超市进销存管理系统-外文翻译

0外文原文ThebusyJavadevelopersguidetoScala:ClassactionItmakessenseforJavadeveloperstouseobjectsasafirstpointofreferenceforunderstandingScala.InthissecondinstallmentofThebusyJavadevelopersguidetoScalaseries,TedNewardfollowsabasicpremiseoflanguagemeasurement:thatthepowerofalanguagecanbemeasuredindirectrelationtoitsabilitytointegratenewfacilities-inthiscase,supportforcomplexnumbers.AlongthewayyoullseesomeinterestingtidbitsrelatedtoclassdefinitionsandusageinScala.Inlastmonthsarticle,yousawjustatouchofScalassyntax,thebareminimumnecessarytorunaScalaprogramandobservesomeofitssimplerfeatures.TheHelloWorldandTimerexamplesfromthatarticleletyouseeScalasApplicationclass,itssyntaxformethoddefinitionsandanonymousfunctions,justaglimpseofanArray,andabitontype-inferencing.Scalahasagreatdealmoretooffer,sothisarticleinvestigatestheintricaciesofScalacoding.Scalasfunctionalprogrammingfeaturesarecompelling,buttheyrenottheonlyreasonJavadevelopersshouldbeinterestedinthelanguage.Infact,Scalablendsfunctionalconceptsandobjectorientation.InordertolettheJava-cum-Scalaprogrammerfeelmoreathome,itmakessensetolookatScalasobjectfeaturesandseehowtheymapovertoJavalinguistically.Bearinmindthatthereisntadirectmappingforsomeofthesefeatures,orinsomecases,themappingismoreofananalogthanadirectparallel.Butwherethedistinctionisimportant,Illpointitout.Scalahasclass(es),tooRatherthanembarkonalengthyandabstractdiscussionoftheclassfeaturesthatScalasupports,letslookatadefinitionforaclassthatmightbeusedtobringrationalnumbersupporttotheScalaplatform(largelyswipedfromScalaByExample-seeResources):Listing1.rational.scalaclassRational(n:Int,d:Int)1privatedefgcd(x:Int,y:Int):Int=if(x=0)yelseif(xjavap-private-classpathclassesRationalCompiledfromrational.scalapublicclassRationalextendsjava.lang.Objectimplementsscala.ScalaObjectprivateintdenom;privateintnumer;privateintg;publicRational(int,int);publicRationalunary_$tilde();publicjava.lang.StringtoString();publicRational$div(Rational);publicRational$times(Rational);publicRational$minus(Rational);publicRational$plus(Rational);publicintdenom();publicintnumer();privateintg();privateintgcd(int,int);8publicRational(int);publicint$tag();C:Projectsscala-classescodeTheoperatorsdefinedintheScalaclasstransmogrifyintomethodcallsinthebesttraditionofJavaprogramming,thoughtheydoseemtobebasedonfunnynames.Twoconstructorsaredefinedontheclass:onetakinganintandonetakingapairofints.And,ifyouhappentobeatallconcernedthattheuseoftheupper-caseInttypeissomehowajava.lang.Integerindisguise,notethattheScalacompilerissmartenoughtotransformthemintoregularJavaprimitiveintsintheclassdefinition.Testing,testing,1-2-3.Itisawell-knownmemethatgoodprogrammerswritecode,andgreatprogrammerswritetests;thusfar,IhavebeenlaxinexercisingthisruleformyScalacode,soletsseewhathappenswhenyouputthisRationalclassinsideofatraditionalJUnittestsuite,asshowninListing10:Listing10.RationalTest.javaimportorg.junit.*;importstaticorg.junit.Assert.*;publicclassRationalTestTestpublicvoidtest2ArgRationalConstructor()Rationalr=newRational(2,5);assertTrue(r.numer()=2);assertTrue(r.denom()=5);Testpublicvoidtest1ArgRationalConstructor()9Rationalr=newRational(5);assertTrue(r.numer()=0);assertTrue(r.denom()=1);/1becauseofgcd()invocationduringconstruction;/0-over-5isthesameas0-over-1TestpublicvoidtestAddRationals()Rationalr1=newRational(2,5);Rationalr2=newRational(1,3);Rationalr3=(Rational)reflectInvoke(r1,$plus,r2);/r1.