPG

ManualKilometers

dadsCar

MakeColor

HorsepowerMPGManual

Kilometers

momsCar

MakeColor

HorsepowerMPG

ManualKilometers

Car


Whyuseclasses?

myCar dadsCar momsCar

797morecars…

puioBuilding pcriBuilding

Morebuildings

Car

Building


Whyuseclasses?

myCar dadsCar momsCar

797morecars…

road

puioBuilding pcriBuilding

Morebuildings


Whyuseclasses?

cars

buildings

[]

road

[]


ObjectandClass(1)

▪ Everythingisanobject.

▪ Aprogramisacollectionofobjectsthattelleachotherwhattodo(passmessages)

▪ Similarobjectsaregroupedtogetherinanobjectclass.▪ Aclassisa“patterntemplate”forcreatingsimilarobjects.


ObjectandClass(2)

▪ Withthekeywordclasswecandefineabstractdatatypes,i.e.classes(fundamentalconceptofOOP).

▪ WeusetheseabstractdatatypesinthesamewayasweusethepredefinedJavadatatypes.

▪Cancreatevariablesofthespecifictype(class),andgivethemconcretevalues(object/instance)

▪Theinstances(objects)ofaclasssharethesamecharacteristics(e.g.,allcarshaveamake,model,color,etc.).

▪Buteachobjecthasitsownstate,somycarisapinkpeugeot104,mymother’scarisagreenvolkswagenbeetle,etc.


Definingclasses


Let’sdeclareacarclass

public class Car {

}


Let’sdeclareacarclass

public class Car {

}

fields:data/characteristics

methods:datamanipulation

msgtreatment


Classfields

public class Car {

TYPE varname;TYPE varname = some_value;

}


Classfields

public class Car {

String make;String color;int horsepower;double mpg;boolean manual;int kilometers = 0;

}


Classfields

public class Car {

String make;String color;int horsepower;double mpg;boolean manual;int kilometers = 0;XXXX YYYY;

}


Classfields

public class Car {

String make;String color;int horsepower;double mpg;boolean manual;int kilometers = 0;Car[] overtaken;

}


Classmethods

public class Car {


void move (int speed, int time) {kilometers += speed*time;System.out.println(“Covered ” + kilometers + “km”);

}}


Classoverview

public class Car {


void move (int speed, int time) {kilometers += speed*time;System.out.println(“Covered ” + kilometers + “km”);

}}

ClassDefinition


NoteonClassdeclaration

Injava:

▪ ClassnamesareCapitalized

▪ 1Class=1file

▪ aClasswithamainfunctioncanberun


Classoverview

Car myCar = new Car();

ClassInstance

(aconcreteobjectofthatclass)

Butwhatmake,color,…?


ClassConstructor

public class CLASSNAME {

CLASSNAME ( ) {}

CLASSNAME ([ARGUMENTS]) {}

}

CLASSNAME obj1 = new CLASSNAME();CLASSNAME obj2 = new CLASSNAME([ARGUMENTS]);


Carconstructorspublic class Car {

String _make;String _color;int _horsepower;double _mpg;boolean _manual;int _kilometers = 0;

public Car(String mymake, String mycolor){_make = mymake; // or this._name = myname;_color = mycolor; // or this._color = mycolor;

}

void move (int speed, int time) {_kilometers += speed*time;System.out.println(“Covered ” + _kilometers + “km”);

}}


ClassConstructorcharacteristics(1)

▪ Aconstructorisaparticularmethod

▪ Itisusedtoinitializeanobject,itinitializesfields,andguaranteesinvariantsattheobjects’creation

▪ Itisinvokedwiththekeywordnewonceperobject,anditreservesmemoryforthenewlycreatedobject


ClassConstructorcharacteristics(2)

▪ Constructorname=Classname

▪ Noreturntype,neverreturnsanything

▪ Allclassesneedatleastoneconstructor,ifnonedefinedthanJavaaddsadefaultone

CLASSNAME ( ) {}

▪ Ifaconstructorisdefined,theJavacompilernolongercreatesadefaultconstructor


Inter-constructorcalls

▪ Cancalloneconstructormethodfromanotherusingthekeywordthis(morelater).Thisprocessiscalledconstructoroverloading.

