畢業(yè)設(shè)計外文文獻-對象入門

附錄A外文翻譯—原文部分Excerptfrom《ThinkinginJava(FouthEdition)》:IntroductiontoObjects“Wecutnatureup,organizeitintoconcepts,andascribesignificancesaswedo,largelybecausewearepartiestoanagreementthatholdsthroughoutourspeechcommunityandiscodifiedinthepatternsofourlanguage…wecannottalkatallexceptbysubscribingtotheorganizationandclassificationofdatawhichtheagreementdecrees.”BenjaminLeeWhorf(1897-1941)Thegenesisofthecomputerrevolutionwasinamachine.Thegenesisofourprogramminglanguagesthustendstolooklikethatmachine.Butcomputersarenotsomuchmachinesastheyaremindamplificationtools(“bicyclesforthemind,”asSteveJobsisfondofsaying)andadifferentkindofexpressivemedium.Asaresult,thetoolsarebeginningtolooklesslikemachinesandmorelikepartsofourminds,andalsolikeotherformsofexpressionsuchaswriting,painting,sculpture,animation,andfilmmaking.Object-orientedprogramming(OOP)ispartofthismovementtowardusingthecomputerasanexpressivemedium.ThischapterwillintroduceyoutothebasicconceptsofOOP,includinganoverviewofdevelopmentmethods.Thischapter,andthisbook,assumesthatyouhavesomeprogrammingexperience,althoughnotnecessarilyinC.Ifyouthinkyouneedmorepreparationinprogrammingbeforetacklingthisbook,youshouldworkthroughtheThinkinginCmultimediaseminar,downloadablefromwww.MindV.Thischapterisbackgroundandsupplementarymaterial.Manypeopledonotfeelcomfortablewadingintoobject-orientedprogrammingwithoutunderstandingthebigpicturefirst.Thus,therearemanyconceptsthatareintroducedheretogiveyouasolidoverviewofOOP.However,otherpeoplemaynotgetthebigpictureconceptsuntilthey’veseensomeofthemechanicsfirst;thesepeoplemaybecomeboggeddownandlostwithoutsomecodetogettheirhandson.Ifyou’repartofthislattergroupandareeagertogettothespecificsofthelanguage,feelfreetojumppastthischapter—skippingitatthispointwillnotpreventyoufromwritingprogramsorlearningthelanguage.However,youwillwanttocomebackhereeventuallytofillinyourknowledgesoyoucanunderstandwhyobjectsareimportantandhowtodesignwiththem.TheprogressofabstractionAllprogramminglanguagesprovideabstractions.Itcanbearguedthatthecomplexityoftheproblemsyou’reabletosolveisdirectlyrelatedtothekindandqualityofabstraction.By“kind”Imean,“Whatisitthatyouareabstracting?”Assemblylanguageisasmallabstractionoftheunderlyingmachine.Manyso-called“imperative”languagesthatfollowed(suchasFORTRAN,BASIC,andC)wereabstractionsofassemblylanguage.Theselanguagesarebigimprovementsoverassemblylanguage,buttheirprimaryabstractionstillrequiresyoutothinkintermsofthestructureofthecomputerratherthanthestructureoftheproblemyouaretryingtosolve.Theprogrammermustestablishtheassociationbetweenthemachinemodel(inthe“solutionspace,”whichistheplacewhereyou’reimplementingthatsolution,suchasacomputer)andthemodeloftheproblemthatisactuallybeingsolved(inthe“problemspace,”whichistheplacewheretheproblemexists,suchasabusiness).Theeffortrequiredtoperformthismapping,andthefactthatitisextrinsictotheprogramminglanguage,producesprogramsthataredifficulttowriteandexpensivetomaintain,andasasideeffectcreatedtheentire“programmingmethods”industry.Thealternativetomodelingthemachineistomodeltheproblemyou’retryingtosolve.EarlylanguagessuchasLISPandAPLchoseparticularviewsoftheworld(“Allproblemsareultimatelylists”or“Allproblemsarealgorithmic,”respectively).Prologcastsallproblemsintochainsofdecisions.Languageshavebeencreatedforconstraint-basedprogrammingandforprogrammingexclusivelybymanipulatinggraphicalsymbols.