计算机专业毕业设计论文外文文献中英文翻译(Object)

计算机专业毕业设计论文外文文献中英文翻译(Object)

计算机专业毕业设计论文外文文献中英文翻译(Object)

外文资料

ObjectlandscapesandlifetimesTechnically,OOPisjustaboutabstractdatatyping,inheritance,andpolymorphism,butotherissuescanbeatleastasimportant.Theremainderofthissectionwillcovertheseissues.Oneofthemostimportantfactorsisthewayobjectsarecreatedanddestroyed.Whereisthedataforanobjectandhowisthelifetimeoftheobjectcontrolled?Therearedifferentphilosophiesatworkhere.C++takestheapproachthatcontrolofefficiencyisthemostimportantissue,soitgivestheprogrammerachoice.Formaximumrun-timespeed,thestorageandlifetimecanbedeterminedwhiletheprogramisbeingwritten,byplacingtheobjectsonthestack(thesearesometimescalledautomaticorscopedvariables)orinthestaticstoragearea.Thisplacesapriorityonthespeedofstorageallocationandrelease,andcontrolofthesecanbeveryvaluableinsomesituations.However,yousacrificeflexibilitybecauseyoumustknowtheexactquantity,lifetime,andtypeofobjectswhileyou'rewritingtheprogram.Ifyouaretryingtosolveamoregeneralproblemsuchascomputer-aideddesign,warehousemanagement,orair-trafficcontrol,thisistoorestrictive.Thesecondapproachistocreateobjectsdynamicallyinapoolofmemorycalledtheheap.Inthisapproach,youdon'tknowuntilrun-timehowmanyobjectsyouneed,whattheirlifetimeis,orwhattheirexacttypeis.Thosearedeterminedatthespurofthemomentwhiletheprogramisrunning.Ifyouneedanewobject,yousimplymakeitontheheapatthepointthatyouneedit.Becausethestorageismanageddynamically,atrun-time,theamountoftimerequiredtoallocatestorageontheheapissignificantlylongerthanthetimetocreatestorageonthestack.(Creatingstorageonthestackisoftenasingleassemblyinstructiontomovethestackpointerdown,andanothertomoveitbackup.)Thedynamicapproachmakesthegenerallylogicalassumptionthatobjectstendtobecomplicated,sotheextraoverheadoffindingstorageandreleasingthatstoragewillnothaveanimportantimpactonthecreationofanobject.Inaddition,thegreaterflexibilityisessentialtosolvethegeneral

programmingproblem.Javausesthesecondapproach,exclusively].Everytimeyouwanttocreateanobject,youusethenewkeywordtobuildadynamicinstanceofthatobject.There'sanotherissue,however,andthat'sthelifetimeofanobject.Withlanguagesthatallowobjectstobecreatedonthestack,thecompilerdetermineshowlongtheobjectlastsandcanautomaticallydestroyit.However,ifyoucreateitontheheapthecompilerhasnoknowledgeofitslifetime.InalanguagelikeC++,youmustdetermineprogrammaticallywhentodestroytheobject,whichcanleadtomemoryleaksifyoudon’tdoitcorrectly(andthisisacommonprobleminC++programs).Javaprovidesafeaturecalledagarbagecollectorthatautomaticallydiscoverswhenanobjectisnolongerinuseanddestroysit.Agarbagecollectorismuchmoreconvenientbecauseitreducesthenumberofissuesthatyoumusttrackandthecodeyoumustwrite.Moreimportant,thegarbagecollectorprovidesamuchhigherlevelofinsuranceagainsttheinsidiousproblemofmemoryleaks(whichhasbroughtmanyaC++projecttoitsknees).Therestofthissectionlooksatadditionalfactorsconcerningobjectlifetimesandlandscapes.1CollectionsanditeratorsIfyoudon’tknowhowmanyobjectsyou’regoingtoneedtosolveaparticularproblem,orhowlongtheywilllast,youalsodon’tknowhowtostorethoseobjects.Howcanyouknowhowmuchspacetocreateforthoseobjects?Youcan’t,sincethatinformationisn’tknownuntilrun-time.Thesolutiontomostproblemsinobject-orienteddesignseemsflippant:youcreateanothertypeofobject.Thenewtypeofobjectthatsolvesthisparticularproblemholdsreferencestootherobjects.Ofcourse,youcandothesamethingwithanarray,whichisavailableinmostlanguages.Butthere’smore.Thisnewobject,generallycalledacontainer(alsocalledacollection,buttheJavalibraryusesthatterminadifferentsensesothisbookwilluse“container”),willexpanditselfwhenevernecessarytoaccommodateeverythingyouplaceinsideit.Soyoudon’tneedtoknowhowmanyobjectsyou’regoingtoholdinacontainer.Justcreatea