▪ Thecalltotheconstructormustbethefirstcallpublic Car(){

this("nomake", "nocolor"); // I want a default value}

public Car(String myname, String mycolor){_name = myname; // or this.name = myname;_color = mycolor; // or this.color = mycolor;

}


Usingclasses


ClassesandInstances

// class Definition (pattern)public class Car {

…}

// class Instances (concrete objects)Car myCar = new Car ("peugeot”, “pink”); Car momsCar = new Car (”volkswagen”, “green”);


ClassesandInstances

// class Definitionpublic class Car {

…}

// class InstancesCar myCar = new Car ("peugeot”, “pink”); Car momsCar = new Car (”volkswagen”, “green”);

▪ Eachinstance(myCar,momsCar)hastheirownvaluesfortheirfields(e.g.,differentmakeorcolor).


Accessingmembersofaclass

▪ Membersofaclass(fields,methods)canbeaccessedwiththenotation“.”appliedafterthereferencetoanobject




▪ ForfieldsObject.Fieldname

Car myCar = new Car ("peugeot”, “pink”); System.out.println(myCar._make); // returns peugeot




▪ FormethodsObject.Methodname([ARGUMENTS ])

Car myCar = new Car ("peugeot”, “pink”);

myCar.move(85,30); // updates the variable kilometers



▪ Withrespecttoclassicfunctions(e.g.inC),wechoseoneoftheobjectstoacton:▪ C:equals(o1,o2)▪ Java:o1.equals(o2)

Class Car{boolean equals (Car m){... }

}

class Main{public void todo{

Car c1 = ...Car c2 = ...if ( c1.equals(c2)){...}

}}


this

▪ this(orself,it)correspondstoareferencetotheobjectonwhichamethodiscalled

▪ Clarifiesthescopeofthevariable(ifsamename)

Herethistakesthevalueofc1andcofc2

Class Car{boolean equals (Car c){

if (this == c)return true;

}}

class Main{public void todo{

Car c1 = ...Car c2 = ...if ( c1.equals(c2)){...}

}}


thisexample

▪ Onlymethod-levelkilometersisupdated

public class Car {int kilometers = 0;

Car(){move(30);System.out.println(“Covered ” + kilometers + “km”);

}void move (int kilometers) {

kilometers += kilometers;System.out.println(“Covered ” + kilometers + “km”);

}

public static void main (String args[]){Car car1 = new Car();

}}


thisexample

▪ Object-levelkilometersisupdated

public class Car {int kilometers = 0;

Car(){move(30);System.out.println(“Covered ” + kilometers + “km”);

}void move (int kilometers) {

this.kilometers += kilometers;System.out.println(“Covered ” + kilometers + “km”);

}

public static void main (String args[]){Car car1 = new Car();

}}


Exercise1-part1www.lri.fr/~anab/teaching/remedialJava/ex-session2.pdf

Hint:useBook[]insideLibrary

Reminder:

▪ Classdefinition

▪ Classuse

public class CLASSNAME {TYPE varname; // fieldTYPE varname = some_value; // initialized fieldCLASSNAME ([ARGUMENTS]) { … } // constructor methodTYPE method_name ([ARGUMENTS]) {…} // another method

}

CLASSNAME obj = new CLASSNAME([ARGUMENTS]);obj.method_name([ARGUMENTS]);


ReferencesvsValues


PrimitivesvsReferences

▪ PrimitivesarebasicJavatypes▪ int,char,double,float,long,short,boolean,byte▪ Theactualvalueisstoredinthevariable(memory)

▪ Thesearenotobjects(almosteverythingelseis)


PrimitivesvsReferences

▪ PrimitivesarebasicJavatypes▪ int,char,double,float,long,short,boolean,byte▪ Theactualvalueisstoredinthevariable(memory)

▪ Thesearenotobjects(almosteverythingelseis)

