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ABOUT SOTIRIOS-EFSTATHIOS MANEASSotiriosEfstathios(Stathis)ManeasisapostgraduatestudentattheDepartmentofInformaticsandTelecommunicationsofTheNationalandKapodistrianUniversityofAthens.Hismaininterestsincludedistributedsystems,webcrawling,modelchecking,operatingsystems,programminglanguagesandwebapplications.

115 Java Interview Questions and Answers The ULTIMATE List (PDFDownload) Postedby:SotiriosEfstathiosManeas inCoreJava April7th,2014

InthistutorialwewilldiscussaboutdifferenttypesofquestionsthatcanbeusedinaJavainterview,inorderfortheemployertotestyourskillsinJavaandobjectorientedprogrammingingeneral.

Inthefollowingsectionswewilldiscussaboutobjectorientedprogramminganditscharacteristics,generalquestionsregardingJavaanditsfunctionality,collectionsinJava,garbagecollectors,exceptionhandling,Javaapplets,Swing,JDBC,RemoteMethodInvocation(RMI),ServletsandJSP.

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ObjectOrientedProgramming(OOP)GeneralQuestionsaboutJavaJavaThreadsJavaCollectionsGarbageCollectorsExceptionHandlingJavaAppletsSwingJDBCRemoteMethodInvocation(RMI)ServletsJSP

ObjectOrientedProgramming(OOP)Javaisacomputerprogramminglanguagethatisconcurrent,classbasedandobjectoriented.Theadvantagesofobjectorientedsoftwaredevelopmentareshownbelow:

Modulardevelopmentofcode,whichleadstoeasymaintenanceandmodification.

Reusabilityofcode.

Improvedreliabilityandflexibilityofcode.

Increasedunderstandingofcode.

Objectorientedprogrammingcontainsmanysignificantfeatures,suchasencapsulation,inheritance,polymorphismandabstraction.Weanalyzeeachfeatureseparatelyinthefollowingsections.

EncapsulationEncapsulationprovidesobjectswiththeabilitytohidetheirinternalcharacteristicsandbehavior.Eachobjectprovidesanumberofmethods,whichcanbeaccessedbyotherobjectsandchangeitsinternaldata.InJava,therearethreeaccessmodifiers:public,privateandprotected.Eachmodifierimposesdifferentaccessrightstootherclasses,eitherinthesameorinexternalpackages.Someoftheadvantagesofusingencapsulationarelistedbelow:

Theinternalstateofeveryobjectedisprotectedbyhidingitsattributes.

Itincreasesusabilityandmaintenanceofcode,becausethebehaviorofanobjectcanbeindependentlychangedorextended.

Itimprovesmodularitybypreventingobjectstointeractwitheachother,inanundesiredway.

Youcanrefertoourtutorialhereformoredetailsandexamplesonencapsulation.

PolymorphismPolymorphismistheabilityofprogramminglanguagestopresentthesameinterfacefordifferingunderlyingdatatypes.Apolymorphictypeisatypewhoseoperationscanalsobeappliedtovaluesofsomeothertype.

InheritanceInheritanceprovidesanobjectwiththeabilitytoacquirethefieldsandmethodsofanotherclass,calledbaseclass.Inheritanceprovidesreusabilityofcodeandcanbeusedtoaddadditionalfeaturestoanexistingclass,withoutmodifyingit.

AbstractionAbstractionistheprocessofseparatingideasfromspecificinstancesandthus,developclassesintermsoftheirownfunctionality,insteadoftheirimplementationdetails.Javasupportsthecreationandexistenceofabstractclassesthatexposeinterfaces,withoutincludingtheactualimplementationofallmethods.Theabstractiontechniqueaimstoseparatetheimplementationdetailsofaclassfromitsbehavior.

DifferencesbetweenAbstractionandEncapsulationAbstractionandencapsulationarecomplementaryconcepts.Ontheonehand,abstractionfocusesonthebehaviorofanobject.Ontheotherhand,encapsulationfocusesontheimplementationofanobjectsbehavior.Encapsulationisusuallyachievedbyhidinginformationabouttheinternalstateofanobjectandthus,canbeseenasastrategyusedinordertoprovideabstraction.

GeneralQuestionsaboutJava1.WhatisJVM?WhyisJavacalledthePlatformIndependentProgrammingLanguage?AJavavirtualmachine(JVM)isaprocessvirtualmachinethatcanexecuteJavabytecode.EachJavasourcefileiscompiledintoabytecodefile,whichisexecutedbytheJVM.Javawasdesignedtoallowapplicationprogramstobebuiltthatcouldberunonanyplatform,withouthavingtoberewrittenor

With 1,240,600uniquevisitorsandover500 authorsweareplacedamongthetopJavarelatedsitesaround.Constantlybeingonthelookoutforpartnersweencourageyoutojoinus.SoIfyouhaveablogwith

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recompiledbytheprogrammerforeachseparateplatform.AJavavirtualmachinemakesthispossible,becauseitisawareofthespecificinstructionlengthsandotherparticularitiesoftheunderlyinghardwareplatform.

2.WhatistheDifferencebetweenJDKandJRE?TheJavaRuntimeEnvironment(JRE)isbasicallytheJavaVirtualMachine(JVM)whereyourJavaprogramsarebeingexecuted.Italsoincludesbrowserpluginsforappletexecution.TheJavaDevelopmentKit(JDK)isthefullfeaturedSoftwareDevelopmentKitforJava,includingtheJRE,thecompilersandtools(likeJavaDoc

,andJava Debugger

),inorderforausertodevelop,compileandexecuteJavaapplications.

3.Whatdoesthestatickeywordmean?CanyouoverrideprivateorstaticmethodinJava?Thestatickeyworddenotesthatamembervariableormethodcanbeaccessed,withoutrequiringaninstantiationoftheclasstowhichitbelongs.AusercannotoverridestaticmethodsinJava,becausemethodoverridingisbasedupondynamicbindingatruntimeandstaticmethodsarestaticallybindedatcompiletime.Astaticmethodisnotassociatedwithanyinstanceofaclasssotheconceptisnotapplicable.

4.Canyouaccessnonstaticvariableinstaticcontext?AstaticvariableinJavabelongstoitsclassanditsvalueremainsthesameforallitsinstances.AstaticvariableisinitializedwhentheclassisloadedbytheJVM.Ifyourcodetriestoaccessanonstaticvariable,withoutanyinstance,thecompilerwillcomplain,becausethosevariablesarenotcreatedyetandtheyarenotassociatedwithanyinstance.

5.WhataretheDataTypessupportedbyJava?WhatisAutoboxingandUnboxing?TheeightprimitivedatatypessupportedbytheJavaprogramminglanguageare:

byte

short

int

long

float

double

boolean

char

Autoboxing

istheautomaticconversionmadebytheJavacompilerbetweentheprimitivetypesandtheircorrespondingobjectwrapperclasses.Forexample,thecompilerconvertsaninttoanInteger

,adoubletoaDouble

,andsoon.Iftheconversiongoestheotherway,thisoperationiscalledunboxing

.

6.WhatisFunctionOverridingandOverloadinginJava?MethodoverloadinginJavaoccurswhentwoormoremethodsinthesameclasshavetheexactsamename,butdifferentparameters.Ontheotherhand,methodoverridingisdefinedasthecasewhenachildclassredefinesthesamemethodasaparentclass.Overriddenmethodsmusthavethesamename,argumentlist,andreturntype.Theoverridingmethodmaynotlimittheaccessofthemethoditoverrides.

7.WhatisaConstructor,ConstructorOverloadinginJavaandCopyConstructor?Aconstructorgetsinvokedwhenanewobjectiscreated.Everyclasshasaconstructor.Incasetheprogrammerdoesnotprovideaconstructorforaclass,theJavacompiler(Javac)createsadefaultconstructorforthatclass.TheconstructoroverloadingissimilartomethodoverloadinginJava.Differentconstructorscanbecreatedforasingleclass.Eachconstructormusthaveitsownuniqueparameterlist.Finally,JavadoessupportcopyconstructorslikeC++,butthedifferenceliesinthefactthatJavadoesntcreateadefaultcopyconstructorifyoudontwriteyourown.

8.DoesJavasupportmultipleinheritance?No,Javadoesnotsupportmultipleinheritance.Eachclassisabletoextendonlyononeclass,butisabletoimplementmorethanoneinterfaces.

9.WhatisthedifferencebetweenanInterfaceandanAbstractclass?Javaprovidesandsupportsthecreationbothofabstractclassesandinterfaces.Bothimplementationssharesomecommoncharacteristics,buttheydifferinthefollowingfeatures:

Allmethodsinaninterfaceareimplicitlyabstract.Ontheotherhand,anabstractclassmaycontainbothabstractandnonabstractmethods.

AclassmayimplementanumberofInterfaces,butcanextendonlyoneabstractclass.

Inorderforaclasstoimplementaninterface,itmustimplementallitsdeclaredmethods.However,aclassmaynotimplementalldeclaredmethodsofanabstractclass.Though,inthiscase,thesubclassmustalsobedeclaredasabstract.

