Object Class in Java - GeeksforGeeks

Object class is present in java.lang package. Every class in Java is directly or indirectly derived from the Object class. If a class does ... 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ChangeLanguage DataStructuresAlgorithmsInterviewPreparationTopic-wisePracticeC++JavaPythonCompetitiveProgrammingMachineLearningJavaScriptSDESheetPuzzlesGFGSchoolProjects RelatedArticles ▲RelatedArticlesAssigningvaluestostaticfinalvariablesinJavaCovariantReturnTypesinJavaObjectClassinJavaStaticclassinJavaFlexiblenatureofjava.lang.ObjectOverridingequalsmethodinJavaOverridingtoString()MethodinJavaInstanceVariableHidinginJavaStaticBlocksinJavaTheInitializerBlockinJavaInstanceInitializationBlock(IIB)inJavaStaticvsDynamicBindinginJavaWhyJavaisnotapurelyObject-OrientedLanguage?UnderstandingClassesandObjectsinJavastaticKeywordinJavafinalKeywordinJavafinal,finallyandfinalizeinJavaabstractkeywordinjavaAbstractClassinJavaDifferencebetweenAbstractClassandInterfaceinJavaInterfacesinJavaAccessspecifierofmethodsininterfacesAccessmodifiersforclassesorinterfacesinJavaClassesandObjectsinJavaHowareJavaobjectsstoredinmemory?ArraysinJavaSplit()StringmethodinJavawithexamplesArrays.sort()inJavawithexamplesFor-eachloopinJavaReverseastringinJavaAssigningvaluestostaticfinalvariablesinJavaCovariantReturnTypesinJavaObjectClassinJavaStaticclassinJavaFlexiblenatureofjava.lang.ObjectOverridingequalsmethodinJavaOverridingtoString()MethodinJavaInstanceVariableHidinginJavaStaticBlocksinJavaTheInitializerBlockinJavaInstanceInitializationBlock(IIB)inJavaStaticvsDynamicBindinginJavaWhyJavaisnotapurelyObject-OrientedLanguage?UnderstandingClassesandObjectsinJavastaticKeywordinJavafinalKeywordinJavafinal,finallyandfinalizeinJavaabstractkeywordinjavaAbstractClassinJavaDifferencebetweenAbstractClassandInterfaceinJavaInterfacesinJavaAccessspecifierofmethodsininterfacesAccessmodifiersforclassesorinterfacesinJavaClassesandObjectsinJavaHowareJavaobjectsstoredinmemory?ArraysinJavaSplit()StringmethodinJavawithexamplesArrays.sort()inJavawithexamplesFor-eachloopinJavaReverseastringinJavaObjectClassinJavaViewDiscussion ImproveArticle SaveArticle LikeArticle DifficultyLevel: MediumLastUpdated: 03Jan,2022ReadDiscussViewDiscussion ImproveArticle SaveArticle LikeArticle Objectclassispresentinjava.langpackage.EveryclassinJavaisdirectlyorindirectlyderivedfromtheObjectclass.IfaclassdoesnotextendanyotherclassthenitisadirectchildclassofObjectandifextendsanotherclassthenitisindirectlyderived.ThereforetheObjectclassmethodsareavailabletoallJavaclasses.HenceObjectclassactsasarootofinheritancehierarchyinanyJavaProgram.UsingObjectClassMethodsTherearemethodsintheObjectclass: 1.toString():ThetoString()providesaStringrepresentationofanobjectandisusedtoconvertanobjecttoString.ThedefaulttoString()methodforclassObjectreturnsastringconsistingofthenameoftheclassofwhichtheobjectisaninstance,theat-signcharacter`@’,andtheunsignedhexadecimalrepresentationofthehashcodeoftheobject.Inotherwords,itisdefinedas://DefaultbehavioroftoString()istoprintclassname,then //@,thenunsignedhexadecimalrepresentationofthehashcode //oftheobject publicStringtoString() { returngetClass().getName()+"@"+Integer.toHexString(hashCode()); }ItisalwaysrecommendedtooverridethetoString()methodtogetourownStringrepresentationofObject.FormoreonoverrideoftoString()methodrefer–OverridingtoString()inJava Note:WheneverwetrytoprintanyObjectreference,theninternallytoString()methodiscalled.Students=newStudent(); //Belowtwostatementsareequivalent