JAVA Interview Questions (1)..........pdf

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1. BasicJavaInterviewQuestions
1.Whatis?
Javaisahigh-level,object-oriented,platform-independentprogramminglanguage.Key
features:
●Platform-independent:Codecanrunondifferentplatformswithoutmodification
usingJVM.
●Highperformance:JITcompilerconvertsbytecodetomachinecodeforfaster
execution.
●Multi-threaded:Supportsconcurrenttaskhandling.
●Nomultipleinheritancethroughclasses:Achievedusinginterfaces.
●Security:Noexplicitpointers;strongmemorymanagementwithgarbagecollection.
2.OOPConcepts
●Inheritance:Codereusebetweensuperclassesandsubclasses;privatemembersare
notinherited.
●Encapsulation:Protectscodebymakingvariablesprivateandusinggetter/setter
methods.
●Polymorphism:Objectsbehavedifferentlybasedontheirtype(methodoverriding).
●Abstraction:Hidesimplementationdetailswhileexposingessentialfeatures.
●Interface:Providesmultipleinheritancebydefiningmethoddeclarations.
3.AccessModifiers
●Default:Accessiblewithinthesamepackage.
●Private:Accessibleonlywithinthedeclaringclass.
●Protected:Accessibleinthesamepackageorsubclassesinotherpackages.
●Public:Accessibleeverywhere.
4.SOLIDPrinciples
●SingleResponsibility:Eachclasshasoneresponsibility.
●Open-Closed:Classesshouldbeopentoextensionbutclosedtomodification.
●LiskovSubstitution:Subtypesmustbesubstitutablefortheirbasetypes.

2. ●InterfaceSegregation:Avoidforcingclientstoimplementunusedmethods.
●DependencyInversion:High-levelmodulesdependonabstractions,notconcrete
implementations.
5.PrimitiveDataTypes
●byte(1byte),short(2bytes),char(2bytes),int(4bytes),long(8bytes)
●float(4bytes),double(8bytes),boolean(1byte)
6.OrderofInitialization
1.StaticBlocks:Runoncewhentheclassisloaded.
2.InstanceInitializationBlocks:Executebeforeconstructors.
3.Constructors:Initializeobjects.
7.LambdaExpressionsvs.Closures
●LambdaExpressions:Anonymousfunctionsthattreatactionsasobjects;can't
accessvariablesoutsidetheirscopeunlessfinaloreffectivelyfinal.
●Closures:Notdirectlysupportedin;allowreferencingvariablesoutsidethe
parameterlist.
8.ObjectClassMethods
●Examples:clone(),equals(),toString(),hashCode(),notify(),wait(),getClass()
9.Atomic,Volatile,Synchronized
●Volatile:Ensuresvisibilityofvariableupdatesacrossthreads.
●AtomicInteger:Providesthread-safeoperationswithoutsynchronizationusingCAS.
●Synchronized:Allowsonlyonethreadatatimetoaccessablockormethod.
10.Serialization
●Convertsobjectstobytestreamsforpersistenceornetworktransfer.

3. ●ImplementSerializableinterfacefordefaultserialization.
●CustomizeusingreadObjectandwriteObject.
●Usetransientorstaticforvariablesnottobeserialized.
11.Cloning
●ShallowCopy:Onlycopiesprimitivefields;objectreferencespointtothesame
memory.
●DeepCopy:Createsnewobjectsforallreferences,duplicatingalldata.
12.StringBuffervs.StringBuilder
●StringBuffer:Thread-safe,slower.
●StringBuilder:Non-thread-safe,faster.
13.XMLParsers
●DOM:Tree-based,memory-intensive.
●SAX:Event-driven,forward-only,lowmemory.
●StAX:Pull-based,allowsparsingandmodification.
14.NewFeaturesin
●8:Lambdaexpressions,StreamAPI,Optional,FunctionalInterfaces,DateAPI.
●11:isBlank(),lines(),strip(),HTTPclientupdates,readString(),writeString().
15.LambdaExpressions
Usedforinlineimplementationoffunctionalinterfaces,improvingcodereadability
andcompactness.
16.Exceptions
Abnormaleventsdisruptingprogramflow,managedusingtry-catchorthrows.
Ifyouneedfurtherexplanationonanyspecifictopic,letmeknow!