▪ Referencetypesarearraysandobjects▪ String,int[],Car,…


HowPrimitivesarestored

▪ Variablesarelikefixedsizememorybuckets

▪ Primitivesaresmallenoughtofitinthem

intdoublebooleanchar


HowObjectsarestored

▪ Objectsaretoobigtofitinavariable▪ Instead▪ Theyarestoredsomewhereelse

▪ Variablesstoreanumberthatlocatestheobject

object objectlocationobject object2

object3 object4


References

▪ Theobject’slocation(address)iscalledareference

▪ Theoperator==comparesreferences

▪ Doescar1==car2?Car car1 = new Car ("peugeot”, “pink”); Car car2 = new Car ("peugeot”, “pink”);

NO


References

▪ Theobject’slocation(address)iscalledareference

▪ Theoperator==comparesreferences

▪ Doescar1==car2?

Thisiswhyweusestring1.equals(string2)

Car car1 = new Car ("peugeot”, “pink”); Car car2 = new Car ("peugeot”, “pink”);

NO


References

Car car1 = new Car ("peugeot”, “pink”); Car car2 = new Car ("peugeot”, “pink”);

Referenceforobjectcar1

car1


car2

Object1peugeotpink

Object2peugeotpink


References

▪ Using=onafieldupdatesitsvalueontheobject

Car car1 = new Car ("peugeot”, “pink”);car1.color = “red”;


car1

Object1peugeotpink


References


car1




References


car1

Object1peugeotred




▪ Using=onanobjectvariableupdatesthereferencetotheobject

References


car1

Car car1 = new Car ("peugeot”, “pink”);Car car2 = new Car ("peugeot”, “pink”); car1= car2;


car2

Object1peugeotpink

Object2peugeotpink



References


car1



car2

Object1peugeotpink

Object2peugeotpink



References


car1



car2

Object1peugeotpink

Object2peugeotpink


▪ Using[]or•▪ Followsthereferencetotheobject▪Maymodifytheobject,butneverthereference

Imaginefollowingdirectionstoahouse,youcanmodifythefurnitureinside(analogoustofollowingthereferenceofanobjectandchangingitsfields)

References


MethodsandReferences

void dofoo (int x, int[] many_ints, Car c){x = 100;many_ints[0] = 100;c.make = “100”;

}

…

int i = 0;int[] j = {0};Car k = new Car (“peugeot”, “pink”);dofoo(i,j,k);

Whatarethevaluesofi,j,k?


MethodsandReferences

▪Primitives(int,double,boolean)

▪Theactualvalueisstoredinthevariable(memory)

▪Sothescopeofthevariableaffectsvalue(passbyvalue)

▪Referencetypes(arraysandobjects)▪Theaddress(ref)totheobjectisstoredinthevariable▪Soevenifthevariablescopechanges,anychangestotheobjectremain(asthevariablepointstotheobject)

(passbyreference)


TypeandClass

public static void main (String args[]){int x = 0; // type is int, value is 0

String s = args[0];Car k = new Car (“peugeot”, “pink”);

}

Asidenote:

▪ VariableshaveaType▪ Basictypeshaveavalue▪ ReferenceshaveaClass

Type Class


Exercise1-part2&3www.lri.fr/~anab/teaching/remedialJava/ex-session2.pdf

Hint:useBook[]insideLibrary

Reminder:

▪ Classdefinition

▪ Classuse

public class CLASSNAME {TYPE varname; // fieldTYPE varname = some_value; // initialized fieldCLASSNAME ([ARGUMENTS]) { … } // constructor methodTYPE method_name ([ARGUMENTS]) {…} // another method

}

CLASSNAME obj = new CLASSNAME([ARGUMENTS]);obj.method_name([ARGUMENTS]);


Car car1; // variable car1 stores a ref // that points nowhere (null)

car1 = new Car ("peugeot”, “pink”); // now it points to a new object

NullReference

Referencetonull

car1

Asidenote:null

▪ AsyoucontinuewithJavayoumaycomeacrossthekeywordnull,thisisthevalueofareferencethatisnotpointinganywhere

