Understand the concept of Java Classes and Objects with an example. · Class – Dogs · Data members or objects– size, age, color, breed, etc.
Skiptocontent
ClassesandObjectsinJavaarethefundamentalcomponentsofOOP’s.Oftenthereisaconfusionbetweenclassesandobjects.Inthistutorial,wetrytotellyouthedifferencebetweenClassandObjectinJava.
First,let’sunderstandwhattheyare,
WhatisClassinJava?
WhatisanObjectinJava?
WhatistheDifferenceBetweenObjectandClassinJava?
ConceptofClassesandObjects
ClassesandObjectsinJavaExamplePrograms
JavaObjectandClassExample:mainoutsideclass
WhatisClassinJava?
Classareablueprintorasetofinstructionstobuildaspecifictypeofobject.ItisabasicconceptofObject-OrientedProgrammingwhichrevolvearoundthereal-lifeentities.ClassinJavadetermineshowanobjectwillbehaveandwhattheobjectwillcontain.
SyntaxofClassinJava
class{
field;
method;
}
WhatisObjectinJava?
Objectisaninstanceofaclass.AnobjectinOOPSisnothingbutaself-containedcomponentwhichconsistsofmethodsandpropertiestomakeaparticulartypeofdatauseful.Forexamplecolorname,table,bag,barking.Whenyousendamessagetoanobject,youareaskingtheobjecttoinvokeorexecuteoneofitsmethodsasdefinedintheclass.
Fromaprogrammingpointofview,anobjectinOOPScanincludeadatastructure,avariable,orafunction.Ithasamemorylocationallocated.JavaObjectsaredesignedasclasshierarchies.
ObjectSyntaxinJava
ClassNameReferenceVariable=newClassName();
WhatistheDifferenceBetweenObjectandClassinJava?
AClassinobjectorientedprogrammingisablueprintorprototypethatdefinesthevariablesandthemethods(functions)commontoallJavaObjectsofacertainkind.
AnobjectinOOPSisaspecimenofaclass.Softwareobjectsareoftenusedtomodelreal-worldobjectsyoufindineverydaylife.
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UnderstandtheconceptofJavaClassesandObjectswithanexample.
Let’stakeanexampleofdevelopingapetmanagementsystem,speciallymeantfordogs.Youwillneedvariousinformationaboutthedogslikedifferentbreedsofthedogs,theage,size,etc.
Youneedtomodelreal-lifebeings,i.e.,dogsintosoftwareentities.
Moreover,themilliondollarquestionis,howyoudesignsuchsoftware?
Hereisthesolution-
First,let’sdoanexercise.
Youcanseethepictureofthreedifferentbreedsofdogsbelow.
Stophererightnow!Listdownthedifferencesbetweenthem.
Someofthedifferencesyoumighthavelistedoutmaybebreed,age,size,color,etc.Ifyouthinkforaminute,thesedifferencesarealsosomecommoncharacteristicssharedbythesedogs.Thesecharacteristics(breed,age,size,color)canformadatamembersforyourobject.
Next,listoutthecommonbehaviorsofthesedogslikesleep,sit,eat,etc.Sothesewillbetheactionsofoursoftwareobjects.
Sofarwehavedefinedfollowingthings,
Class–Dogs
Datamembersorobjects–size,age,color,breed,etc.
Methods–eat,sleep,sitandrun.
Now,fordifferentvaluesofdatamembers(breedsize,age,andcolor)inJavaclass,youwillgetdifferentdogobjects.
YoucandesignanyprogramusingthisOOPsapproach.
Whilecreatingaclass,onemustfollowthefollowingprinciples.
SingleResponsibilityPrinciple(SRP)-Aclassshouldhaveonlyonereasontochange
OpenClosedResponsibility(OCP)-Itshouldbeabletoextendanyclasseswithoutmodifyingit
LiskovSubstitutionResponsibility(LSR)-Derivedclassesmustbesubstitutablefortheirbaseclasses
DependencyInversionPrinciple(DIP)-Dependonabstractionandnotonconcretions
InterfaceSegregationPrinciple(ISP)-Preparefinegrainedinterfacesthatareclientspecific.
ClassesandObjectsinJavaExamplePrograms
//ClassDeclaration
publicclassDog{
//InstanceVariables
Stringbreed;
Stringsize;
intage;
Stringcolor;
//method1
publicStringgetInfo(){
return("Breedis:"+breed+"Sizeis:"+size+"Ageis:"+age+"coloris:"+color);
}
publicstaticvoidmain(String[]args){
Dogmaltese=newDog();
maltese.breed="Maltese";
maltese.size="Small";
maltese.age=2;
maltese.color="white";
System.out.println(maltese.getInfo());
}
}
Output:
Breedis:MalteseSizeis:SmallAgeis:2coloris:white
JavaObjectandClassExample:mainoutsideclass
Inpreviousprogram,wearecreatingmain()methodinsidetheclass.Now,wecreateclassesanddefinemain()methodinanotherclass.Thisisabetterwaythanpreviousone.
//ClassDeclaration
classDog{
//InstanceVariables
Stringbreed;
Stringsize;
intage;
Stringcolor;
//method1
publicStringgetInfo(){
return("Breedis:"+breed+"Sizeis:"+size+"Ageis:"+age+"coloris:"+color);
}
}
publicclassExecute{
publicstaticvoidmain(String[]args){
Dogmaltese=newDog();
maltese.breed="Maltese";
maltese.size="Small";
maltese.age=2;
maltese.color="white";
System.out.println(maltese.getInfo());
}
}
Output:
Breedis:MalteseSizeis:SmallAgeis:2coloris:white
Summary:
JavaClassisanentitythatdetermineshowJavaObjectswillbehaveandwhatobjectswillcontain
AJavaobjectisaself-containedcomponentwhichconsistsofmethodsandpropertiestomakecertaintypeofdatauseful
Aclasssystemallowstheprogramtodefineanewclass(derivedclass)intermsofanexistingclass(superclass)byusingatechniquelikeinheritance,overridingandaugmenting.
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