Abstractclassescanimplementinterfaceswithoutevenprovidingtheimplementationofinterfacemethods.

VariablesdeclaredinaJavainterfaceisbydefaultfinal.Anabstractclassmaycontainnonfinalvariables.

MembersofaJavainterfacearepublicbydefault.Amemberofanabstractclasscaneitherbeprivate,protectedorpublic.

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Aninterfaceisabsolutelyabstractandcannotbeinstantiated.Anabstractclassalsocannotbeinstantiated,butcanbeinvokedifitcontainsamainmethod.

AlsocheckouttheAbstractclassandInterfacedifferencesforJDK8.

10.Whatarepassbyreferenceandpassbyvalue?Whenanobjectispassedbyvalue,thismeansthatacopyoftheobjectispassed.Thus,evenifchangesaremadetothatobject,itdoesntaffecttheoriginalvalue.Whenanobjectispassedbyreference,thismeansthattheactualobjectisnotpassed,ratherareferenceoftheobjectispassed.Thus,anychangesmadebytheexternalmethod,arealsoreflectedinallplaces.

JavaThreads11.Whatisthedifferencebetweenprocessesandthreads?Aprocessisanexecutionofaprogram,whileaThread

isasingleexecutionsequencewithinaprocess.Aprocesscancontainmultiplethreads.AThread

issometimescalledalightweightprocess.

12.Explaindifferentwaysofcreatingathread.Whichonewouldyoupreferandwhy?TherearethreewaysthatcanbeusedinorderforaThread

tobecreated:

AclassmayextendtheThread

class.

AclassmayimplementtheRunnable

interface.

AnapplicationcanusetheExecutor

framework,inordertocreateathreadpool.

TheRunnable

interfaceispreferred,asitdoesnotrequireanobjecttoinherittheThread

class.Incaseyourapplicationdesignrequiresmultipleinheritance,onlyinterfacescanhelpyou.Also,thethreadpoolisveryefficientandcanbeimplementedandusedveryeasily.

13.Explaintheavailablethreadstatesinahighlevel.Duringitsexecution,athreadcanresideinoneofthefollowingstates

:

NEW

:Thethreadbecomesreadytorun,butdoesnotnecessarilystartrunningimmediately.

RUNNABLE

:TheJavaVirtualMachine(JVM)isactivelyexecutingthethreadscode.

BLOCKED

:Thethreadisinablockedstatewhilewaitingforamonitorlock.

WAITING

:Thethreadwaitsforanotherthreadtoperformaparticularaction.

TIMED_WAITING

:Thethreadwaitsforanotherthreadtoperformaparticularactionuptoaspecifiedwaitingtime.



TERMINATED

:Thethreadhasfinisheditsexecution.

14.Whatisthedifferencebetweenasynchronizedmethodandasynchronizedblock?InJavaprogramming,eachobjecthasalock.Athreadcanacquirethelockforanobjectbyusingthesynchronizedkeyword.Thesynchronizedkeywordcanbeappliedinamethodlevel(coarsegrainedlock)orblocklevelofcode(finegrainedlock).

15.Howdoesthreadsynchronizationoccursinsideamonitor?Whatlevelsofsynchronizationcanyouapply?TheJVMuseslocksinconjunctionwithmonitors.Amonitorisbasicallyaguardianthatwatchesoverasequenceofsynchronizedcodeandensuringthatonlyonethreadatatimeexecutesasynchronizedpieceofcode.Eachmonitorisassociatedwithanobjectreference.Thethreadisnotallowedtoexecutethecodeuntilitobtainsthelock.

16.Whatsadeadlock?Aconditionthatoccurswhentwoprocessesarewaitingforeachothertocomplete,beforeproceeding.Theresultisthatbothprocesseswaitendlessly.

17.HowdoyouensurethatNthreadscanaccessNresourceswithoutdeadlock?AverysimplewaytoavoiddeadlockwhileusingNthreadsistoimposeanorderingonthelocksandforceeachthreadtofollowthatordering.Thus,ifallthreadslockandunlockthemutexesinthesameorder,nodeadlockscanarise.

JavaCollections18.WhatarethebasicinterfacesofJavaCollectionsFramework?Java Collections Framework

providesawelldesignedsetofinterfacesandclassesthatsupportoperationsonacollectionsofobjects.ThemostbasicinterfacesthatresideintheJavaCollectionsFrameworkare:

Collection

,whichrepresentsagroupofobjectsknownasitselements.

Set

,whichisacollectionthatcannotcontainduplicateelements.

List

,whichisanorderedcollectionandcancontainduplicateelements.

Map

,whichisanobjectthatmapskeystovaluesandcannotcontainduplicatekeys.

19.WhyCollectiondoesntextendCloneableandSerializableinterfaces?TheCollection

interfacespecifiesgroupsofobjectsknownaselements.EachconcreteimplementationofaCollection

canchooseitsownwayofhowtomaintainandorderitselements.Somecollectionsallowduplicatekeys,whilesomeothercollectionsdont.Thesemanticsandtheimplicationsofeithercloningorserializationcomeintoplaywhendealingwithactualimplementations.Thus,theconcreteimplementationsofcollectionsshoulddecidehowtheycanbeclonedorserialized.

20.WhatisanIterator?TheIterator

interfaceprovidesanumberofmethodsthatareabletoiterateoveranyCollection

.EachJavaCollection

containstheiterator

methodthatreturnsanIterator

instance.Iteratorsarecapableofremovingelementsfromtheunderlyingcollectionduringtheiteration.21.WhatdifferencesexistbetweenIteratorandListIterator?Thedifferencesoftheseelementsarelistedbelow:

An



Iterator

canbeusedtotraversetheSet

andList

collections,whiletheListIterator

canbeusedtoiterateonlyoverLists

.

TheIterator

cantraverseacollectiononlyinforwarddirection,whiletheListIterator

cantraverseaList

inbothdirections.

TheListIterator

implementstheIterator

interfaceandcontainsextrafunctionality,suchasaddinganelement,replacinganelement,gettingtheindexpositionforpreviousandnextelements,etc.

22.Whatisdifferencebetweenfailfastandfailsafe?TheIterator's

failsafepropertyworkswiththecloneoftheunderlyingcollectionandthus,itisnotaffectedbyanymodificationinthecollection.Allthecollectionclassesinjava.utilpackagearefailfast,whilethecollectionclassesinjava.util.concurrentarefailsafe.FailfastiteratorsthrowaConcurrentModificationException

,whilefailsafeiteratorneverthrowssuchanexception.

23.HowHashMapworksinJava?AHashMapinJavastoreskeyvaluepairs.TheHashMap

requiresahashfunctionanduseshashCode

andequalsmethods,inordertoputandretrieveelementstoandfromthecollectionrespectively.Whentheputmethodisinvoked,theHashMap

calculatesthehashvalueofthekeyandstoresthepairintheappropriateindexinsidethecollection.Ifthekeyexists,itsvalueisupdatedwiththenewvalue.SomeimportantcharacteristicsofaHashMap

areitscapacity,itsloadfactorandthethresholdresizing.

24.WhatistheimportanceofhashCode()andequals()methods?InJava,aHashMap

usesthehashCode

andequals



methodstodeterminetheindexofthekeyvaluepairandtodetectduplicates.Morespecifically,thehashCode

methodisusedinordertodeterminewherethespecifiedkeywillbestored.Sincedifferentkeysmayproducethesamehashvalue,theequals

methodisused,inordertodeterminewhetherthespecifiedkeyactuallyexistsinthecollectionornot.Therefore,theimplementationofbothmethodsiscrucialtotheaccuracyandefficiencyoftheHashMap

.

25.WhatdifferencesexistbetweenHashMapandHashtable?BoththeHashMap

andHashtable

classesimplementtheMapinterfaceandthus,haveverysimilarcharacteristics.However,theydifferinthefollowingfeatures:

AHashMap

allowstheexistenceofnullkeysandvalues,whileaHashtable

doesntallowneithernullkeys,nornullvalues.

AHashtable

issynchronized,whileaHashMap

isnot.Thus,HashMap

ispreferredinsinglethreadedenvironments,whileaHashtable

issuitableformultithreadedenvironments.

AHashMap

providesitssetofkeysandaJavaapplicationcaniterateoverthem.Thus,aHashMap

isfailfast.Ontheotherhand,aHashtable

providesanEnumeration

ofitskeys.

TheHashtable

classisconsideredtobealegacyclass.

26.WhatisdifferencebetweenArrayandArrayList?WhenwillyouuseArrayoverArrayList?TheArray

andArrayList

classesdifferonthefollowingfeatures:

Arrays



cancontainprimitiveorobjects,whileanArrayList

cancontainonlyobjects.

Arrays

havefixedsize,whileanArrayList

isdynamic.

AnArrayList

providesmoremethodsandfeatures,suchasaddAll

,removeAll

,iterator

,etc.

Foralistofprimitivedatatypes,thecollectionsuseautoboxingtoreducethecodingeffort.However,thisapproachmakesthemslowerwhenworkingonfixedsizeprimitivedatatypes.