Referencetonull

car1




NullReference

Referencetonull

car1

ObjectCarpeugeotred

Asidenote:null


ReferencetothenewCarobject

car1




NullReferenceAsidenote:null


Youcouldtestifareferenceisnull(e.g.,if(car1==null),butthisisalastresortpractice,thereareusuallywaystowriteyourcodesothisisnotneeded


Statictypesandmethods


▪ Thekeywordstaticcanbeappliedtofieldsandmethods

▪ Itstatesthatthefieldormethod:▪ Isdefinedattheclassdeclaration(nottheobject)

▪ Isnotuniqueforeachinstance(object),butissharedbetweenallobjectsoftheclass

static


static

public class Car {static int noOfCarsMade = 0;

}

Car car1 = new Car ();Car car2 = new Car (); Car.noOfCarsMade = 2;

▪Whatiscar1.noOfCarsMade?car2.noOfCarsMade?


▪ Keeptrackofcarsmade

▪WhatdidIforget?

staticexample

public class Car {int noOfCarsMade = 0;

Car() {++noOfCarsMade;

}}


▪ Keeptrackofcarsmade

staticfieldexample

public class Car {static int noOfCarsMade = 0;

Car() {++noOfCarsMade;

}}


staticmethodexample

▪ Dependingonwhetherthecallismadebyastaticmethod

public class Car {static void carColor( Car mycar){

System.out.println(mycar.color); // or this.color}

}

public class Car {void carColor(){

System.out.println(color);}

}

▪ Or…


staticmethod

public class Car {String numberPlate;static void whoami(){

System.out.println(numberPlate);}

}

▪ Non-staticmethodscanreferencestaticmethods,butnottheotherwayround,why?


main

public static void main (String[] args) {

}

▪ Whyismainstatic?


main

class Car {void move (int x){

_kilometers += x;}

public static void main (String[] args) {// I cannot call move(100); // as it needs to be static,// BUT I can run it through an object

Car c = new Car();c.move(100);

}}

▪ Whyismainstatic?


PrinciplesofOOP


PrinciplesofOOP

▪ Abstraction(classes…)▪ Groupingfunctionalityrelatedtooriginalobjects/conceptswearetryingtorepresent(hidingtheimplementationdetails,relatedtoencapsulation…)

▪ Oneobject,oneresponsibility

▪ Encapsulation

▪ Inheritance(later)

▪ Polymorphism(later)


Oneobject,oneresponsibility

▪ Ifwewanttoreuse(typesof)objects,theyshouldnotholdtoomuchfunctionality

▪ 2object(types)=>2responsibilities

▪ Weusedelegation/msgpassingbetweenobjects(i.e.thefactthatobjectscancommunicate)todivideresponsibilities


Oneobject,oneresponsibility(example)

▪ Wewanttocreateanapplicationthatmanagesagarage,andcalculateshowmuchmoneythegaragewouldmakeifallcarsleftthegarageatonce(i.e.expectedgain)

▪ Thepricedependsontheparkingtimeandonthecarbrand

▪ Responsibilities?



▪ 4possibleresponsibilities(so4objects)▪ Garage:storecars▪ Car:thecaritself☺

▪ Brand:thecarbrandthataffectsprice▪ BrandPricing:findingthepricebasedonthebrand,timeandamodel



▪ Aclassdiagram

Parking------------------------Car[]cars-----------------------+park(int,Car)+unpark(int):Car

BrandPricer----------------------------------------------+getPrice(Brand,time):double

Car--------------------------idPlate:String-brand:Brand-------------------------

Brand--------------------------name:String-------------------------+getName():String+setName(String)


Encapsulation

▪ Theprinciplethatguaranteesthatnofieldinanobjectcanbemodified(andpossiblybecorrupted)withoutverification

▪ Thestateofafieldcanonlybechanged(ifitismodifiable),bymethodsthatverifythechanges


Encapsulation

▪ Anexampleofpolarcoordinates

▪ ClassPointdoesnotguaranteeencapsulation

class oneClass{void oneMethod (Point p){

p.init(2.0,3.0);

p.rho = -1; // oh, no!!!}

}

class Point{ double rho; double theta;

void init (double x, double y) {rho = Math.hypot(x,y);theta = Math.atan2(x,y);