4. CollectionsInterviewQuestions
1.CollectionsTypeHierarchy
TheCollectionsFrameworkisstructuredaroundseveralcoreinterfaces:
●Iterable:Therootinterfaceenablingiterationoverelements(e.g.,for-eachloop).
●Collection:ExtendsIterableanddefinesmethodslikeadd,remove,size,andmore.
○List:Ordered,index-basedcollections(e.g.,ArrayList,LinkedList).
○Set:Unorderedcollectionswithuniqueelements(e.g.,HashSet,TreeSet).
○Queue:Supportselementorderingforprocessing(e.g.,PriorityQueue).
●Map:Key-valuepairs,distinctfromCollection,emphasizingmappingratherthan
collections.
Eachinterfaceservesauniquepurpose,ensuringflexibilityandscalabilityindesigning
applications.
2.MapImplementationsandUseCases
●HashMap:Bestforgeneral-purposeusewhereorderisnotaconcern.O(1)time
complexityforoperations.
●LinkedHashMap:Retainsinsertionorder,idealforaccess-order-sensitiveapplications
(e.g.,LRUcaches).
●TreeMap:Maintainskeysinsortedorder.Suitableforrangequeriesorordereddata
(logarithmicaccesstime).
●ConcurrentHashMap:Optimizedforconcurrentaccess,avoidinggloballocksfor
betterperformanceinmulti-threadedenvironments.
●Hashtable:Synchronizedbutoutdated;typicallyreplacedbyConcurrentHashMap.
3.LinkedListvs.ArrayList
●ArrayList:
○Backedbyaresizablearray.
○Fastrandomaccess(O(1)).
○Costlyinsertions/removalsinthemiddle(O(n))duetoshiftingelements.
●LinkedList:
○Usesadoubly-linkedlist.
○Efficientinsertions/removals(O(1)).
○Slowertraversal(O(n)).
○Highermemoryoverheadduetonodereferences.

5. UseCase:UseArrayListforfrequentaccess;preferLinkedListforheavy
insertions/deletions.
4.HashSetvs.TreeSet
●HashSet:
○BackedbyHashMap.
○Noorderguarantee.
○Fastoperations(O(1)).
●TreeSet:
○BackedbyTreeMap.
○Maintainsnaturalorcustomorder.
○Sloweroperations(O(logn)).
UseCase:UseHashSetforperformance;preferTreeSetfororderedsets.
5.HashMapImplementation
AHashMapemploys:
●Hashing:Calculatesthebucketindexfromthehashcodeofkeys.
●Buckets:Holdkey-valuepairs.Collisionsarehandledusinglinkedlistsorred-black
trees(since8).
●Resizing:Automaticallydoublesinsizewhentheloadfactorexceedsthethreshold.
KeyoperationslikeputandgetareoptimizedforO(1),butexcessivecollisionscandegrade
performancetoO(logn).
6.InitialCapacityandLoadFactor
●InitialCapacity:Startingsizeoftheinternalarray(roundedtothenearestpowerof2).
●LoadFactor:Threshold(default0.75)forresizingwhentheratioofelementsto
bucketsisexceeded.
Optimizingtheseparametersminimizesresizingoverhead.
7.SpecialCollectionsforEnums
●EnumSet:ExtremelyefficientSetforenumsusingbitwiseoperations.
●EnumMap:Usesenumsaskeysandarraysforstorage,providingfastlookups
withouthashing.

6. 8.Fail-Fastvs.Fail-SafeIterators
●Fail-Fast:
○Detectsconcurrentmodificationsandthrows
ConcurrentModificationException.
○FoundincollectionslikeArrayList,HashMap.
●Fail-Safe:
○Operatesonacopyofthecollection.
○FoundinCopyOnWriteArrayList,ConcurrentHashMap.
9.Comparablevs.Comparator
●Comparable:
○NaturalorderingdefinedbycompareTomethod.
○Example:Sortingintegersinascendingorder.
●Comparator:
○Customorderingviacomparemethod.
○Example:Sortingstringsbylength.
BothinterfacesarepivotalforsortingcollectionsusingmethodslikeCollections.sortorfor
maintainingorderinTreeSet/TreeMap.
ConcurrencyInterviewQuestions
CreatingandRunningaThreadInstance:
UsingRunnable:
Threadthread1=newThread(()->System.out.println("Hellofrom
Runnable!"));
thread1.start();
1.
UsingaSubclassofThread:
Threadthread2=newThread(){
@Override