27.WhatisdifferencebetweenArrayListandLinkedList?BoththeArrayList

andLinkedList

classesimplementtheListinterface,buttheydifferonthefollowingfeatures:

AnArrayList

isanindexbaseddatastructurebackedbyanArray

.ItprovidesrandomaccesstoitselementswithaperformanceequaltoO(1).Ontheotherhand,aLinkedList

storesitsdataaslistofelementsandeveryelementislinkedtoitspreviousandnextelement.Inthiscase,thesearchoperationforanelementhasexecutiontimeequaltoO(n).

TheInsertion,additionandremovaloperationsofanelementarefasterinaLinkedList

comparedtoanArrayList

,becausethereisnoneedofresizinganarrayorupdatingtheindexwhenanelementisaddedinsomearbitrarypositioninsidethecollection.

ALinkedList

consumesmorememorythananArrayList

,becauseeverynodeinaLinkedList

storestworeferences,oneforitspreviouselementandoneforitsnextelement.

CheckalsoourarticleArrayListvs.LinkedList.

28.WhatisComparableandComparatorinterface?Listtheirdifferences.Javaprovidesthe



Comparable

interface,whichcontainsonlyonemethod,calledcompareTo

.Thismethodcomparestwoobjects,inordertoimposeanorderbetweenthem.Specifically,itreturnsanegativeinteger,zero,orapositiveintegertoindicatethattheinputobjectislessthan,equalorgreaterthantheexistingobject.JavaprovidestheComparator

interface,whichcontainstwomethods,calledcompare

andequals

.Thefirstmethodcomparesitstwoinputargumentsandimposesanorderbetweenthem.Itreturnsanegativeinteger,zero,orapositiveintegertoindicatethatthefirstargumentislessthan,equalto,orgreaterthanthesecond.Thesecondmethodrequiresanobjectasaparameterandaimstodecidewhethertheinputobjectisequaltothecomparator.Themethodreturnstrue,onlyifthespecifiedobjectisalsoacomparatoranditimposesthesameorderingasthecomparator.

29.WhatisJavaPriorityQueue?ThePriorityQueue

isanunboundedqueue,basedonapriorityheapanditselementsareorderedintheirnaturalorder.Atthetimeofitscreation,wecanprovideaComparatorthatisresponsiblefororderingtheelementsofthePriorityQueue

.APriorityQueue

doesntallownullvalues,thoseobjectsthatdoesntprovidenaturalordering,orthoseobjectsthatdonthaveanycomparatorassociatedwiththem.Finally,theJavaPriorityQueue

isnotthreadsafeanditrequiresO(log(n))timeforitsenqueinganddequeingoperations.

30.WhatdoyouknowaboutthebigOnotationandcanyougivesomeexampleswithrespecttodifferentdatastructures?TheBigOnotationsimplydescribeshowwellanalgorithmscalesorperformsintheworstcasescenarioasthenumberofelementsinadatastructureincreases.TheBigOnotationcanalsobeusedtodescribeotherbehaviorsuchasmemoryconsumption.Sincethecollectionclassesareactuallydatastructures,weusuallyusetheBigOnotationtochosethebestimplementationtouse,basedontime,memoryandperformance.BigOnotationcangiveagoodindicationaboutperformanceforlargeamountsofdata.

31.Whatisthetradeoffbetweenusinganunorderedarrayversusanorderedarray?ThemajoradvantageofanorderedarrayisthatthesearchtimeshavetimecomplexityofO(logn),comparedtothatofanunorderedarray,whichisO(n).ThedisadvantageofanorderedarrayisthattheinsertionoperationhasatimecomplexityofO(n),becausetheelementswithhighervaluesmustbemovedtomakeroomforthenewelement.Instead,theinsertionoperationforanunorderedarraytakesconstanttimeofO(1).

32.WhataresomeofthebestpracticesrelatingtotheJavaCollectionframework?

Choosingtherighttypeofthecollectiontouse,basedontheapplicationsneeds,isverycrucialforitsperformance.Forexampleifthesizeoftheelementsisfixedandknowapriori,weshalluseanArray

,insteadofanArrayList

.

Somecollectionclassesallowustospecifytheirinitialcapacity.Thus,ifwehaveanestimationonthenumberofelementsthatwillbestored,wecanuseittoavoidrehashingorresizing.

AlwaysuseGenericsfortypesafety,readability,androbustness.Also,byusingGenericsyouavoidtheClassCastException

duringruntime.

UseimmutableclassesprovidedbytheJavaDevelopmentKit(JDK)asakeyinaMap,inordertoavoidtheimplementationofthehashCode

andequalsmethodsforourcustomclass.

Programintermsofinterfacenotimplementation.

Returnzerolengthcollectionsorarraysasopposedtoreturninganullincasetheunderlyingcollectionisactuallyempty.



33.WhatsthedifferencebetweenEnumerationandIteratorinterfaces?Enumeration

istwiceasfastascomparedtoanIteratorandusesverylessmemory.However,theIterator

ismuchsafercomparedtoEnumeration

,becauseotherthreadsarenotabletomodifythecollectionobjectthatiscurrentlytraversedbytheiterator.Also,Iterators

allowthecallertoremoveelementsfromtheunderlyingcollection,somethingwhichisnotpossiblewithEnumerations

.

34.WhatisthedifferencebetweenHashSetandTreeSet?TheHashSet

isImplementedusingahashtableandthus,itselementsarenotordered.Theadd,remove,andcontainsmethodsofaHashSet

haveconstanttimecomplexityO(1).Ontheotherhand,aTreeSet

isimplementedusingatreestructure.TheelementsinaTreeSet

aresorted,andthus,theadd,remove,andcontainsmethodshavetimecomplexityofO(logn).

GarbageCollectors35.WhatisthepurposeofgarbagecollectioninJava,andwhenisitused?Thepurposeofgarbagecollectionistoidentifyanddiscardthoseobjectsthatarenolongerneededbytheapplication,inorderfortheresourcestobereclaimedandreused.

36.WhatdoesSystem.gc()andRuntime.gc()methodsdo?ThesemethodscanbeusedasahinttotheJVM,inordertostartagarbagecollection.However,thisitisuptotheJavaVirtualMachine(JVM)tostartthegarbagecollectionimmediatelyorlaterintime.

37.Whenisthefinalize()called?Whatisthepurposeoffinalization?Thefinalizemethodiscalledbythegarbagecollector,justbeforereleasingtheobjectsmemory.Itisnormallyadvisedtoreleaseresourcesheldbytheobjectinsidethefinalizemethod.

38.Ifanobjectreferenceissettonull,willtheGarbageCollectorimmediatelyfreethememoryheldbythatobject?No,theobjectwillbeavailableforgarbagecollectioninthenextcycleofthegarbagecollector.

39.WhatisstructureofJavaHeap?WhatisPermGenspaceinHeap?TheJVMhasaheapthatistheruntimedataareafromwhichmemoryforallclassinstancesandarraysisallocated.ItiscreatedattheJVMstartup.Heapmemoryforobjectsisreclaimedbyanautomaticmemorymanagementsystemwhichisknownasagarbagecollector.Heapmemoryconsistsofliveanddeadobjects.Liveobjectsareaccessiblebytheapplicationandwillnotbeasubjectofgarbagecollection.Deadobjectsarethosewhichwillneverbeaccessiblebytheapplication,buthavenotbeencollectedbythegarbagecollectoryet.Suchobjectsoccupytheheapmemoryspaceuntiltheyareeventuallycollectedbythegarbagecollector.

40.WhatisthedifferencebetweenSerialandThroughputGarbagecollector?Thethroughputgarbagecollectorusesaparallelversionoftheyounggenerationcollectorandismeanttobeusedwithapplicationsthathavemediumtolargedatasets.Ontheotherhand,theserialcollectorisusuallyadequateformostsmallapplications(thoserequiringheapsofuptoapproximately100MBonmodernprocessors).

41.WhendoesanObjectbecomeseligibleforGarbagecollectioninJava?AJavaobjectissubjecttogarbagecollectionwhenitbecomesunreachabletotheprograminwhichitiscurrentlyused.

42.DoesGarbagecollectionoccurinpermanentgenerationspaceinJVM?GarbageCollectiondoesoccurinPermGenspaceandifPermGenspaceisfullorcrossathreshold,itcantriggerafullgarbagecollection.Ifyoulookcarefullyattheoutputofthegarbagecollector,youwillfindthatPermGenspaceisalsogarbagecollected.ThisisthereasonwhycorrectsizingofPermGenspaceisimportanttoavoidfrequentfullgarbagecollections.AlsocheckourarticleJava8:PermGentoMetaspace.

ExceptionHandling43.WhatarethetwotypesofExceptionsinJava?Whicharethedifferencesbetweenthem?Javahastwotypesofexceptions:checkedexceptionsanduncheckedexceptions.Uncheckedexceptionsdonotneedtobedeclaredinamethodoraconstructorsthrowsclause,iftheycanbethrownbytheexecutionofthemethodortheconstructor,andpropagateoutsidethemethodorconstructorboundary.Ontheotherhand,checkedexceptionsmustbedeclaredinamethodoraconstructorsthrowsclause.SeeherefortipsonJavaexceptionhandling.