}}


Visibility

▪ Somelanguages(likeSmaltalk)donotallowaccesstofieldsfromoutsidetheobject

▪ Others,likeC++,Java,requiresustodeterminethevisibilityoffieldsusingvisibilitymodifiers

▪ Visibilitycanensureencapsulation,byrestrictingthedirectaccesstofieldsofanobject


Modifyingvisibility

▪ Thereare4visibilitymodifiers:▪ Apublicmemberisvisiblebyeveryone▪ Aprotectedmemberisvisiblebyinheritedclasses(later)andclassesinsidethesamepackage

▪ Amemberwithoutmodifier(orfriend)isvisiblebyclassesinsidethesamepackage

▪ Aprivatemember,isonlyvisibleinsideaclass

Thevisibilityorderis: private<protected<public


Encapsulation

▪ Trytokeepfieldsprivate

class oneClass{void oneMethod (Point p){

p.init(2.0,3.0);System.out.println(p.theta);p.rho = -1; // last 2 lines

// do not compile}

}

class Point{ private double rho; private double theta;

public void init (double x, double y) {rho = Math.hypot(x,y);theta = Math.atan2(x,y);

}}


Whymodifyvisibility

▪ Controllingaccesstoclassfieldscan:▪ Protectprivateclassinformation(sorta)andavoiderrorsinstate

▪ Clarifieshowotherscanuseyourclass▪ Keepsimplementationseparatefromclassinterface(i.e.viewtotheworld)


Visibilityandmaintenance

▪ Someadvice:▪ Avoidfieldsthatarepublicorprotected(apartfromconstants)

▪ Makemethodspubliconlywhennecessary▪ Aprotectedmethodallowsustoimplementacommonserviceforsubclasses(later),ifthepackagevisibilityisnotavailable


Encapsulation,aproblem

▪ Imaginewewantrhotoneverbelessthan0▪ Possibility1:controlaccesstorho


public void init (double x, double y) {…

}

public void setRho (double rho){// do some tests

}}



▪ Imaginewewantrhotoneverbelessthan0▪ Possibility2:whataboutinitialization?


public void init (double x, double y) {…

}

public void setRho (double rho){// do some tests

}

public Point (double rho, double theta){setRho(rho);

}}



▪ Imaginewewantrhotoneverbelessthan0▪ Possibility2b(advanced):initialization(constructor)

▪ Canmakethefieldconstant(final)andthecompilerverifiesvaluesareassignedbyeachconstructor,andcannotbechangedafter.

class Point{ private final double rho; private final double theta;

public Point (double rho, double theta){if (rho <= 0) {

throw new IllegalArgumentException ("Illegal rho" + rho); } this.rho = rho; // else will not compile this.theta = theta; // else will not compile}


DifferenttypesofmethodsClassmethodscanbe:

▪ Constructors:initializeobjectfields

▪ Mutator&Accessormethods:▪ Mutator/Setter:controlandverifychangesinafield.OftenhavetheformsetX()

▪ Accessor/Getter:returnthevalueofaprivatefield(usuallyaccompaniesasettermethod)

▪ Businessmethods:▪ …:performotherneededcalculationsandactions


Setters/Gettersclass Point{ private double x; private double y;

public Point (double x, double y){this.x = x;this.y = y;

} public double getX (){

return x; } public void setX (double x){

this.x = x; } public double getY (){

return y; } public void setY (double y){

this.y = y; }}

Methodsthatallowaccessto(readandmodify)fields…

Point p = new Point (1.0,1.0);System.out.println(“%d %d\n”,

p.getX(), p.getY() );


Setters/Gettersclass Point{ private double rho; private double theta;

public Point (double x, double y){init(x,y);

} private void init (double x, double y){

rho = hypot(x,y);theta = atan2(x,y);

} public double getX (){

return rho*cos(theta); } public void setX (double x){

init(x,getY()); } public double getY (){

return rho*sin(theta); } public void setY (double y){

init(getX(),y); }}

Methodsthatallowaccessto(readandmodify)fields…

Andchangetheimplementation,withoutchangingthemethodcall

Point p = new Point (1.0,1.0);System.out.println(“%d %d\n”,

p.getX(), p.getY() );