System.out.println(s); System.out.println(s.toString());2.hashCode():Foreveryobject,JVMgeneratesauniquenumberwhichishashcode.Itreturnsdistinctintegersfordistinctobjects.AcommonmisconceptionaboutthismethodisthatthehashCode()methodreturnstheaddressoftheobject,whichisnotcorrect.Itconvertstheinternaladdressoftheobjecttoanintegerbyusinganalgorithm.ThehashCode()methodisnativebecauseinJavaitisimpossibletofindtheaddressofanobject,soitusesnativelanguageslikeC/C++tofindtheaddressoftheobject.UseofhashCode()method:Itreturnsahashvaluethatisusedtosearchobjectsinacollection.JVM(JavaVirtualMachine)usesthehashcodemethodwhilesavingobjectsintohashing-relateddatastructureslikeHashSet,HashMap,Hashtable,etc.Themainadvantageofsavingobjectsbasedonhashcodeisthatsearchingbecomeseasy. Note:OverrideofhashCode()methodneedstobedonesuchthatforeveryobjectwegenerateauniquenumber.Forexample,foraStudentclass,wecanreturntherollno.ofastudentfromthehashCode()methodasitisunique. Java//Javaprogramtodemonstrateworkingof//hashCode()andtoString() publicclassStudent{    staticintlast_roll=100;    introll_no;     //Constructor    Student()    {        roll_no=last_roll;        last_roll++;    }     //OverridinghashCode()    @OverridepublicinthashCode(){returnroll_no;}     //Drivercode    publicstaticvoidmain(Stringargs[])    {        Students=newStudent();         //Belowtwostatementsareequivalent        System.out.println(s);        System.out.println(s.toString());    }}Output:Student@64 Student@64Notethat4*160+6*161=100 3.equals(Objectobj):Itcomparesthegivenobjectto“this”object(theobjectonwhichthemethodiscalled).Itgivesagenericwaytocompareobjectsforequality.Itisrecommendedtooverridetheequals(Objectobj)methodtogetourownequalityconditiononObjects.Formoreonoverrideofequals(Objectobj)methodrefer–OverridingequalsmethodinJavaNote:ItisgenerallynecessarytooverridethehashCode()methodwheneverthismethodisoverridden,soastomaintainthegeneralcontractforthehashCodemethod,whichstatesthatequalobjectsmusthaveequalhashcodes. 4.getClass():Itreturnstheclassobjectof“this”objectandisusedtogettheactualruntimeclassoftheobject.Itcanalsobeusedtogetmetadataofthisclass.ThereturnedClassobjectistheobjectthatislockedbystaticsynchronizedmethodsoftherepresentedclass.Asitisfinalsowedon’toverrideit.Java//JavaprogramtodemonstrateworkingofgetClass() publicclassTest{    publicstaticvoidmain(String[]args)    {        Objectobj=newString("GeeksForGeeks");        Classc=obj.getClass();        System.out.println("ClassofObjectobjis:"                           +c.getName());    }}Output: ClassofObjectobjis:java.lang.StringNote:Afterloadinga.classfile,JVMwillcreateanobjectofthetypejava.lang.ClassintheHeaparea.WecanusethisclassobjecttogetClasslevelinformation.ItiswidelyusedinReflection 5.finalize()method:Thismethodiscalledjustbeforeanobjectisgarbagecollected.ItiscalledtheGarbageCollectoronanobjectwhenthegarbagecollectordeterminesthattherearenomorereferencestotheobject.Weshouldoverridefinalize()methodtodisposeofsystemresources,performclean-upactivitiesandminimizememoryleaks.Forexample,beforedestroyingServletobjectswebcontainer,alwayscalledfinalizemethodtoperformclean-upactivitiesofthesession. Note:Thefinalizemethodiscalledjustonceonanobjecteventhoughthatobjectiseligibleforgarbagecollectionmultipletimes. Java//Javaprogramtodemonstrateworkingoffinalize() publicclassTest{    publicstaticvoidmain(String[]args)    {        Testt=newTest();        System.out.println(t.hashCode());         t=null;         //callinggarbagecollector        System.gc();         System.out.println("end");    }     @Overrideprotectedvoidfinalize()    {        System.out.println("finalizemethodcalled");    }}Output: 366712642 finalizemethodcalled end6.clone():Itreturnsanewobjectthatisexactlythesameasthisobject.Forclone()methodreferClone().Theremainingthreemethodswait(),notify()notifyAll()arerelatedtoConcurrency.Referto Inter-threadCommunicationinJavafordetails.MyPersonalNotes arrow_drop_upSave LikePreviousCovariantReturnTypesinJavaNext StaticclassinJavaRecommendedArticlesPage:09,Nov2027,Dec1906,Apr1603,Sep1607,Jan1407,Jan1424,Dec1511,Jan1611,Jan1612,Jan1612,Jan1626,Oct1724,Mar2130,Mar1703,Apr1726,May1725,Apr1820,Sep1820,Sep1820,Sep1820,Sep1824,Sep1824,Sep1824,Sep18ArticleContributedBy:GeeksforGeeksVotefordifficultyCurrentdifficulty: MediumEasy Normal Medium Hard ExpertImprovedBy:JobAlertHariCharanReddy_Vadnanirshad158nishkarshgandhiArticleTags:Java-ClassandObjectJavaPracticeTags:JavaJava-ClassandObjectReportIssueWritingcodeincomment? 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