计算机专业毕业设计论文外文文献中英文翻译Object.docx

计算机专业毕业设计论文外文文献中英文翻译Object.docx

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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

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.c:

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*****************？

Subject=[TIJ3]Chap01_179

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?

c:

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TIJ3＠MindV?

Subject=[TIJ3％5dChap01_182

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.c:

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TIJ3＠MindView。

net?

Subject=[TIJ3％5dChap01_185

2Thesinglyrootedhierarchy

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

Allobjectsinasinglyrootedhierarchyhaveaninterfaceincommon,sotheyareallultimatelythesametype。

Thealternative（providedbyC++）isthatyoudon’tknowthateverythingisthesamefundamentaltype.Fromabackward-compatibilitystandpointthisfitsthemodelofCbetterandcanbethoughtofaslessrestrictive，butwhenyouwanttodofull—onobject-orientedprogrammingyoumustthenbuildyourownhierarchytoprovidethesameconveniencethat’sbuiltintootherO