44.WhatisthedifferencebetweenExceptionandErrorinjava?Exception

andError

classesarebothsubclassesoftheThrowable

class.TheException

classisusedforexceptionalconditionsthatausersprogramshouldcatch.TheError

classdefinesexceptionsthatarenotexceptedtobecaughtbytheuserprogram.

45.Whatisthedifferencebetweenthrowandthrows?Thethrowkeywordisusedtoexplicitlyraiseaexceptionwithintheprogram.Onthecontrary,thethrowsclauseisusedtoindicatethoseexceptionsthatarenothandledbyamethod.Eachmethodmustexplicitlyspecifywhichexceptionsdoesnothandle,sothecallersofthatmethodcanguardagainstpossibleexceptions.Finally,multipleexceptionsareseparatedbyacomma.

45.Whatistheimportanceoffinallyblockinexceptionhandling?Afinallyblockwillalwaysbeexecuted,whetherornotanexceptionisactuallythrown.Eveninthecasewherethecatchstatementismissingandanexceptionisthrown,thefinallyblockwillstillbeexecuted.LastthingtomentionisthatthefinallyblockisusedtoreleaseresourceslikeI/Obuffers,databaseconnections,etc.

46.WhatwillhappentotheExceptionobjectafterexceptionhandling?TheException

objectwillbegarbagecollectedinthenextgarbagecollection.

47.Howdoesfinallyblockdifferfromfinalize()method?Afinallyblockwillbeexecutedwhetherornotanexceptionisthrownandisusedtoreleasethoseresourcesheldbytheapplication.FinalizeisaprotectedmethodoftheObjectclass,whichiscalledbytheJavaVirtualMachine(JVM)justbeforeanobjectisgarbagecollected.

JavaApplets48.WhatisanApplet?AjavaappletisprogramthatcanbeincludedinaHTMLpageandbeexecutedinajavaenabledclientbrowser.Appletsareusedforcreatingdynamicandinteractivewebapplications.

49.ExplainthelifecycleofanApplet.Anappletmayundergothefollowingstates:

Init

:Anappletisinitializedeachtimeisloaded.

Start

:Begintheexecutionofanapplet.

Stop

:Stoptheexecutionofanapplet.

Destroy

:Performafinalcleanup,beforeunloadingtheapplet.

50.Whathappenswhenanappletisloaded?Firstofall,aninstanceoftheappletscontrollingclassiscreated.Then,theappletinitializesitselfandfinally,itstartsrunning.

51.WhatisthedifferencebetweenanAppletandaJavaApplication?Appletsareexecutedwithinajavaenabledbrowser,butaJavaapplicationisastandaloneJavaprogramthatcanbeexecutedoutsideofabrowser.However,theybothrequiretheexistenceofaJavaVirtualMachine(JVM).Furthermore,aJavaapplicationrequiresamainmethodwithaspecificsignature,inordertostartitsexecution.Javaappletsdontneedsuchamethodtostarttheirexecution.Finally,Javaappletstypicallyusearestrictivesecuritypolicy,whileJavaapplicationsusuallyusemorerelaxedsecuritypolicies.

52.WhataretherestrictionsimposedonJavaapplets?Mostlyduetosecurityreasons,thefollowingrestrictionsareimposedonJavaapplets:

Anappletcannotloadlibrariesordefinenativemethods.

Anappletcannotordinarilyreadorwritefilesontheexecutionhost.

Anappletcannotreadcertainsystemproperties.

Anappletcannotmakenetworkconnectionsexcepttothehostthatitcamefrom.



Anappletcannotstartanyprogramonthehostthatsexecutingit.

53.Whatareuntrustedapplets?UntrustedappletsarethoseJavaappletsthatcannotaccessorexecutelocalsystemfiles.Bydefault,alldownloadedappletsareconsideredasuntrusted.

54.Whatisthedifferencebetweenappletsloadedovertheinternetandappletsloadedviathefilesystem?Regardingthecasewhereanappletisloadedovertheinternet,theappletisloadedbytheappletclassloaderandissubjecttotherestrictionsenforcedbytheappletsecuritymanager.Regardingthecasewhereanappletisloadedfromtheclientslocaldisk,theappletisloadedbythefilesystemloader.Appletsloadedviathefilesystemareallowedtoreadfiles,writefilesandtoloadlibrariesontheclient.Also,appletsloadedviathefilesystemareallowedtoexecuteprocessesandfinally,appletsloadedviathefilesystemarenotpassedthroughthebytecodeverifier.

55.Whatistheappletclassloader,andwhatdoesitprovide?Whenanappletisloadedovertheinternet,theappletisloadedbytheappletclassloader.TheclassloaderenforcestheJavanamespacehierarchy.Also,theclassloaderguaranteesthatauniquenamespaceexistsforclassesthatcomefromthelocalfilesystem,andthatauniquenamespaceexistsforeachnetworksource.Whenabrowserloadsanappletoverthenet,thatappletsclassesareplacedinaprivatenamespaceassociatedwiththeappletsorigin.Then,thoseclassesloadedbytheclassloaderarepassedthroughtheverifier.TheverifierchecksthattheclassfileconformstotheJavalanguagespecification.Amongotherthings,theverifierensuresthattherearenostackoverflowsorunderflowsandthattheparameterstoallbytecodeinstructionsarecorrect.

56.Whatistheappletsecuritymanager,andwhatdoesitprovide?TheappletsecuritymanagerisamechanismtoimposerestrictionsonJavaapplets.Abrowsermayonlyhaveonesecuritymanager.Thesecuritymanagerisestablishedatstartup,anditcannotthereafterbereplaced,overloaded,overridden,orextended.

Swing57.WhatisthedifferencebetweenaChoiceandaList?AChoiceisdisplayedinacompactformthatmustbepulleddown,inorderforausertobeabletoseethelistofallavailablechoices.OnlyoneitemmaybeselectedfromaChoice.AListmaybedisplayedinsuchawaythatseveralListitemsarevisible.AListsupportstheselectionofoneormoreListitems.

58.Whatisalayoutmanager?Alayoutmanageristheusedtoorganizethecomponentsinacontainer.

59.WhatisthedifferencebetweenaScrollbarandaJScrollPane?AScrollbar

isaComponent

,butnotaContainer

.AScrollPane

isaContainer

.AScrollPane

handlesitsowneventsandperformsitsownscrolling.

60.WhichSwingmethodsarethreadsafe?Thereareonlythreethreadsafemethods:repaint,revalidate,andinvalidate.

61.NamethreeComponentsubclassesthatsupportpainting.TheCanvas

,Frame

,Panel

,andAppletclassessupportpainting.

62.Whatisclipping?Clippingisdefinedastheprocessofconfiningpaintoperationstoalimitedareaorshape.

63.WhatisthedifferencebetweenaMenuItemandaCheckboxMenuItem?TheCheckboxMenuItem

classextendstheMenuItem

classandsupportsamenuitemthatmaybeeithercheckedorunchecked.



64.HowaretheelementsofaBorderLayoutorganized?TheelementsofaBorderLayout

areorganizedattheborders(North,South,East,andWest)andthecenterofacontainer.

65.HowaretheelementsofaGridBagLayoutorganized?TheelementsofaGridBagLayout

areorganizedaccordingtoagrid.Theelementsareofdifferentsizesandmayoccupymorethanoneroworcolumnofthegrid.Thus,therowsandcolumnsmayhavedifferentsizes.

66.WhatisthedifferencebetweenaWindowandaFrame?TheFrame

classextendstheWindowclassanddefinesamainapplicationwindowthatcanhaveamenubar.

67.Whatistherelationshipbetweenclippingandrepainting?WhenawindowisrepaintedbytheAWTpaintingthread,itsetstheclippingregionstotheareaofthewindowthatrequiresrepainting.

68.Whatistherelationshipbetweenaneventlistenerinterfaceandaneventadapterclass?Aneventlistenerinterfacedefinesthemethodsthatmustbeimplementedbyaneventhandlerforaparticularevent.Aneventadapterprovidesadefaultimplementationofaneventlistenerinterface.

69.HowcanaGUIcomponenthandleitsownevents?AGUIcomponentcanhandleitsownevents,byimplementingthecorrespondingeventlistenerinterfaceandaddingitselfasitsowneventlistener.

70.WhatadvantagedoJavaslayoutmanagersprovideovertraditionalwindowingsystems?Javauseslayoutmanagerstolayoutcomponentsinaconsistentmanner,acrossallwindowingplatforms.Sincelayoutmanagersarenttiedtoabsolutesizingandpositioning,theyareabletoaccomodateplatformspecificdifferencesamongwindowingsystems.

71.WhatisthedesignpatternthatJavausesforallSwingcomponents?ThedesignpatternusedbyJavaforallSwingcomponentsistheModelViewController(MVC)pattern.