Don’toverdoit

Don’tputsetters/gettersforeverything

▪ Agettermeansmorecodetomaintain,soaddthemonlywhenneededtoseeafield

▪ Asetterisonlyneededifwearesureweneedtochangethefieldofanobject

▪ Limitthenumberofobjectswithmutators,i.e.objectsthatcanchange(mutablevsimmutableobjects)


Exercise2www.lri.fr/~anab/teaching/remedialJava/ex-session2.pdf

Reminder:

▪ Visibilitykeywordsprivate<protected<public

▪ Membersofaclass(fields,methods)shouldbeprivateunlessnecessary

▪ Setter/gettermethodscanhelpaccessprivatefieldsifneeded

▪ secours

▪ fkns922j


Notesonstyle


Somenotesonstyle

▪ Usegood/expressivenamesforvariablesString a1;int a2;double b; // BAD!!

String firstName; // GOOD String lastName; // GOOD int temperature; // GOOD


Somenotesonstyle

▪ Usegood/expressivenamesforvariables▪ Differentiatefieldvariablenamesfromothervariables

public class TimeOfDay{private int _hour;private int _minute;

public TimeOfDay(int hour, int minute){_hour = hour;_minute = minute;

}}


Somenotesonstyle


▪ Javanamingconventions:▪ Variables: Nouns, lowercase first letter, capitals separating words

e.g., x, shape, highScore, fileName

▪ Methods: Verbs, lowercase first letter e.g., getSize(), draw(), drawWithColor()

▪ Classes: Nouns, uppercase first letter e.g., Shape, WebPage, EmailAddress


Somenotesonstyle


▪ Useindentation

▪ Usewhitespacesincomplexexpressions


Somenotesonstyle


▪ Useindentation

▪ Usewhitespacesincomplexexpressions

▪ Donotduplicatetests


Somenotesonstyle


if (basePay < 9.0) { ...

} else if (hours > 60) { ...

} else if (basePay >= 9.0 && hours <= 60) { ...

}


Somenotesonstyle


if (basePay < 9.0) { ...

} else if (hours > 60) { ...

} else { ...

}


Exercise3-Homeworkwww.lri.fr/~anab/teaching/remedialJava/ex-session2.pdf

Reminder:▪ Considerencapsulation:privatefields(andmethodsunless

needed),useofgetter/settermethods

▪ Remembernamingconventions/styles(differentiatefield_variables,namevariablesaslowerCaseNouns,andmethodsaslowerCaseVerbs()

▪ Thestatickeywordcreatesfields/methodsthatarecommonforallobjectsofaclass


AquickOOPsummary


OOP–Object(1)1.Everythingisanobject.▪ Anobjectisanimprovedvariable:itstoresdata,wecanqueryit,requestthatitactsuponitself.

▪ Wecantakeanyconceptofaproblemwewanttosolve(abook,adog,anadministrativeservice,…)andrepresentitasanobjectintheprogram


OOP–Object(2)2.Aprogramisacollectionofobjectsthattelleachotherwhattodo,bypassingmessages.▪ Tomakearequesttoanobject,wepassitamessage.Morespecifically,arequestcanbeseenasacalltoapublicmethod/functionthatbelongstothetargetobject.


OOP–Object(3)3.Eachobjecthasitsowndedicatedmemoryspace,madeupofotherobjects(attributes)▪ Anewobjectismadeupbyotherexistingobjects(orcollectionsofthem).Thisway,thecomplexityofaprogramishiddeninthesimplicityoftheroleofeachobject.


OOP–Object(4)

4.Eachobjectisofaspecifictype.▪ InOOP,eachobjectisaninstanceofaclass(canbeconsideredasatype,orapattern).Themostimportantcharacteristicofaclassis:▪Whatmessagescanwesendtoit?


OOP–Object(5)

5.Allobjectsofthesametype(class)canreceivethesamemessages.▪ Anobjectofthetype“circle”isalsounobjectofthetype“geometricform”,soacirclecanacceptmessagesdestinedtogeometricforms.


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