$plus(r2);assertTrue(r3.numer()=11);assertTrue(r3.denom()=15);/.somedetailsomittedAsidefromconfirmingthattheRationalclassbehaves,well,rationally,theabovetestsuitealsoprovesthatitispossibletocallScalacodefromJavacode(albeitwithalittlebitofanimpedancemismatchwhenitcomestotheoperators).Thecoolthingaboutthis,ofcourse,isthatitletsyoutryoutScalaslowly,bymigratingJavaclassesovertoScalaclasseswithouteverhavingtochangetheteststhatbackthem.Theonlyweirdnessyoumightnoticeinthetestcodehastodowithoperatorinvocation,inthiscase,the+methodontheRationalclass.Lookingbackatthejavapoutput,Scalahasobviouslytranslatedthe+functionintotheJVMmethod$plus,buttheJavaLanguageSpecificationdoesnotallowthe$characterinidentifiers(whichiswhyitsusedinnestedandanonymousnestedclassnames).10Inordertoinvokethosemethods,youeitherhavetowritethetestsinGroovyorJRuby(orsomeotherlanguagethatdoesntposearestrictiononthe$character),oryoucanwritealittlebitofReflectioncodetoinvokeit.Igowiththelatterapproach,whichisntallthatinterestingfromaScalaperspective,buttheresultisincludedinthisarticlescodebundle,shouldyoubecurious.(SeeDownload.)NotethatworkaroundsliketheseareonlynecessaryforfunctionnamesthatarentalsolegitimateJavaidentifiers.AbetterJavaBackwhenIwasfirstlearningC+,BjarneStroustrupsuggestedthatonewaytolearnC+wastoseeitasabetterC(seeResources).Insomeways,JavadeveloperstodaymightcometoseeScalaasabetterJava,becauseitprovidesamoreterseandsuccinctwayofwritingtraditionalJavaPOJOs.ConsiderthetraditionalPersonPOJOshowninListing11:Listing11.JavaPerson.java(originalPOJO)publicclassJavaPersonpublicJavaPerson(StringfirstName,StringlastName,intage)this.firstName=firstName;this.lastName=lastName;this.age=age;publicStringgetFirstName()returnthis.firstName;publicvoidsetFirstName(Stringvalue)11this.firstName=value;publicStringgetLastName()returnthis.lastName;publicvoidsetLastName(Stringvalue)this.lastName=value;publicintgetAge()returnthis.age;publicvoidsetAge(intvalue)this.age=value;publicStringtoString()returnPerson:firstName+firstName+lastName:+lastName+age:+age+;privateStringfirstName;privateStringlastName;privateintage;12NowconsideritsequivalentwritteninScala:Listing12.person.scala(threadsafePOJO)classPerson(firstName:String,lastName:String,age:Int)defgetFirstName=firstNamedefgetLastName=lastNamedefgetAge=ageoverridedeftoString=PersonfirstName:+firstName+lastName:+lastName+age:+age+Itisntacompletedrop-inreplacement,giventhattheoriginalPersonhadsomemutablesetters.ButconsideringtheoriginalPersonalsohadnosynchronizationcodearoundthosemutablesetters,theScalaversionissafertouse.Also,ifthegoalistotrulyreducethenumberoflinesofcodeinPerson,youcouldremovethegetFoopropertymethodsentirelybecauseScalawillgenerateaccessormethodsaroundeachoftheconstructorparameters-firstName()returnsaString,lastName()returnsaString,andage()returnsanint).Eveniftheneedforthosemutablesettermethodsisundeniable,theScalaversionisstillsimpler,asyoucanseeinListing13:Listing13.person.scala(fullPOJO)classPerson(varfirstName:String,varlastName:String,varage:Int)defgetFirstName=firstNamedefgetLastName=lastNamedefgetAge=agedefsetFirstName(value:String):Unit=firstName=valuedefsetLastName(value:String)=lastName=value13defsetAge(value:Int)=age=valueoverridedeftoString=PersonfirstName:+firstName+lastName:+lastName+age:+age+Asanaside,noticetheintroductionofthevarkeywordontheconstructorparameters.Withoutgoingintotoomuchdetail,vartellsthecompilerthatthevalueismutable.Asaresult,Scalageneratesbothaccessor(StringfirstName(void)andmutator(voidfirstName_$eq(String