JDBC72.WhatisJDBC?JDBCisanabstractionlayerthatallowsuserstochoosebetweendatabases.JDBCenablesdeveloperstowritedatabaseapplicationsinJava,withouthavingtoconcernthemselveswiththeunderlyingdetailsofaparticulardatabase.

73.ExplaintheroleofDriverinJDBC.TheJDBCDriverprovidesvendorspecificimplementationsoftheabstractclassesprovidedbytheJDBCAPI.Eachdrivermustprovideimplementationsforthefollowingclassesofthejava.sqlpackage:Connection

,Statement

,PreparedStatement

,CallableStatement

,ResultSet

andDriver

.

74.WhatisthepurposeClass.forNamemethod?Thismethodisusedtomethodisusedtoloadthedriverthatwillestablishaconnectiontothedatabase.

75.WhatistheadvantageofPreparedStatementoverStatement?PreparedStatementsareprecompiledandthus,theirperformanceismuchbetter.Also,PreparedStatementobjectscanbereusedwithdifferentinputvaluestotheirqueries.

76.WhatistheuseofCallableStatement?Namethemethod,whichisusedtoprepareaCallableStatement.ACallableStatement

isusedtoexecutestoredprocedures.Storedproceduresarestoredandofferedbyadatabase.Storedproceduresmaytakeinputvaluesfromtheuserandmayreturnaresult.Theusageofstoredproceduresishighlyencouraged,becauseitofferssecurityandmodularity.ThemethodthatpreparesaCallableStatement



isthefollowing:

77.WhatdoesConnectionpoolingmean?Theinteractionwithadatabasecanbecostly,regardingtheopeningandclosingofdatabaseconnections.Especially,whenthenumberofdatabaseclientsincreases,thiscostisveryhighandalargenumberofresourcesisconsumed.Apoolofdatabaseconnectionsisobtainedatstartupbytheapplicationserverandismaintainedinapool.Arequestforaconnectionisservedbyaconnectionresidinginthepool.Intheendoftheconnection,therequestisreturnedtothepoolandcanbeusedtosatisfyfuturerequests.

RemoteMethodInvocation(RMI)78.WhatisRMI?TheJavaRemoteMethodInvocation(JavaRMI)isaJavaAPIthatperformstheobjectorientedequivalentofremoteprocedurecalls(RPC),withsupportfordirecttransferofserializedJavaclassesanddistributedgarbagecollection.RemoteMethodInvocation(RMI)canalsobeseenastheprocessofactivatingamethodonaremotelyrunningobject.RMIofferslocationtransparencybecauseauserfeelsthatamethodisexecutedonalocallyrunningobject.ChecksomeRMITipshere.

79.WhatisthebasicprincipleofRMIarchitecture?TheRMIarchitectureisbasedonaveryimportantprinciplewhichstatesthatthedefinitionofthebehaviorandtheimplementationofthatbehavior,areseparateconcepts.RMIallowsthecodethatdefinesthebehaviorandthecodethatimplementsthebehaviortoremainseparateandtorunonseparateJVMs.

80.WhatarethelayersofRMIArchitecture?TheRMIarchitectureconsistsofthefollowinglayers:

Stub and Skeleton layer

:Thislayerliesjustbeneaththeviewofthedeveloper.ThislayerisresponsibleforinterceptingmethodcallsmadebytheclienttotheinterfaceandredirectthesecallstoaremoteRMIService.

Remote Reference Layer

:ThesecondlayeroftheRMIarchitecturedealswiththeinterpretationofreferencesmadefromtheclienttotheserversremoteobjects.Thislayerinterpretsandmanagesreferencesmadefromclientstotheremoteserviceobjects.Theconnectionisaonetoone(unicast)link.

Transport layer

:ThislayerisresponsibleforconnectingthetwoJVMparticipatingintheservice.ThislayerisbasedonTCP/IPconnectionsbetweenmachinesinanetwork.Itprovidesbasicconnectivity,aswellassomefirewallpenetrationstrategies.

81.WhatistheroleofRemoteInterfaceinRMI?TheRemoteinterfaceservestoidentifyinterfaceswhosemethodsmaybeinvokedfromanonlocalvirtualmachine.Anyobjectthatisaremoteobjectmustdirectlyorindirectlyimplementthisinterface.Aclassthatimplementsaremoteinterfaceshoulddeclaretheremoteinterfacesbeingimplemented,definetheconstructorforeachremoteobjectandprovideanimplementationforeachremotemethodinallremoteinterfaces.

82.Whatistheroleofthejava.rmi.NamingClass?Thejava.rmi.Namingclassprovidesmethodsforstoringandobtainingreferencestoremoteobjectsintheremoteobjectregistry.EachmethodoftheNamingclasstakesasoneofitsargumentsanamethatisaStringinURLformat.

83.WhatismeantbybindinginRMI?Bindingistheprocessofassociatingorregisteringanameforaremoteobject,whichcanbeusedatalatertime,inordertolookupthatremoteobject.AremoteobjectcanbeassociatedwithanameusingthebindorrebindmethodsoftheNamingclass.

84.Whatisthedifferencebetweenusingbind()andrebind()methodsofNamingClass?Thebindmethodbindisresponsibleforbindingthespecifiednametoaremoteobject,whiletherebindmethodisresponsibleforrebindingthespecifiednametoanewremoteobject.Incaseabindingexistsforthatname,thebindingisreplaced.

85.WhatarethestepsinvolvedtomakeworkaRMIprogram?ThefollowingstepsmustbeinvolvedinorderforaRMIprogramtoworkproperly:

Compilationofallsourcefiles.

Generatationofthestubsusingrmic.

Startthermiregistry.

StarttheRMIServer.

Runtheclientprogram.

86.WhatistheroleofstubinRMI?Astubforaremoteobjectactsasaclientslocalrepresentativeorproxyfortheremoteobject.Thecallerinvokesamethodonthelocalstub,whichisresponsibleforexecutingthemethodontheremoteobject.Whenastubsmethodisinvoked,itundergoesthefollowingsteps:

ItinitiatesaconnectiontotheremoteJVMcontainingtheremoteobject.

ItmarshalstheparameterstotheremoteJVM.

Itwaitsfortheresultofthemethodinvocationandexecution.

Itunmarshalsthereturnvalueoranexceptionifthemethodhasnotbeensuccessfullyexecuted.

Itreturnsthevaluetothecaller.

1 CallableStament.prepareCall();



87.WhatisDGC?Andhowdoesitwork?DGCstandsforDistributedGarbageCollection.RemoteMethodInvocation(RMI)usesDGCforautomaticgarbagecollection.SinceRMIinvolvesremoteobjectreferencesacrossJVMs,garbagecollectioncanbequitedifficult.DGCusesareferencecountingalgorithmtoprovideautomaticmemorymanagementforremoteobjects.

88.WhatisthepurposeofusingRMISecurityManagerinRMI?RMISecurityManagerprovidesasecuritymanagerthatcanbeusedbyRMIapplications,whichusedownloadedcode.TheclassloaderofRMIwillnotdownloadanyclassesfromremotelocations,ifthesecuritymanagerhasnotbeenset.

89.ExplainMarshallinganddemarshalling.Whenanapplicationwantstopassitsmemoryobjectsacrossanetworktoanotherhostorpersistittostorage,theinmemoryrepresentationmustbeconvertedtoasuitableformat.Thisprocessiscalledmarshallingandtherevertoperationiscalleddemarshalling.

90.ExplainSerializationandDeserialization.Javaprovidesamechanism,calledobjectserializationwhereanobjectcanberepresentedasasequenceofbytesandincludestheobjectsdata,aswellasinformationabouttheobjectstype,andthetypesofdatastoredintheobject.Thus,serializationcanbeseenasawayofflatteningobjects,inordertobestoredondisk,andlater,readbackandreconstituted.Deserialisationisthereverseprocessofconvertinganobjectfromitsflattenedstatetoaliveobject.

Servlets91.WhatisaServlet?TheservletisaJavaprogramminglanguageclassusedtoprocessclientrequestsandgeneratedynamicwebcontent.ServletsaremostlyusedtoprocessorstoredatasubmittedbyanHTMLform,providedynamiccontentandmanagestateinformationthatdoesnotexistinthestatelessHTTPprotocol.

92.ExplainthearchitechureofaServlet.Thecoreabstractionthatmustbeimplementedbyallservletsisthejavax.servlet.Servletinterface.Eachservletmustimplementiteitherdirectlyorindirectly,eitherbyextendingjavax.servlet.GenericServletorjavax.servlet.http.HTTPServlet.Finally,eachservletisabletoservemultiplerequestsinparallelusingmultithreading.

93.WhatisthedifferencebetweenanAppletandaServlet?AnAppletisaclientsidejavaprogramthatrunswithinaWebbrowserontheclientmachine.Ontheotherhand,aservletisaserversidecomponentthatrunsonthewebserver.Anappletcanusetheuserinterfaceclasses,whileaservletdoesnothaveauserinterface.Instead,aservletwaitsforclientsHTTPrequestsandgeneratesaresponseineveryrequest.

