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本科毕业设计外文文献翻译

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XXX工业大学本科毕业设计外文文献翻译

Objectlandscapesandlifetimes

Technically,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,thegreaterflexibilityisessentialtosolvethegeneralprogrammingproblem.

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.

1Collectionsanditerators

Ifyoudon’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.Justcreateacontainerobjectandletittakecareofthedetails.

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（inversion1.0and1.1）withastandarditerator,calledEnumeration,forallofitscontainerclasses.Java2hasaddedamuchmorecompletecontainerlibrarythatcontainsaniteratorcalledIteratorthatdoesmorethantheolderEnumeration.

Fromadesignstandpoint,allyoureallywantisasequencethatcanbemanipulatedtosolveyourproblem.Ifasingletypeofsequencesatisfiedallofyourneeds,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.

2Thesinglyrootedhierarchy

OneoftheissuesinOOPthathasbecomeespeciallyprominentsincetheintroductionofC++iswhetherallclassesshouldultimatelybeinheritedfromasinglebaseclass.InJava（aswithvirtuallyallotherOOPlanguages）theansweris“yes”andthenameofthisultimatebaseclassissimplyObject.Itturnsoutthatthebenefitsofthesinglyrootedhierarchyaremany.

Allobjectsinasinglyrootedhierarchyhaveaninterfaceincommon,sotheyareallultimatelythesametype.Thealternative（providedbyC++）isthatyoudon’tknowthateverythingisthesamefundamentaltype.Fromabackward-compatibilitystandpointthisfitsthemodelofCbetterandcanbethoughtofaslessrestrictive,butwhenyouwanttodofull-onobject-orientedprogrammingyoumustthenbuildyourownhierarchytoprovidethesameconveniencethat’sbuiltintootherOOPlanguages.Andinanynewclasslibrary