7. publicvoidrun(){
System.out.println("Hellofromsubclass!");
}
};
thread2.start();
2.
ThreadStatesandTransitions:
1.NEW:Threadcreatedbutnotstarted(Threadt=newThread()).
2.RUNNABLE:Aftercallingstart(),threadisreadyforexecution.
3.BLOCKED:Threadwaitingtoenterasynchronizedblock.
4.WAITING:Threadwaitingindefinitelyforanotherthread'ssignal.
5.TIMED_WAITING:Threadwaitingforaspecifiedduration.
6.TERMINATED:Threadcompletesexecution.
RunnablevsCallable:
●Runnable:Doesnotreturnaresultorthrowcheckedexceptions.
Callable:Returnsaresultandcanthrowcheckedexceptions.Example:
Callable<Integer>callable=()->42;
●
DaemonThreads:
●Definition:Threadssupportingnon-daemonthreads.JVMexitsonceallnon-daemon
threadsfinish.
Creation:
Threaddaemon=newThread(()->System.out.println("Daemon
thread!"));
daemon.setDaemon(true);
daemon.start();

8. ●
InterruptFlagandExceptions:
●SetInterrupt:thread.interrupt().
●CheckInterrupt:Thread.interrupted()(clearsflag)orisInterrupted()
(doesn'tclearflag).
●Behavior:ThrowsInterruptedExceptionifinblockingoperationlikesleep().
ExecutorvsExecutorService:
●Executor:SimpleinterfaceforexecutingRunnabletasks.
●ExecutorService:Addslifecyclemanagement,tasktracking,andadvancedexecution
controls.
ExecutorServiceImplementations:
1.ThreadPoolExecutor:Managesapoolofthreads.
2.ScheduledThreadPoolExecutor:Schedulestaskswithdelaysorperiodicexecution.
3.ForkJoinPool:Efficientfordivide-and-conquertasksusingwork-stealing.
MemoryModel(JMM):
●Defineshowthreadsinteractthroughsharedmemory.
●Ensurespredictablememoryvisibilityandorderingforconcurrentthreads.
●KeyConcepts:Actions,Synchronizationactions,Happens-beforerelationships,and
Happens-beforeconsistency.
VolatileFieldsinJMM:
●Guarantees:
○Visibility:Changestovolatilevariablesareimmediatelyvisibletoother
threads.
○Atomicity:Singleread/writeoflongordoublevariables.
AtomicityofOperations:

9. ●Atomic:Writestointorvolatilelong.
●Non-atomic:Incrementingavolatilelong.
FinalFieldGuarantees:
●finalensuresproperinitializationvisibilityacrossthreads.
SynchronizedKeyword:
●Synchronizesaccessusing:
○Instancemethods:Lockontheobject.
○Staticmethods:Lockontheclassobject.
○Block:Lockonaspecifiedobject.
StaticvsInstanceSynchronizedMethods:
●Instancemethods:Threadsaccessingdifferentinstancesdon'tblock.
●Staticmethods:Threadsshareasingleclass-levellock.
wait(),notify(),notifyAll():
●Purpose:Forthreadcoordinationinsynchronizedblocks.
●Behavior:
○wait():Releaseslockandwaitsfornotification.
○notify():Wakesasinglewaitingthread.
○notifyAll():Wakesallwaitingthreads.