94.WhatisthedifferencebetweenGenericServletandHttpServlet?GenericServletisageneralizedandprotocolindependentservletthatimplementstheServletandServletConfiginterfaces.ThoseservletsextendingtheGenericServletclassshalloverridetheservicemethod.Finally,inordertodevelopanHTTPservletforuseontheWebthatservesrequestsusingtheHTTPprotocol,yourservletmustextendtheHttpServletinstead.CheckServletexampleshere.

95.ExplainthelifecycleofaServlet.Oneveryclientsrequest,theServletEngineloadstheservletsandinvokesitsinitmethods,inorderfortheservlettobeinitialized.Then,theServletobjecthandlesallsubsequentrequestscomingfromthatclient,byinvokingtheservicemethodforeachrequestseparately.Finally,theservletisremovedbycallingtheserversdestroymethod.

96.WhatisthedifferencebetweendoGet()anddoPost()?doGET

:TheGETmethodappendsthenamevaluepairsontherequestsURL.Thus,thereisalimitonthenumberofcharactersandsubsequentlyonthenumberofvaluesthatcanbeusedinaclientsrequest.Furthermore,thevaluesoftherequestaremadevisibleandthus,sensitiveinformationmustnotbepassedinthatway.doPOST

:ThePOSTmethodovercomesthelimitimposedbytheGETrequest,bysendingthevaluesoftherequestinsideitsbody.Also,thereisnolimitationsonthenumberofvaluestobesentacross.Finally,thesensitiveinformationpassedthroughaPOSTrequestisnotvisibletoanexternalclient.

97.WhatismeantbyaWebApplication?AWebapplicationisadynamicextensionofaWeborapplicationserver.Therearetwotypesofwebapplications:presentationorientedandserviceoriented.ApresentationorientedWebapplicationgeneratesinteractivewebpages,whichcontainvarioustypesofmarkuplanguageanddynamiccontentinresponsetorequests.Ontheotherhand,aserviceorientedwebapplicationimplementstheendpointofawebservice.Ingeneral,aWebapplicationcanbeseenasacollectionofservletsinstalledunderaspecificsubsetoftheserversURLnamespace.

98.WhatisaServerSideInclude(SSI)?ServerSideIncludes(SSI)isasimpleinterpretedserversidescriptinglanguage,usedalmostexclusivelyfortheWeb,andisembeddedwithaservlettag.ThemostfrequentuseofSSIistoincludethecontentsofoneormorefilesintoaWebpageonaWebserver.WhenaWebpageisaccessedbyabrowser,theWebserverreplacestheservlettaginthatWebpagewiththehypertextgeneratedbythecorrespondingservlet.

99.WhatisServletChaining?ServletChainingisthemethodwheretheoutputofoneservletissenttoasecondservlet.Theoutputofthesecondservletcanbesenttoathirdservlet,andsoon.Thelastservletinthechainisresponsibleforsendingtheresponsetotheclient.

100.Howdoyoufindoutwhatclientmachineismakingarequesttoyourservlet?TheServletRequestclasshasfunctionsforfindingouttheIPaddressorhostnameoftheclientmachine.getRemoteAddr()getstheIPaddressoftheclientmachineandgetRemoteHost()getsthehostnameoftheclientmachine.Seeexamplehere.

101.WhatisthestructureoftheHTTPresponse?TheHTTPresponseconsistsofthreeparts:

StatusCode:describesthestatusoftheresponse.Itcanbeusedtocheckiftherequesthasbeensuccessfullycompleted.Incasetherequestfailed,thestatuscodecanbeusedtofindoutthereasonbehindthefailure.Ifyourservletdoesnotreturnastatuscode,thesuccessstatuscode,HttpServletResponse.SC_OK,isreturnedbydefault.

HTTPHeaders:theycontainmoreinformationabouttheresponse.Forexample,theheadersmayspecifythedate/timeafterwhichtheresponseisconsideredstale,ortheformofencodingusedtosafelytransfertheentitytotheuser.Seehowtoretrieveheadersin



Servlethere.Body:itcontainsthecontentoftheresponse.ThebodymaycontainHTMLcode,animage,etc.ThebodyconsistsofthedatabytestransmittedinanHTTPtransactionmessageimmediatelyfollowingtheheaders.

102.Whatisacookie?Whatisthedifferencebetweensessionandcookie?AcookieisabitofinformationthattheWebserversendstothebrowser.ThebrowserstoresthecookiesforeachWebserverinalocalfile.Inafuturerequest,thebrowser,alongwiththerequest,sendsallstoredcookiesforthatspecificWebserver.Thedifferencesbetweensessionandacookiearethefollowing:

Thesessionshouldwork,regardlessofthesettingsontheclientbrowser.Theclientmayhavechosentodisablecookies.However,thesessionsstillwork,astheclienthasnoabilitytodisablethemintheserverside.

Thesessionandcookiesalsodifferintheamountofinformationthecanstore.TheHTTPsessioniscapableofstoringanyJavaobject,whileacookiecanonlystoreStringobjects.

103.Whichprotocolwillbeusedbybrowserandservlettocommunicate?ThebrowsercommunicateswithaservletbyusingtheHTTPprotocol.

104.WhatisHTTPTunneling?HTTPTunnelingisatechniquebywhich,communicationsperformedusingvariousnetworkprotocolsareencapsulatedusingtheHTTPorHTTPSprotocols.TheHTTPprotocolthereforeactsasawrapperforachannelthatthenetworkprotocolbeingtunneledusestocommunicate.ThemaskingofotherprotocolrequestsasHTTPrequestsisHTTPTunneling.

105.WhatsthedifferencebetweensendRedirectandforwardmethods?ThesendRedirectmethodcreatesanewrequest,whiletheforwardmethodjustforwardsarequesttoanewtarget.Thepreviousrequestscopeobjectsarenotavailableafteraredirect,becauseitresultsinanewrequest.Ontheotherhand,thepreviousrequestscopeobjectsareavailableafterforwarding.FInally,ingeneral,thesendRedirectmethodisconsideredtobeslowercomparetotheforwardmethod.

106.WhatisURLEncodingandURLDecoding?TheURLencodingprocedureisresponsibleforreplacingallthespacesandeveryotherextraspecialcharacterofaURL,intotheircorrespondingHexrepresentation.Incorrespondence,URLdecodingistheexactoppositeprocedure.

JSP107.WhatisaJSPPage?AJavaServerPage(JSP)isatextdocumentthatcontainstwotypesoftext:staticdataandJSPelements.Staticdatacanbeexpressedinanytextbasedformat,suchasHTMLorXML.JSPisatechnologythatmixesstaticcontentwithdynamicallygeneratedcontent.SeeJSPexamplehere.

108.HowaretheJSPrequestshandled?OnthearrivalofaJSPrequest,thebrowserfirstrequestsapagewitha.jspextension.Then,theWebserverreadstherequestandusingtheJSPcompiler,theWebserverconvertstheJSPpageintoaservletclass.NoticethattheJSPfileiscompiledonlyonthefirstrequestofthepage,oriftheJSPfilehaschanged.Thegeneratedservletclassisinvoked,inordertohandlethebrowsersrequest.Oncetheexecutionoftherequestisover,theservletsendsaresponsebacktotheclient.SeehowtogetRequestparametersinaJSP.

109.WhataretheadvantagesofJSP?TheadvantagesofusingtheJSPtechnologyareshownbelow:

JSPpagesaredynamicallycompiledintoservletsandthus,thedeveloperscaneasilymakeupdatestopresentationcode.

JSPpagescanbeprecompiled.

JSPpagescanbeeasilycombinedtostatictemplates,includingHTMLorXMLfragments,withcodethatgeneratesdynamiccontent.

DeveloperscanoffercustomizedJSPtaglibrariesthatpageauthorsaccessusinganXMLlikesyntax.

Developerscanmakelogicchangesatthecomponentlevel,withouteditingtheindividualpagesthatusetheapplicationslogic.

110.WhatareDirectives?WhatarethedifferenttypesofDirectivesavailableinJSP?DirectivesareinstructionsthatareprocessedbytheJSPengine,whenthepageiscompiledtoaservlet.Directivesareusedtosetpagelevelinstructions,insertdatafromexternalfiles,andspecifycustomtaglibraries.Directivesaredefinedbetween< %@ and % >

.Thedifferenttypesofdirectivesareshownbelow:

Include directive

:itisusedtoincludeafileandmergesthecontentofthefilewiththecurrentpage.

Page directive

:itisusedtodefinespecificattributesintheJSPpage,likeerrorpageandbuffer.

Taglib

:itisusedtodeclareacustomtaglibrarywhichisusedinthepage.