(Thelatterprovedtobetoorestrictive.)Eachoftheseapproachesmaybeagoodsolutiontotheparticularclassofproblemthey’redesignedtosolve,butwhenyoustepoutsideofthatdomaintheybecomeawkward.Theobject-orientedapproachgoesastepfurtherbyprovidingtoolsfortheprogrammertorepresentelementsintheproblemspace.Thisrepresentationisgeneralenoughthattheprogrammerisnotconstrainedtoanyparticulartypeofproblem.Werefertotheelementsintheproblemspaceandtheirrepresentationsinthesolutionspaceas“objects.”(Youwillalsoneedotherobjectsthatdon’thaveproblem-spaceanalogs.)Theideaisthattheprogramisallowedtoadaptitselftothelingooftheproblembyaddingnewtypesofobjects,sowhenyoureadthecodedescribingthesolution,you’rereadingwordsthatalsoexpresstheproblem.Thisisamoreflexibleandpowerfullanguageabstractionthanwhatwe’vehadbefore.1Thus,OOPallowsyoutodescribetheproblemintermsoftheproblem,ratherthanintermsofthecomputerwherethesolutionwillrun.There’sstillaconnectionbacktothecomputer:Eachobjectlooksquiteabitlikealittlecomputer—ithasastate,andithasoperationsthatyoucanaskittoperform.However,thisdoesn’tseemlikesuchabadanalogytoobjectsintherealworld—theyallhavecharacteristicsandbehaviors.AlanKaysummarizedfivebasiccharacteristicsofSmalltalk,thefirstsuccessfulobject-orientedlanguageandoneofthelanguagesuponwhichJavaisbased.Thesecharacteristicsrepresentapureapproachtoobject-orientedprogramming:1.Everythingisanobject.Thinkofanobjectasafancyvariable;itstoresdata,butyoucan“makerequests”tothatobject,askingittoperformoperationsonitself.Intheory,youcantakeanyconceptualcomponentintheproblemyou’retryingtosolve(dogs,buildings,services,etc.)andrepresentitasanobjectinyourprogram.2.Aprogramisabunchofobjectstellingeachotherwhattodobysendingmessages.Tomakearequestofanobject,you“sendamessage”tothatobject.Moreconcretely,youcanthinkofamessageasarequesttocallamethodthatbelongstoaparticularobject.3.Eachobjecthasitsownmemorymadeupofotherobjects.Putanotherway,youcreateanewkindofobjectbymakingapackagecontainingexistingobjects.Thus,youcanbuildcomplexityintoaprogramwhilehidingitbehindthesimplicityofobjects.4.Everyobjecthasatype.Usingtheparlance,eachobjectisaninstanceofaclass,inwhich“class”issynonymouswith“type.”Themostimportantdistinguishingcharacteristicofaclassis“Whatmessagescanyousendtoit?”5.Allobjectsofaparticulartypecanreceivethesamemessages.Thisisactuallyaloadedstatement,asyouwillseelater.Becauseanobjectoftype“circle”isalsoanobjectoftype“shape,”acircleisguaranteedtoacceptshapemessages.Thismeansyoucanwritecodethattalkstoshapesandautomaticallyhandleanythingthatfitsthedescriptionofashape.ThissubstitutabilityisoneofthepowerfulconceptsinOOP.Boochoffersanevenmoresuccinctdescriptionofanobject:Anobjecthasstate,behaviorandidentity.Thismeansthatanobjectcanhaveinternaldata(whichgivesitstate),methods(toproducebehavior),andeachobjectcanbeuniquelydistinguishedfromeveryotherobject—toputthisinaconcretesense,eachobjecthasauniqueaddressinmemory.AnobjecthasaninterfaceAristotlewasprobablythefirsttobeginacarefulstudyoftheconceptoftype;hespokeof“theclassoffishesandtheclassofbirds.”Theideathatallobjects,whilebeingunique,arealsopartofaclassofobjectsthathavecharacteristicsandbehaviorsincommonwasuseddirectlyinthefirstobject-orientedlanguage,Simula-67,withitsfundamentalkeywordclassthatintroducesanewtypeintoaprogram.Simula,asitsnameimplies,wascreatedfordevelopingsimulationssuchastheclassic“banktellerproblem.”Inthis,youhavenumeroustellers,customers,accounts,transactions,andunitsofmoney—alotof“objects.”O(jiān)bjectsthatareidenticalexceptfortheirstateduringaprogram’sexecutionaregroupedtogetherinto“classesofobjects,”andthat’swherethekeywordclasscamefrom.Creatingabstractdatatypes(classes)isafundamentalconceptinobject-orientedprogramming.Abstractdatatypesworkalmostexactlylikebuilt-intypes:Youcancreatevariablesofatype(calledobjectsorinstancesinobject-orientedparlance)andmanipulatethosevariables(calledsendingmessagesorrequests;yousendamessageandtheobjectfiguresoutwhattodowithit).Themembers(elements)ofeachclasssharesomecommonality:Everyaccounthasabalance,everytellercanacceptadeposit,etc.Atthesametime,eachmemberhasitsownstate:Eachaccounthasadifferentbalance,eachtellerhasaname.Thus,thetellers,customers,accounts,transactions,etc.,caneachberepresentedwithauniqueentityinthecomputerprogram.Thisentityistheobject,andeachobjectbelongstoaparticularclassthatdefinesitscharacteristicsandbehaviors.So,althoughwhatwereallydoinobject-orientedprogrammingiscreatenewdatatypes,virtuallyallobject-orientedprogramminglanguagesusethe“class”keyword.Whenyouseetheword“type”think“class”andviceversa.3Sinceaclassdescribesasetofobjectsthathaveidenticalcharacteristics(dataelements)andbehaviors(functionality),aclassisreallyadatatypebecauseafloatingpointnumber,forexample,alsohasasetofcharacteristicsandbehaviors.Thedifferenceisthataprogrammerdefinesaclasstofitaproblemratherthanbeingforcedtouseanexistingdatatypethatwasdesignedtorepresentaunitofstorageinamachine.Youextendtheprogramminglanguagebyaddingnewdatatypesspecifictoyourneeds.Theprogrammingsystemwelcomesthenewclassesandgivesthemallthecareandtypecheckingthatitgivestobuilt-intypes.Theobject-orientedapproachisnotlimitedtobuildingsimulations.Whetherornotyouagreethatanyprogramisasimulationofthesystemyou’redesigning,theuseofOOPtechniquescaneasilyreducealargesetofproblemstoasimplesolution.Onceaclassisestablished,youcanmakeasmanyobjectsofthatclassasyoulike,andthenmanipulatethoseobjectsasiftheyaretheelementsthatexistintheproblemyouaretryingtosolve.Indeed,oneofthechallengesofobject-orientedprogrammingistocreateaone-to-onemappingbetweentheelementsintheproblemspaceandobjectsinthesolutionspace.Buthowdoyougetanobjecttodousefulworkforyou?Thereneedstobeawaytomakearequestoftheobjectsothatitwilldosomething,suchascompleteatransaction,drawsomethingonthescreen,orturnonaswitch.Andeachobjectcansatisfyonlycertainrequests.Therequestsyoucanmakeofanobjectaredefinedbyitsinterface,andthetypeiswhatdeterminestheinterface.Asimpleexamplemightbearepresentationofalightbulb:Lightlt=newLight();lt.on();Theinterfacedeterminestherequeststhatyoucanmakeforaparticularobject.However,theremustbecodesomewheretosatisfythatrequest.This,alongwiththehiddendata,comprisestheimplementation.Fromaproceduralprogrammingstandpoint,it’snotthatcomplicated.Atypehasamethodassociatedwitheachpossiblerequest,andwhenyoumakeaparticularrequesttoanobject,thatmethodiscalled.Thisprocessisusuallysummarizedbysayingthatyou“sendamessage”(makearequest)toanobject,andtheobjectfiguresoutwhattodowiththatmessage(itexecutescode).Here,thenameofthetype/classisLight,thenameofthisparticularLightobjectislt,andtherequeststhatyoucanmakeofaLightobjectaretoturniton,turnitoff,makeitbrighter,ormakeitdimmer.YoucreateaLightobjectbydefininga“reference”(lt)forthatobjectandcallingnewtorequestanewobjectofthattype.Tosendamessagetotheobject,youstatethenameoftheobjectandconnectittothemessagerequestwithaperiod(dot).Fromthestandpointoftheuserofapredefinedclass,that’sprettymuchallthereistoprogrammingwithobjects.TheprecedingdiagramfollowstheformatoftheUnifiedModelingLanguage(UML).Eachclassisrepresentedbyabox,withthetypenameinthetopportionofthebox,anydatamembersthatyoucaretodescribeinthemiddleportionofthebox,andthemethods(thefunctionsthatbelongtothisobject,whichreceiveanymessagesyousendtothatobject)inthebottomportionofthebox.附錄B外文翻譯—譯文部分摘自《Java編程思想（第四版）》：對象入門面向?qū)ο缶幊蹋∣OP）具有多方面的吸引力。對管理人員，它實現(xiàn)了更快和更廉價的開發(fā)與維護過程。對分析與設(shè)計人員，建模處理變得更加簡單，能生成清晰、易于維護的設(shè)計方案。對程序員，對象模型顯得如此高雅和淺顯。此外，面向?qū)ο蠊ぞ咭约皫斓木薮笸κ咕幊坛蔀橐豁椄谷擞鋹偟娜蝿?wù)。每個人都可從中獲益，至少表面如此。如果說它有缺點，那就是掌握它需付出的代價。思考對象的時候，需要采用形象思維，而不是程序化的思維。與程序化設(shè)計相比，對象的設(shè)計過程更具挑戰(zhàn)性——特別是在嘗試創(chuàng)建可重復(fù)使用（可再生）的對象時。過去，那些初涉面向?qū)ο缶幊填I(lǐng)域的人都必須進行一項令人痛苦的選擇：選擇一種諸如Smalltalk的語言，“出師”前必須掌握一個巨型的庫。選擇幾乎根本沒有庫的C++，然后深入學習這種語言，直至能自行編寫對象庫。事實上，很難很好地設(shè)計出對象——從而很難設(shè)計好任何東西。因此，只有數(shù)量相當少的“專家”能設(shè)計出最好的對象，然后讓其他人享用。對于成功的OOP語言，它們不僅集成了這種語言的語法以及一個編譯程序（編譯器），而且還有一個成功的開發(fā)環(huán)境，其中包含設(shè)計優(yōu)良、易于使用的庫。所以，大多數(shù)程序員的首要任務(wù)就是用現(xiàn)有的對象解決自己的應(yīng)用問題。本章的目標就是向大家揭示出面向?qū)ο缶幊痰母拍睿⒆C明它有多么簡單。本章將向大家解釋Java的多項設(shè)計思想，并從概念上解釋面向?qū)ο蟮某绦蛟O(shè)計。但要注意在閱讀完本章后，并不能立即編寫出全功能的Java程序。所有詳細的說明和示例會在本書的其他章節(jié)慢慢道來。抽象的進步所有編程語言的最終目的都是提供一種“抽象”方法。一種較有爭議的說法是：解決問題的復(fù)雜程度直接取決于抽象的種類及質(zhì)量。這兒的“種類”是指準備對什么進行“抽象”？匯編語言是對基礎(chǔ)機器的少量抽象。后來的許多“命令式”語言（如FORTRAN，BASIC和C）是對匯編語言的一種抽象。與匯編語言相比，這些語言已有了長足的進步，但它們的抽象原理依然要求我們著重考慮計算機的結(jié)構(gòu)，而非考慮問題本身的結(jié)構(gòu)。在機器模型（位于“方案空間”）與實際解決的問題模型（位于“問題空間”）之間，程序員必須建立起一種聯(lián)系。這個過程要求人們付出較大的精力，而且由于它脫離了編程語言本身的范圍，造成程序代碼很難編寫，而且要花較大的代價進行維護。由此造成的副作用便是一門完善的“編程方法”學科。為機器建模的另一個方法是為要解決的問題制作模型。對一些早期語言來說，如LISP和APL，它們的做法是“從不同的角度觀察世界”——“所有問題都歸納為列表”或“所有問題都歸納為算法”。PROLOG則將所有問題都歸納為決策鏈。對于這些語言，我們認為它們一部分是面向基于“強制”的編程，另一部分則是專為處理圖形符號設(shè)計的。每種方法都有自己特殊的用途，適合解決某一類的問題。但只要超出了它們力所能及的范圍，就會顯得非常笨拙。面向?qū)ο蟮某绦蛟O(shè)計在此基礎(chǔ)上則跨出了一大步，程序員可利用一些工具表達問題空間內(nèi)的元素。由于這種表達非常普遍，所以不必受限于特定類型的問題。我們將問題空間中的元素以及它們在方案空間的表示物稱作“對象”（Object）。當然，還有一些在問題空間沒有對應(yīng)體的其他對象。通過添加新的對象類型，程序可進行靈活的調(diào)整，以便與特定的問題配合。所以在閱讀方案的描述代碼時，會讀到對問題進行表達的話語。與我們以前見過的相比，這無疑是一種更加靈活、更加強大的語言抽象方法?？傊琌OP允許我們根據(jù)問題來描述問題，而不是根據(jù)方案。然而，仍有一個聯(lián)系途徑回到計算機。每個對象都類似一臺小計算機；它們有自己的狀態(tài)，而且可要求它們進行特定的操作。與現(xiàn)實世界的“對象”或者“物體”相比，編程“對象”與它們也存在共通的地方：它們都有自己的特征和行為。AlanKay總結(jié)了Smalltalk的五大基本特征。這是第一種成功的面向?qū)ο蟪绦蛟O(shè)計語言，也是Java的基礎(chǔ)語言。通過這些特征，我們可理解“純粹”的面向?qū)ο蟪绦蛟O(shè)計方法是什么樣的：1、所有東西都是對象。可將對象想象成一種新型變量；它保存著數(shù)據(jù)，但可要求它對自身進行操作。理論上講，可從要解決的問題身上提出所有概念性的組件，然后在程序中將其表達為一個對象。2、程序是一大堆對象的組合；通過消息傳遞，各對象知道自己該做些什么。為了向?qū)ο蟀l(fā)出請求，需向那個對象“