10. JavaGarbageCollectionQuestions
WhatisGarbageCollection?
Garbagecollection(GC)istheprocessofautomaticallyreclaimingmemorybyidentifying
andremovingobjectsnolongerreferencedbytheapplication,thuspreventingmemory
leaks.
WhichPartoftheMemoryisInvolvedinGarbageCollection?
GarbagecollectionoccursintheHeapMemory,specificallyintheYoungGenerationand
OldGeneration.
MinorvsMajorGarbageCollection
1.MinorGC:
○CleanstheYoungGeneration(EdenandSurvivorspaces).
○Occursmorefrequentlyandisfaster.
○TriggerswhentheEdenspaceisfull.
2.MajorGC:
○CleanstheOldGeneration.
○Occurslessfrequentlyandismoretime-consuming.
○TriggerswhentheOldGenerationisfulloratJVM-definedthresholds.
AlgorithmforGarbageCollectioninJava
1.Mark-and-Sweep:
○MarkPhase:Identifiesreachableobjects.
○SweepPhase:Reclaimsmemoryoccupiedbyunreachableobjects.
2.GenerationalGC:
○Objectsarecategorizedintogenerations(YoungandOld).
○Differentstrategiesareappliedtoeachgeneration.
3.Stop-the-World:
○Allapplicationthreadspauseduringgarbagecollection.

11. 4.CompactPhase(Optional):
○Fragmentedmemoryisconsolidatedforefficiency.
finalize()MethodinJava
●Purpose:Providesawaytoperformcleanupoperationsbeforeanobjectis
reclaimedbythegarbagecollector.
●WhenCalled:Thegarbagecollectorinvokesfinalize()ifitdetectsthatthe
objectiseligibleforGCandoverridesthemethod.
●Issues:
○Noguaranteethatfinalize()willbecalled.
○Cancauseperformanceissuesandunpredictablebehavior.
○DeprecatedasofJava9.
MakinganObjectEligibleforGarbageCollection
NullifyReferences:Setobjectreferencestonull.
obj=null;
1.
ReassignReferences:Pointthereferencetoanotherobject.
obj1=obj2;
2.
ScopeLimitation:Lettheobjectgooutofscope.
//Objectdeclaredinsideamethodisnolongeraccessibleoutsideit.
3.
WaystoCallGarbageCollector
1.ExplicitlySuggest:
○UsingSystem.gc()orRuntime.getRuntime().gc().
○NoguaranteethatGCwillrunimmediately.
2.JVMTriggered:
○HappensautomaticallybasedonJVM'smemorythresholdsand
requirements.

12. HeapvsStackMemoryinJava
Aspect HeapMemory StackMemory
Purpose StoresobjectsandJREclasses.Storesmethodcalls,local
variables,andreferences.
Allocation Dynamic(atruntime). Static(basedonthemethod
lifecycle).
Lifetime Existsthroughouttheapplication
lifecycle.
Existsonlyduringmethod
execution.
ThreadScopeSharedacrossthreads.Thread-local(eachthreadhasits
stack).
Speed Slowerduetoglobalaccessand
dynamicallocation.
Fasterbecauseofsimpler
memoryaccesspatterns.
Memory
Management
ManagedbyGarbageCollector.Managedautomaticallyvia
methodcalls.
PartsoftheHeapMemory
1.YoungGeneration:
○DividedintoEdenSpaceandSurvivorSpaces.
○Newobjectsareallocatedhere,andminorgarbagecollectionoccursto
removeshort-livedobjects.
2.OldGeneration(TenuredSpace):
○Storeslong-livedobjects.
○Majorgarbagecollectionhappenshere.
3.Metaspace(Java8andlater):
○Storesmetadataaboutclasses,methods,andotherreflectivedata.
4.PermanentGeneration(Java7andearlier):
○SimilartoMetaspacebuthadafixedsizeandwaspartoftheheap.
PermGenvsMetaspace
AspectPermGen(Java7andearlier)Metaspace(Java8andlater)
Purpose Storesclassmetadataand
reflectivedata.
Alsostoresclassmetadata,
replacingPermGen.