111.WhatareJSPactions?JSPactionsuseconstructsinXMLsyntaxtocontrolthebehavioroftheservletengine.JSPactionsareexecutedwhenaJSPpageisrequested.Theycanbedynamicallyinsertedintoafile,reuseJavaBeanscomponents,forwardtheusertoanotherpage,orgenerateHTMLfortheJavaplugin.Someoftheavailableactionsarelistedbelow:

jsp:include



includesafile,whentheJSPpageisrequested.

jsp:useBean

findsorinstantiatesaJavaBean.

jsp:setProperty

setsthepropertyofaJavaBean.

jsp:getProperty

getsthepropertyofaJavaBean.

jsp:forward

forwardstherequestertoanewpage.

jsp:plugin

generatesbrowserspecificcode.

112.WhatareScriptlets?InJavaServerPages(JSP)technology,ascriptletisapieceofJavacodeembeddedinaJSPpage.Thescriptletiseverythinginsidethetags.Betweenthesetags,ausercanaddanyvalidscriplet.

113.WhatareDecalarations?DeclarationsaresimilartovariabledeclarationsinJava.Declarationsareusedtodeclarevariablesforsubsequentuseinexpressionsorscriptlets.Toaddadeclaration,youmustusethesequencestoencloseyourdeclarations.

114.WhatareExpressions?AJSPexpressionisusedtoinsertthevalueofascriptinglanguageexpression,convertedintoastring,intothedatastreamreturnedtotheclient,bythewebserver.Expressionsaredefinedbetween

tags.

115.Whatismeantbyimplicitobjectsandwhatarethey?JSPimplicitobjectsarethoseJavaobjectsthattheJSPContainermakesavailabletodevelopersineachpage.Adevelopercancallthemdirectly,withoutbeingexplicitlydeclared.JSPImplicitObjectsarealsocalledpredefinedvariables.ThefollowingobjectsareconsideredimplicitinaJSPpage:

application

page

request

response

session

exception

out

config

pageContext

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86 COMMENTS

April11th,2014at10:05amaerobless

Thatlistseemsreallygood.IstudycomputerscienceanditlldefinitelycomeinhandyonceIstartlookingforajob.

Reply

April13th,2014at12:51pmleon

OnesmallerroronhashCodeandequals:if2instancesareequal,theymustprovidethesamehashCode,BUTtwodifferentinstancesmayreturnthesamehashCode.

HavinghashCodealwaysreturn1orsomeotherfixednumbersatisfiesthecontract,althoughitwouldkilltheperformanceofHashMapandotherhashrelatedfunctionality.

ThehashCodejustdeterminesinwhichhashmapbuckettheinstanceisput,andwithinthatbucketthecorrectinstanceisthenlookedupwiththemuchmoreexpensiveequals.

Reply

April15th,2014at1:55amteoman

perfectpost,thanksforsharing.

Reply

June19th,2014at10:55amMadirajuKrishnaChaitanya

NicePost.Thanksforsharingthiswithus.

Reply

July2nd,2014at3:21pmSumitBisht

Thisiscorrect,butmanytopicsareoutdatedhere.

Reply

August13th,2014at12:13pmJeffreyBurch



Yeah,thereshouldbesomequestionsaboutJava8features.Itwilltellifthecandidateputsanyeffortintoselfimprovement.

Reply

July8th,2014at1:28pmNikitha

Reallyperfectpost,thanksforsharingwithus.

Reply

July17th,2014at4:24pmRobertMartin

Thisisaqualitypeaceofwork.Thankyoufortheeffort.

Reply

August18th,2014at7:12pmVlad

ThankstotheauthorIappreciateyoureffortstowriteeverythinginoneplace

Reply

August26th,2014at7:18pmraghukumarchalla

goodveryusefultoallfreshengineeringgraduatesandexperiancedtogetintojobeasily.

Reply

September1st,2014at1:48pmM.Hagras

Thanksalot,itisveryuseful

Reply

September1st,2014at9:34pmTk

Verygoodwork.ItwouldbereallygreatifyoucouldaddmorequestionsespeciallyonthenewversionsofJava.Also,fewimplementationexamplescanbepostedfordifferentconcept.Youhavedoneagreatjob.

Reply

September11th,2014at10:57amRAVIKUMAR.M

Itisveryusefulforjobseekerstogetagreatjob.thanksforsharingwithus.

Reply

September15th,2014at5:40pmRuth

Thankssomuchforthiscompilation.Itisveryuseful.

Reply

September17th,2014at10:28pmNazim

Thanks,butfoundsomeinaccuracies:Q33OtherthreadsareabletomodifyCollectionofsomeIterator,butnextcalledmethodonIteratorwillthrowConcurrentModificationExceptionQ37Itisrecommendednottooverridefinalize()methodinordertoreleaseresourceasJVMdoesnotguaranteethismethodtobeinvoked.

Reply



September26th,2014at12:05pmsatishshirale

Thanks,Irefreshedmyjavaknowledgeonceagain.

Reply

September27th,2014at9:46amGuarav

Thanksveryuseful

Reply

October19th,2014at11:27amMadhu

Thankstotheauthor!Keepupdating!

Reply

October23rd,2014at4:55pmJun

nicejob.ThankYou.Thistutorialisveryusefulforpeoplewhoseekjobinprogramming.Ijustlovecoding.

Reply

October24th,2014at6:38pmAkansha

niceinfrmation

Reply

October26th,2014at1:43amAhmedMansour

niceinfrmation

Reply

October27th,2014at2:50pmManjunath

Superinformation

Reply

October27th,2014at2:51pmManjunath

Veryusefultous

Reply

November17th,2014at8:35pmSotiriosEfstathiosManeas

Thanksalotforyourcomments!

Reply

October28th,2014at6:07pmYS

Greatsourceofupdatedinfo.Keepitup.Thanks.

Reply


Thanksalot!



Reply

October31st,2014at1:47pmvalentin

whereisthelinktodownloadthislist?

Reply

May21st,2015at4:24pmpriyanga

linkfordownload

Reply

November1st,2014at2:55pmRainerAlfldi

YouranswertotheHashMapquestion(24)iswrong.AHashMapworksevenifthekeysallreturnhashcode1.Hashcodedefinesthebuckets,equalsisusedforequalitychecking.Therearemoreproblemswithboththequestionsandtheanswers.

Reply


Thanksalotforyourfeedback,Icorrectedmyresponseinquestion24.

Moreover,youaremorethanwelcometodiscussyourproblemsaboutboththequestionsandanswers.

Reply

November6th,2014at7:29amAmiruddin

WhereisthedownloadLink?

Reply

November12th,2014at3:05amsachdeva

wherecanwedownloadthis?plzsendmethelinkonmymailid.thanks

Reply

November13th,2014at1:49amtn

ThereisadetailedexplanationofHashMapinternalsathttp://javahungry.blogspot.com/2013/08/hashinghowhashmapworksinjavaor.htmlandJDK6sourcecodeisathttp://hg.openjdk.java.net/jdk6/jdk6/jdk/file/a42d6999734b/src/share/classes/java/util/HashMap.java.

RaineriscorrectinthatHashMapwillacceptmultiplekeyswiththesamehashCode.ItestedthiswithaclasswhereallinstanceshaveahashCodeof1.TosomeextentHashMapcompensatesforthisbyhashingthehashCodesofkeys.MultiplekeyscanhavethesamehashCodesinceitisusedtodeterminethebucketonlyinwhichisstoredalistofEntityobjectswhicharedistinguishedbytheresultofequalsonthevalues.TheEntityclassisdefinedasanimplementationofMap.Entrywiththeadditionofanextfieldwhichisincludedinitsconstructorandusedforiteration.

Reply


Thanksalotforyourfeedback,Icorrectedmyresponseinquestion24.

Reply

November16th,2014at10:27pmJose

Hi,

Youshouldcorrecttheanswertothe13question(Explaintheavailablethreadstatesinahighlevel?).Itisquitewrong.

ItdoesnotexistastatecalledRunningorSleepeing.



Takealook:https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.State.html

BR

Reply


Iwasdescribingthestatesofathreadintheperspectiveoftheoperatingsystem,butitwasnotclear.

Iupdatedmyresponse,inordertodescribethestatesofathread,asbeingregardedbytheJavaVirtualMachine(JVM).

Reply

November17th,2014at10:10pmtn

Question39actuallyincludestwoquestionsthesecondofwhichisnotanswered.Pleaseanswerthissecondquestion,whichisWhatisPermGenspaceinHeap?.Thanks

Reply

November17th,2014at11:07pmtn

Regardingquestion39WhatisPermGenspaceinHeap?,accordingtoonereplyathttp://stackoverflow.com/questions/4223809/isjavapermgenspacepartofthetotalvmmemory,InSunsJVM,thepermanentgenerationisnotpartoftheheap.Itsadifferentspaceforclassdefinitionsandrelateddata,aswellaswhereinternedstringslive.TheJavaVMSpecificationJavaSE7Edition(https://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf)doesnotmentionpermanentgeneration,PermGen.orPermGen(caseinvariant).HoweveritdoesmentiontheMethodAreaintermsthatresemblePermGenfunctionalityandaccordingtoareplyathttp://stackoverflow.com/questions/9095748/methodareaandpermgen,theMethodAreacouldbeconsideredtobeasubsetofPermGen.ThisinducesmethinkthatPermGenisalogicalconstructthatmapstomorethanonerealmemoryareaorsubarea.