containerobjectandletittakecareofthedetails.Fortunately,agoodOOPlanguagecomeswithasetofcontainersaspartofthepackage.InC++,it’spartoftheStandardC++LibraryandissometimescalledtheStandardTemplateLibrary(STL).ObjectPascalhascontainersinitsVisualComponentLibrary(VCL).Smalltalkhasaverycompletesetofcontainers.Javaalsohascontainersinitsstandardlibrary.Insomelibraries,agenericcontainerisconsideredgoodenoughforallneeds,andinothers(Java,forexample)thelibraryhasdifferenttypesofcontainersfordifferentneeds:avector(calledanArrayListinJava)forconsistentaccesstoallelements,andalinkedlistforconsistentinsertionatallelements,forexample,soyoucanchoosetheparticulartypethatfitsyourneeds.Containerlibrariesmayalsoincludesets,queues,hashtables,trees,stacks,etc.Allcontainershavesomewaytoputthingsinandgetthingsout;thereareusuallyfunctionstoaddelementstoacontainer,andotherstofetchthoseelementsbackout.Butfetchingelementscanbemoreproblematic,becauseasingle-selectionfunctionisrestrictive.Whatifyouwanttomanipulateorcompareasetofelementsinthecontainerinsteadofjustone?Thesolutionisaniterator,whichisanobjectwhosejobistoselecttheelementswithinacontainerandpresentthemtotheuseroftheiterator.Asaclass,italsoprovidesalevelofabstraction.Thisabstractioncanbeusedtoseparatethedetailsofthecontainerfromthecodethat’saccessingthatcontainer.Thecontainer,viatheiterator,isabstractedtobesimplyasequence.Theiteratorallowsyoutotraversethatsequencewithoutworryingabouttheunderlyingstructure?thatis,whetherit’sanArrayList,aLinkedList,aStack,orsomethingelse.Thisgivesyoutheflexibilitytoeasilychangetheunderlyingdatastructurewithoutdisturbingthecodeinyourprogram.Javabegan(inversion

1.0and

1。

1)withastandarditerator,calledEnumeration,forallofitscontainerclasses.Java2hasaddedamuchmorecompletecontainerlibrarythatcontainsaniteratorcalledIteratorthatdoesmorethantheolderEnumeration.Fromadesignstandpoint,allyoureallywantisasequencethatcanbemanipulatedtosolveyourproblem.Ifasingletypeofsequencesatisfiedallofyour

needs,there’dbenoreasontohavedifferentkinds.Therearetworeasonsthatyouneedachoiceofcontainers.First,containersprovidedifferenttypesofinterfacesandexternalbehavior.Astackhasadifferentinterfaceandbehaviorthanthatofaqueue,whichisdifferentfromthatofasetoralist.Oneofthesemightprovideamoreflexiblesolutiontoyourproblemthantheother.Second,differentcontainershavedifferentefficienciesforcertainoperations.ThebestexampleisanArrayListandaLinkedList.Botharesimplesequencesthatcanhaveidenticalinterfacesandexternalbehaviors.Butcertainoperationscanhaveradicallydifferentcosts.RandomlyaccessingelementsinanArrayListisaconstant-timeoperation;ittakesthesameamountoftimeregardlessoftheelementyouselect.However,inaLinkedListitisexpensivetomovethroughthelisttorandomlyselectanelement,andittakeslongertofindanelementthatisfurtherdownthelist.Ontheotherhand,ifyouwanttoinsertanelementinthemiddleofasequence,it’smuchcheaperinaLinkedListthaninanArrayList.Theseandotheroperationshavedifferentefficienciesdependingontheunderlyingstructureofthesequence.Inthedesignphase,youmightstartwithaLinkedListand,whentuningforperformance,changetoanArrayList.Becauseoftheabstractionviaiterators,youcanchangefromonetotheotherwithminimalimpactonyourcode.Intheend,rememberthatacontainerisonlyastoragecabinettoputobjectsin.Ifthatcabinetsolvesallofyourneeds,itdoesn’treallymatterhowitisimplemented(abasicconceptwithmosttypesofobjects).Ifyou’reworkinginaprogrammingenvironmentthathasbuilt-inoverheadduetootherfactors,thenthecostdifferencebetweenanArrayListandaLinkedListmightnotmatter.Youmightneedonlyonetypeofsequence.Youcanevenimaginethe“perfect”containerabstraction,whichcanautomaticallychangeitsunderlyingimplementationaccordingtothewayitisused.2ThesinglyrootedhierarchyOneoftheissuesinOOPthathasbecomeespeciallyprominentsincetheintroductionofC++iswhetherallclassesshouldultimatelybeinheritedfromasinglebaseclass.InJava(aswithvirtuallyallotherOOPlanguages)theansweris“yes”and

thenameofthisultimatebaseclassissimplyObject.Itturnsoutthatthebenefitsofthesinglyrootedhierarchyaremany.Allobjectsinasinglyrootedhierarchyhaveaninterfaceincommon,sotheyareallultimatelythesametype.Thealternative(providedbyC++)isthatyoudon’tknowthateverythingisthesamefundamentaltype.Fromabackward-compatibilitystandpointthisfitsthemodelofCbetterandcanbethoughtofaslessrestrictive,butwhenyouwanttodofull-onobject-orientedprogrammingyoumustthenbuildyourownhierarchytoprovidethesameconveniencethat’sbuiltintootherOOPlanguages.Andinanynewclasslibraryyouacquire,someotherincompatibleinterfacewillbeused.Itrequireseffort(andpossiblymultipleinheritance)toworkthenewinterfaceintoyourdesign.Istheextra“flexibility”ofC++worthit?Ifyouneedit?ifyouhavealargeinvestmentinC?it’squitevaluable.Ifyou’restartingfromscratch,otheralternativessuchasJavacanoftenbemoreproductive.Allobjectsinasinglyrootedhierarchy(suchasJavaprovides)canbeguaranteedtohavecertainfunctionality.Youknowyoucanperformcertainbasicoperationsoneveryobjectinyoursystem.Asinglyrootedhierarchy,alongwithcreatingallobjectsontheheap,greatlysimplifiesargumentpassing(oneofthemorecomplextopicsinC++).Asinglyrootedhierarchymakesitmucheasiertoimplementagarbagecollector(whichisconvenientlybuiltintoJava).Thenecessarysupportcanbeinstalledinthebaseclass,andthegarbagecollectorcanthussendtheappropriatemessagestoeveryobjectinthesystem.Withoutasinglyrootedhierarchyandasystemtomanipulateanobjectviaareference,itisdifficulttoimplementagarbagecollector.Sincerun-timetypeinformationisguaranteedtobeinallobjects,you’llneverendupwithanobjectwhosetypeyoucannotdetermine.Thisisespeciallyimportantwithsystemleveloperations,suchasexceptionhandling,andtoallowgreaterflexibilityinprogramming.3CollectionlibrariesandsupportforeasycollectionuseBecauseacontainerisatoolthatyou’llusefrequently,itmakessensetohavea

libraryofcontainersthatarebuiltinareusablefashion,soyoucantakeoneofftheshelfBecauseacontainerisatoolthatyou’llusefrequently,itmakessensetohavealibraryofcontainersthatarebuiltinareusablefashion,soyoucantakeoneofftheshelfandplugitintoyourprogram.Javaprovidessuchalibrary,whichshouldsatisfymostneeds.Downcastingvs.templates/genericsTomakethesecontainersreusable,theyholdtheoneuniversaltypeinJavathatwaspreviouslymentioned:Object.ThesinglyrootedhierarchymeansthateverythingisanObject,soacontainerthatholdsObjectscanholdanything.Thismakescontainerseasytoreuse.Tousesuchacontainer,yousimplyaddobjectreferencestoit,andlateraskforthemback.But,since