13. MemoryLimitFixedsize,configurableviaJVM
options.
Dynamicallyresized;growsas
needed.
Garbage
Collection
Partoftheheap,managedby
GC.
ManagedbyGC,butpartofnative
memory.
ForMoreClick

14. JavaAnnotationInterviewQuestions
1.WhatareMeta-Annotations,andWhatisthePurposeofEachinthe
java.lang.annotationPackage?
●@Retention:Definesthescopeoftheannotation.
○Policies:
■SOURCE:Annotationisdiscardedduringcompilation.
■CLASS:Annotationisintheclassfilebutnotavailableatruntime.
■RUNTIME:AnnotationisretainedintheJVMandavailablevia
reflection.
●@Target:Specifiesthekindsofelementsanannotationcanapplyto(e.g.,METHOD,
FIELD).
●@Inherited:Allowssubclassestoinheritannotationsfromtheirparentclasses.
●@Documented:Indicatesthattheannotationshouldappearinthegenerated
Javadoc.
●@Repeatable:Allowsmultipleinstancesofthesameannotationonanelement.
2.HowDoYouCreateaCustomAnnotation?
Example:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public@interfaceTest{
Stringdescription()default"Nodescription";
intpriority()default0;
Class<?extendsThrowable>expectedExceptions()defaultNone.class;
}
3.ExplaintheDifferenceBetweenRuntimeandCompile-TimeAnnotations.HowAre
TheyProcessed?
●Compile-TimeAnnotations:
○Examples:@Override,@Deprecated.
○Processedduringcompilation.
○EnsurescorrectnessandcompliancewithJavaconventions.
●RuntimeAnnotations:
○Examples:Customannotationswith@Retention(RUNTIME).
○Accessedusingreflectionduringruntime.
○CommonlyusedinframeworkslikeSpringandHibernate.

15. 4.HowCanYouImplementaCustomAnnotationProcessor?
ExampleImplementation:
@SupportedAnnotationTypes("com.example.MyCustomAnnotation")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
publicclassMyAnnotationProcessorextendsAbstractProcessor{
@Override
publicbooleanprocess(Set<?extendsTypeElement>annotations,RoundEnvironment
roundEnv){
//Customprocessinglogic
returntrue;
}
}
5.DescribeAdvancedUseCasesofJavaReflectionwithAnnotations.
●DynamicMethodInvocation:Executemethodsbasedonannotations.
●RuntimeConfigurations:Modifybehaviordynamicallyusingannotations.
●DependencyInjection:FrameworkslikeSpringuseannotationsforDI.
●ValidationFrameworks:Buildfield-levelvalidatorswithannotations.
●TestFrameworks:Customtestrunnersandassertions.
6.WhatAretheLimitationsofAnnotations,andHowCanTheyBeOvercome?
●Limitations:
○Cannotcontainmethodimplementations.
○Limitedtospecificelementtypes.
○Mayintroduceruntimeoverhead.
●Strategies:
○Combineannotationswithreflectiveorbytecode-basedtools.
○Designminimalisticannotationstoreducecomplexity.
7.CompareandContrastAnnotationswithMarkerInterfaces.
●Annotations:
○Flexiblemetadata,nohierarchyconstraints.
○Bettersuitedfordeclarativeprogramming.
●MarkerInterfaces:
○Usedfortype-checkingandenforceablecontracts.
○Limitedflexibilitycomparedtoannotations.

16. 8.HowWouldYouImplementaCustomValidationFrameworkUsingAnnotations?
Example:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public@interfaceValidate{
booleannotNull()defaultfalse;
intminLength()default0;
intmaxLength()defaultInteger.MAX_VALUE;
}
publicclassValidationProcessor{
publicbooleanvalidate(Objectobject){
//Implementreflection-basedvalidationlogic
returntrue;
}
}
9.HowDoFrameworksLikeSpringUtilizeAnnotationsforDependencyInjectionand
Configuration?
●Annotations:
○@Autowired:Forinjectingdependencies.
○@Component:MarksaclassasaSpringbean.
○@Configuration:Indicatesaconfigurationclass.
○@Bean:Definesbeansinconfigurationclasses.
●Mechanism:
○Frameworksscanannotationsatruntimeorcompile-timetoperforminjections
andconfigurations.
10.DemonstrateaComplexUseCaseofMeta-ProgrammingUsingAnnotations.
●Scenario:CreateaCustomORM(Object-RelationalMapping).Example:
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public@interfaceColumn{
Stringname();
booleanprimaryKey()defaultfalse;
}
publicclassORMProcessor{

17. publicvoidmapObjectToDB(Objectentity){
//Usereflectiontomapfieldswith@Columntodatabasecolumns
}
}