Also,PermGenimplementationmaydependonthespecificJVM.Forexample,accordingtoareplyathttp://stackoverflow.com/questions/4848669/permspacevsheapspace,JVMslikeJRocketdonthavePermGenatall,everythingisstoredinheap.OnlyinsuchcontextcanyoucallPermGenaspecialpartofheap.

Foranotherexample,inOraclesJava8JVMs,PermGenisreplacedbyMetaspace,accordingtohttp://www.javacodegeeks.com/2013/02/java8frompermgentometaspace.html.Howeverneithermetaspace,PermGen,orPermGen(caseinvariant)arementionedintheJavaVMSpecificationJavaSE8Edition(https://docs.oracle.com/javase/specs/jvms/se8/jvms8.pdf).IconcludethatMetaspaceisalogicalconstructlikePermGenwhichhasaJVMoptioninterfaceandcorrespondstooperationsovermorethanonememoryareasorregionsinthem.

ItwouldbeusefultoclarifythedistinctionbetweenactualJVMruntimeareas,asdocumentedintheJVMspecificationmanuals,andlogicalmappingstothemmadeavailablethroughJVMoptions.ButthiswouldbedifficulttodoexhaustivelyandauthoritativelyduetoaprobablelackofdetaileddocumentationanddifficultyinunderstandingJVMsourcecodeifitisavailable.SomeusefulcluesmightbeobtainedbyinspectingOpenJDKsourceswhichareavailableathttp://download.java.net/openjdk/.

However,IbelievethatthislevelofknowledgeisnotnecessaryformostJavadeveloperpositionsandwouldapplyonlyonlyforJavaJVMdeveloperpositions.

Reply

November19th,2014at2:10amyax

thankyou!

Reply

November22nd,2014at2:22amMohit

justAwesome

Reply

November22nd,2014at12:38pmLEo

WhereisthePDFlink?

Reply

November26th,2014at12:18pmHARSHRASTOGI

Hello!sir..IwanttohavesomeknowledgeaboutwhatissomewhenaprogramisrunHowtheobjectiscreated???Howtheloaderswork??Everything.Anyhelpwebepraised.

Reply



November26th,2014at1:06pmadada

whereisthelink?

Reply

November28th,2014at8:48pmAshot

Question37.Itsaverybadideatoreleaseresourcesinfinalize()methodbecausethereisnogarantiesthatthismethodwillbeinvoked.

Reply

December8th,2014at11:26pmTdogg

Thisistrue.Itisbesttoavoidthefinalizemethodasitsbehaviorisunreliable

Reply

December11th,2014at8:27pmabhisheksauda

Mostefficientfortheguys..whostudiesadaybeforeVivathanksman!

Reply

December13th,2014at4:05pmInstanceofjava

reallyhelpful.thnkyousomuch

Reply

December17th,2014at12:30amsmitha

Awesome..ThewayIneeded.Thanksheaps

Reply

December22nd,2014at6:49pmwojtek

Niceonemate.Veryinformative

Reply

December23rd,2014at3:08pmArisha

greatwork..iwishifyoucouldincludethequestionsaboutstrutsandhibernatealso!

Reply

December28th,2014at7:27pmBalajiKorangi

115JavaInterviewQuestionsandAnswersTheULTIMATEList(PDFDownload)

Howandwheretodownload?

Reply

January13th,2015at10:51pmSauravSingh

WhereisthelinktodownloadthePDF?

Iamalreadyasubscriber.#help

Reply



January16th,2015at5:58pmAndrii

SoImnotaloneinfactthatthereisno(promised)pdfaround?

btwthelistisreallygood

Reply

January17th,2015at7:17amJosRomero

Awesomecontent.Veryusefullandinteresting.

Reply

January18th,2015at2:20pmRanga

Perfectstartforbeginners.Ialsofindthisvideoveryusefulforfreshers.Coverstheimportantinterviewquestionsinvideoformat.

https://www.youtube.com/watch?v=njZ48YVkei0

Reply

January18th,2015at5:37pmAsad

Whereisthepffifyoualreadyamember?Hopethistime,Iwouldgetananswerformyquestion

Reply

January26th,2015at3:21pmkevin

verynicejob,thankyoualot.itwillbehelpfulformycomminginterveiws

Reply

January31st,2015at6:42pmPramod

Verynicequestions.veryhelpful

Reply

February6th,2015at5:50amMohdFurkan

Itisanicecollectionofquestions.Thankyoualot.

Reply

February10th,2015at11:55pmJonathan

Nicecollection.Afewcomments:Question4>Astaticmethodmayaccessaninstancevariable,butonlyifgetsaccesstoaninstanceofaclassstaticintmymethod(Aa){returna.instanceVar+2}Question8>Technically,Javadoespermitaformofmultipleinheritance,namelymultipleinheritanceoftype.Seehereformoreinfohttp://docs.oracle.com/javase/tutorial/java/IandI/multipleinheritance.htmlQuestion44>exceptedshouldbeexpected

Reply

February11th,2015at6:04amPramod

Verynicecollectionofinterviewquestions.veryhelpfulforinterview

Reply

February14th,2015at4:45pmYusuf



Really,whereisthePDFLink?Itsbeenaskedformanytimesbutthereisnoreply.

Reply

February22nd,2015at1:26amwahoo

Yoosoofreally?Trytobegratefulforthissiteinsteadofmakingdemandsreally.

Asfortheotherscomplainingitisoutdatedwhydontyoucomeupwithasiteyourselves.

ThanksManeas!

Reply

February22nd,2015at7:28pmYusuf

Thisisnotcompliantforthesite.Exactopposite,Imoneofthefansofthesiteandmanyarticlespublishedonit.

IjusttriedtofindoutwhyIcannotreachPDFversionsofsomearticles,evenhavingmembershipofnewsletter.ButIcouldnotseeanykindofexplanationthatByronnoticedwithbelowcomment.ThisiswhyIwrotemycomment.

Ithink,wahoo,youshouldtrytobemoreunderstandingaboutwhatpeoplearetryingtosay.

Reply

February22nd,2015at3:26amByronKiourtzoglou

Helloall,

InordertogettheeBookthePDFversionofthisarticleyouhavetosubscribetoournewsletter.Thisisclearlystatedatthebeginningofthepostwherethesubscriptionformresides.

IfyouarealreadyamemberofournewsletteranddidnotreceivetheeBook,pleaseshootusanemailatoursupportaddressandwewillhelpyouout.

Bestregards,Byron

Reply

February22nd,2015at7:33pmYusuf

ThanksByron,

Yougavetheexactanswerofmyquestion.AsImentionedinthereplyofwahooscomment,Iamalreadyamemberofnewsletter,andgotmanyPDFversionsofarticles.Butforsomearticles,likethisone,Icouldtgetit.

Iwillsendanemail.

Regards

Reply

February23rd,2015at4:04amJavaRock

PleasemailmethepdffortheJavainterviewQuestionsPDF.Nicearticle

Reply

March11th,2015at5:37pmpreeti

PleasemailmethepdfofJavainterviewquestions

Reply

March13th,2015at7:03amyash

goodeffortitsreallyhelpful

Reply



April1st,2015at12:15amKrishnan

Thankyousomuch!Itshouldbeincrediblyusefulforjobinterviews.

Reply

April1st,2015at1:28pmbharat

Thanksalotguys

Reply

April5th,2015at2:18pmishara

Verygoodpostthankyouverymuch

Reply

April5th,2015at10:35pmAmernath

Goodone

Reply

April7th,2015at11:50amGratian

Very,verygoodlistofquestionsandanswers.Goodjob!

Reply

April9th,2015at4:12amRR

GoodCollectionofJavaInterviewquestions.

Reply

April15th,2015at5:11pmOmar

Wellwrittenandtothepoint.Thankyou!

Reply

April18th,2015at10:24amali

Giveacompelling(technical)argumentwhythetitfortatpolicyasusedinBitTorrentisfarfromoptimalforfilesharingintheInternet

Reply

April20th,2015at7:03pmArnab

thanksithelpedalot

Reply

April28th,2015at11:00amRahulTilloo

Iamreallythankfultotheauthorforasuchagreatresource.TomorrowIhaveaninterviewforJAVASothisguidewouldbereallyhelpful.Inspiredfromyourarticleifyoudoguestpostingdoletmeknow.

Reply

May6th,2015at7:34pmAbdul

GoodJob



Reply

May11th,2015at4:10amd

YoudidntsendthelinktothePDFfilefortheebook.pleasedoso.

Reply

May14th,2015at2:31pmprakash

Verygoodpostthankyouverymuch

Reply

May23rd,2015at3:24pmshobs

Doesnyoneknowwhatkindofinterviewquestionscanbeaskedfrom10+yrsofexpinjava.anysamplecollection?

Reply

May28th,2015at5:31amLongwuXiao

regist

Reply

May28th,2015at5:34amLongwuXiao

canIgetthePDF?

Reply

June1st,2015at2:22pmUrgen

whereismypdfversionplease?

Reply

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