18. JavaDependencyInjection(DI)InterviewQuestions
1.WhatisDependencyInjection(DI),andWhyisitImportant?
●Definition:DependencyInjectionisadesignpatterninwhichanobject’s
dependenciesareprovidedexternallyratherthancreatedinternallybytheobject.
●Purpose:
○Promotesloosecouplingbetweenobjects.
○Makescodeeasiertomaintain,test,andextend.
●ProblemsSolved:
○Avoidshard-codingdependencies.
○Facilitatesunittestingbyallowingtheuseofmockdependencies.
●Comparison:Traditionalobjectcreationcouplesaclasswithitsdependencies,
whereasDIseparatestheirlifecycle.
2.ExplaintheDifferentTypesofDependencyInjection
ConstructorInjection:
●Dependenciesareinjectedthroughtheconstructor.
●Ensuresdependenciesareimmutableandrequiredfortheobjecttofunction.
publicclassUserService{
privatefinalUserRepositoryrepository;
publicUserService(UserRepositoryrepository){
this.repository=repository;
}
}
SetterInjection:
●Dependenciesaresetthroughsettermethods.
●Allowsoptionalorlatedependencyinjection.
publicclassUserService{
privateUserRepositoryrepository;
publicvoidsetUserRepository(UserRepositoryrepository){
this.repository=repository;
}
}

19. InterfaceInjection:
●Dependenciesareprovidedthroughacustommethoddefinedinaninterface.
●Rarelyusedcomparedtoconstructororsetterinjection.
publicinterfaceRepositoryInjector{
voidinjectRepository(UserServiceservice);
}
3.AdvancedDIConcepts
●InversionofControl(IoC):
○Delegatestheresponsibilityofdependencymanagementtoacontainer.
●DependencyInversionPrinciple:
○High-levelmodulesshouldnotdependonlow-levelmodules;bothshould
dependonabstractions.
●ServiceLocatorPatternvsDependencyInjection:
○ServiceLocator:Centralizedaccesstodependencies.
○DI:Dependenciesarepusheddirectlyintoobjects.
●Pros/ConsofInjectionMethods:
○Constructor:Mandatorydependencies,immutability.
○Setter:Optionaldependencies,flexibility.
4.SpringFrameworkDIQuestions
Annotations:
@Component
publicclassUserService{
@Autowired
privateUserRepositoryrepository;
}
●@Autowired:Injectsdependenciesautomatically.
●@Component,@Service,@Repository:DefineSpring-managedbeans.
●@Scope:Definesbeanscope(singleton,prototype,etc.).
●@Qualifier:Resolvesambiguitywhenmultiplebeansofthesametypeexist.
5.ImplementingaCustomDependencyInjectionContainer
ExampleImplementation:

20. publicclassDIContainer{
privateMap<Class<?>,Object>instances=newHashMap<>();
public<T>voidregister(Class<T>type,Tinstance){
instances.put(type,instance);
}
public<T>Tresolve(Class<T>type){
return(T)instances.get(type);
}
}
6.ComplexDependencyInjectionScenarios
●CircularDependencies:
○AdependsonB,andBdependsonA.
○Solvedusing@Lazyinitializationorrefactoring.
●HandlingOptionalDependencies:
○UseOptional<T>orsetterinjection.
●LazyInitialization:
○Delaysthecreationofdependenciesuntilneeded.
●PrototypevsSingletonScopes:
○Prototype:Newinstanceperinjection.
○Singleton:Singlesharedinstance.
7.PerformanceandBestPractices
●ConstructorInjectionPreferred:
○Ensuresalldependenciesareavailableatobjectcreation.
@Service
publicclassUserService{
privatefinalUserRepositoryrepository;
@Autowired
publicUserService(UserRepositoryrepository){
this.repository=repository;
}
}
●BestPractices:
○Avoidtoomanydependencies.
○Usedependencygraphsjudiciously.

21. ○Properlyscopebeanstoavoidmemoryleaks.
8.AdvancedDIwithGenerics
Example:
publicinterfaceGenericRepository<T>{
voidsave(Tentity);
}
@Service
publicclassGenericService<T>{
privatefinalGenericRepository<T>repository;
@Autowired
publicGenericService(GenericRepository<T>repository){
this.repository=repository;
}
}
9.TestingwithDependencyInjection
●UsemockingframeworkslikeMockitofordependencyinjectionduringtests.
publicclassUserServiceTest{
@Mock
privateUserRepositorymockRepository;
@InjectMocks
privateUserServiceuserService;
@Test
publicvoidtestUserCreation(){
//Testlogichere
}
}
●Strategies:
○Mockdependenciesforisolatedunittesting.
○Testintegrationwithactualdependenciesforend-to-endtesting.

22. 10.JavaEEandJakartaEEDIAnnotations
Annotations:
@Named
@ApplicationScoped
publicclassUserService{
@Inject
privateUserRepositoryrepository;
}
●CDI(ContextandDependencyInjection):
○@Inject:Standardforinjectingdependencies.
○@Named:Bindsanametoabean.
○Scopeannotationslike@ApplicationScoped,@RequestScoped.

23. PatternsInterviewQuestions
CreationalPatterns
CreationalDesignPatternsarefocusedontheprocessofobjectcreation.Theyaimto
reducecomplexityandinstabilitybycreatingobjectsinacontrolledmanner.
SingletonDesignPattern
TheSingletonDesignPatternensuresthatonlyoneinstanceofaclassexiststhroughout
theVirtualMachine(JVM)andprovidesaglobalaccesspointtothatinstance.
●LazyInitializationwithStaticInnerClass:
publicclassSingleton{
privateSingleton(){}
privatestaticclassSingletonHolder{
publicstaticfinalSingletoninstance=newSingleton();
}
publicstaticSingletongetInstance(){
returnSingletonHolder.instance;
}
}
●EagerInitialization(NotLazy):

24. publicclassSingleton{
publicstaticfinalSingletonINSTANCE=newSingleton();
privateSingleton(){}
}
ProblemswithSerializationandDeserialization
IfyouserializeaSingletonclass,itcancreatenewinstancesduringdeserialization,violating
theSingletonproperty.ImplementingtheSerializableinterfacealonewon'tsolvethis.
ProblemswithReflection
AdvanceduserscanbreaktheSingletonbyusingreflectiontochangetheaccessmodifierof
theconstructoratruntime,allowingformultipleinstancesoftheSingleton.
SingletonwithEnum
Enumsprovideamuchmorerobustsolution.Sinceenumsareinherentlyserializable,they
avoidtheserializationandreflectionissues.Here’showyoucanimplementaSingletonwith
anenum:
publicenumSingletonEnum{
INSTANCE;
privateintvalue;

25. publicintgetValue(){
returnvalue;
}
publicvoidsetValue(intvalue){
this.value=value;
}
}
Usage:
publicclassEnumDemo{
publicstaticvoidmain(String[]args){
SingletonEnumsingleton=SingletonEnum.INSTANCE;
System.out.println(singleton.getValue());
singleton.setValue(2);
System.out.println(singleton.getValue());
}
}
WhentoUseSingletonDesignPattern
●Forexpensiveresourceslikedatabaseconnections.

26. ●Tooptimizeperformance(e.g.,loggers).
●Forconfigurationsettingsinanapplication.
●Forresourcesaccessedinsharedmode.
FactoryDesignPattern
TheFactoryDesignPatterndefinesaninterfaceforcreatingobjects,butletssubclasses
decidewhichclasstoinstantiate.Thispatternisoftenusedtocreateobjectsbasedoninput,
suchasthenumberofsidesofapolygon.
●Example:
publicinterfacePolygon{
StringgetType();
}
publicclassPolygonFactory{
publicPolygongetPolygon(intnumberOfSides){
switch(numberOfSides){
case3:returnnewTriangle();
case4:returnnewSquare();
case5:returnnewPentagon();
case7:returnnewHeptagon();
case8:returnnewOctagon();
default:returnnull;
}
}

27. }
WhentoUseFactoryDesignPattern
●Whenimplementationsofaninterfaceorabstractclasschangefrequently.
●Whenthecurrentimplementationcannoteasilyaccommodatechanges.
●Whentheinitializationprocessissimpleandrequiresonlyafewparameters.
AbstractFactoryDesignPattern
TheAbstractFactoryDesignPatternisusedtocreatefamiliesofrelatedordependent
objects.It’sessentiallyafactoryoffactories.Thisisusefulwhendealingwithcomplex
objectsthatbelongtoaspecificfamilyandneedtobecreatedtogether.
BuilderDesignPattern
TheBuilderDesignPatternisusedwhenthecreationofanobjectbecomestoocomplex.It
separatestheconstructionofanobjectfromitsrepresentation.
●Example:
publicclassBankAccount{
privateStringname;
privateStringaccountNumber;
privateStringemail;
privatebooleannewsletter;
publicstaticclassBankAccountBuilder{
privateStringname;

28. privateStringaccountNumber;
privateStringemail;
privatebooleannewsletter;
publicBankAccountBuilder(Stringname,StringaccountNumber)
{
this.name=name;
this.accountNumber=accountNumber;
}
publicBankAccountBuilderwithEmail(Stringemail){
this.email=email;
returnthis;
}
publicBankAccountBuilderwantNewsletter(booleannewsletter)
{
this.newsletter=newsletter;
returnthis;
}
publicBankAccountbuild(){
returnnewBankAccount(this);
}
}

29. }
Usage:
BankAccountnewAccount=newBankAccount
.BankAccountBuilder("Jon","22738022275")
.withEmail("[email protected]")
.wantNewsletter(true)
.build();
WhentoUseBuilderPattern
●Whentheobjectcreationprocessiscomplexandhasmanymandatoryandoptional
parameters.
●Whenconstructorswithmanyparametersleadtoalargenumberofconstructors.
●Whenclientsexpectdifferentrepresentationsoftheobject.
StructuralPatterns
StructuralDesignPatternsdealwiththecompositionofclassesandobjects,focusingon
howobjectsarecomposedtoformlargerstructures.
ProxyPattern
TheProxyPatterncreatesanintermediaryobjectthatactsasaninterfacetoanother
resource,suchasafileordatabaseconnection.Itallowsforcontrolledaccessandprotects
theoriginalobjectfromcomplexity.
●Example:
publicinterfaceExpensiveObject{
voidprocess();
}

30. publicclassExpensiveObjectImplimplementsExpensiveObject{
publicExpensiveObjectImpl(){
heavyInitialConfiguration();
}
@Override
publicvoidprocess(){
System.out.println("Processingcomplete.");
}
privatevoidheavyInitialConfiguration(){
System.out.println("Loadinginitialconfiguration...");
}
}
publicclassExpensiveObjectProxyimplementsExpensiveObject{
privatestaticExpensiveObjectobject;
@Override
publicvoidprocess(){
if(object==null){
object=newExpensiveObjectImpl();
}
object.process();
}
}
WhentoUseProxyPattern

31. ●Whenyouwanttosimplifyacomplexorheavyobject.
●Whentheoriginalobjectresidesinadifferentaddressspaceandneedstobe
accessedlocally.
●Whenyouneedtoaddalayerofsecuritytocontrolaccess.
DecoratorPattern
TheDecoratorPatternenhancesthebehaviorofanobjectdynamicallybyaddingadditional
responsibilitieswithoutmodifyingtheoriginalclass.
●Example:
publicinterfaceChristmasTree{
Stringdecorate();
}
publicclassChristmasTreeImplimplementsChristmasTree{
@Override
publicStringdecorate(){
return"Christmastree";
}
}
publicabstractclassTreeDecoratorimplementsChristmasTree{
privateChristmasTreetree;
publicTreeDecorator(ChristmasTreetree){
this.tree=tree;
}
@Override
publicStringdecorate(){

32. returntree.decorate();
}
}
publicclassBubbleLightsextendsTreeDecorator{
publicBubbleLights(ChristmasTreetree){
super(tree);
}
@Override
publicStringdecorate(){
returnsuper.decorate()+"withBubbleLights";
}
}
WhentoUseDecoratorPattern
●Whenyouwanttoadd,enhance,orremovethebehaviorofanobjectdynamically.
●Whenyouneedtomodifythefunctionalityofasingleobjectwithoutchangingothers.
Formorequestionsandanswers,youcan
followtheseadditionallinks:
●Devinterview-ioJavaInterviewQuestions
●AatulJavaInterviewQuestionsandAnswers