)methods.ItthenbecomeseasytocreatesetFoopropertymutatormethodsthatusethegeneratedmutatormethodsunderthehood.ConclusionScalaisanattempttoincorporatefunctionalconceptsandtersenesswithoutlosingtherichnessoftheobjectparadigm.Asyouveperhapsbeguntoseeinthisseries,Scalaalsocorrectssomeoftheegregious(inhindsight)syntacticproblemsfoundintheJavalanguage.ThissecondarticleintheBusyJavadevelopersguidetoScalaserieshasfocusedonScalasobjectfacilities,whichletyoustartusingScalawithouthavingtodivetoodeeplyintothefunctionalpool.Basedonwhatyouvelearnedsofar,youcanalreadystartusingScalatoreduceyourprogrammingworkload.Amongotherthings,youcanuseScalatoproducetheverysamePOJOsneededforotherprogrammingenvironments,suchasSpringorHibernate.Holdontoyourdivingcapsandscubagear,however,becausenextmonthsarticlewillmarkthebeginningofourdescentintothedeependofthefunctionalpool.14外文翻译面向Java开发人员的Scala指南:类操作Java开发人员可以将对象作为理解Scala的出发点。本文是面向Java开发人员的Scala指南系列的第二期，作者TedNeward遵循对一种语言进行评价的基本前提：一种语言的威力可以直接通过它集成新功能的能力衡量，在本文中就是指对复数的支持。跟随本文，您将了解在Scala中与类的定义和使用有关的一些有趣特性。在上一期文章中，您只是稍微了解了一些Scala语法，这些是运行Scala程序和了解其简单特性的最基本要求。通过上一篇文章中的HelloWorld和Timer示例程序，您了解了Scala的Application类、方法定义和匿名函数的语法，还稍微了解Array和一些类型推断方面的知识。Scala还提供了很多其他特性，本文将研究Scala编程中的一些较复杂方面。Scala的函数编程特性非常引人注目，但这并非Java开发人员应该对这门语言感兴趣的惟一原因。实际上，Scala融合了函数概念和面向对象概念。为了让Java和Scala程序员感到得心应手，可以了解一下Scala的对象特性，看看它们是如何在语言方面与Java对应的。记住，其中的一些特性并不是直接对应，或者说，在某些情况下，“对应”更像是一种类比，而不是直接的对应。不过，遇到重要区别时，我会指出来。Scala和Java一样使用类我们不对Scala支持的类特性作冗长而抽象的讨论，而是着眼于一个类的定义，这个类可用于为Scala平台引入对有理数的支持（主要借鉴自“ScalaByExample”，参见参考资料）：清单1.Rational.scalaclassRational(n:Int,d:Int)privatedefgcd(x:Int,y:Int):Int=if(x=0)yelseif(xjavap-private-classpathclassesRational19Compiledfromrational.scalapublicclassRationalextendsjava.lang.Objectimplementsscala.ScalaObjectprivateintdenom;privateintnumer;privateintg;publicRational(int,int);publicRationalunary_$tilde();publicjava.lang.StringtoString();publicRational$div(Rational);publicRational$times(Rational);publicRational$minus(Rational);publicRational$plus(Rational);publicintdenom();publicintnumer();privateintg();privateintgcd(int,int);publicRational(int);publicint$tag();C:Projectsscala-classescodeScala类中定义的“操作符”被转换成传统Java编程中的方法调用，不过它们仍使用看上去有些古怪的名称。类中定义了两个构造函数：一个构造函数带有一个int参数，另一个带有两个int参数。您可能会注意到，大写的Int类型与java.lang.Integer有点相似，Scala编译器非常聪明，会在类定义中将它们转换成常规的Java原语int。测试Rational类一种著名的观点认为，优秀的程序员编写代码，伟大的程序员编写测试；到目前为止，我还没有对我的Scala代码严格地实践这一规则，那么现在看看将这个Rational类放入一个传统的JUnit测试套件中会怎样，如清单10所示：清单10.RationalTest.javaimportorg.junit.*;20importstaticorg.junit.Assert.*;publicclassRationalTestTestpublicvoidtest2ArgRationalConstructor()Rationalr=newRational(2,5);assertTrue(r.numer()=2);assertTrue(r.denom()=5);Testpublicvoidtest1ArgRationalConstructor()Rationalr=newRational(5);assertTrue(r.numer()=0);assertTrue(r.denom()=1);/1becauseofgcd()invocationduringconstruction;/0-over-5isthesameas0-over-1TestpublicvoidtestAddRationals()Rationalr1=newRational(2,5);Rationalr2=newRational(1,3);Rationalr3=(Rational)reflectInvoke(r1,$plus,r2);/r1.$plus(r2);assertTrue(r3.numer()=11);assertTrue(r3.denom()=15);/.somedetailsomitted21除了确认Rational类运行正常之外，上面的测试套件还证明可以从Java代码中调用Scala代码（尽管在操作符方面有点不匹配）。当然，令人高兴的是，您可以将Java类迁移至Scala类，同时不必更改支持这些类的测试，然后慢慢尝试Scala。您惟一可能觉得古怪的地方是操作符调用，在本例中就是Rational类中的+方法。回顾一下javap的输出，Scala显然已经将+函数转换为JVM方法$plus，但是Java语言规范并不允许标识符中出现$字符（这正是它被用于嵌套和匿名嵌套类名称中的原因）。为了调用那些方法，需要用Groovy或JRuby（或者其他对$字符没有限制的语言）编写测试，或者编写Reflection代码来调用它。我采用后一种方法，从Scala的角度看这不是那么有趣，但是如果您有兴趣的话，可以看看本文的代码中包含的结果（参见下载）。注意，只有当函数名称不是合法的Java标识符时才需要用这类方法。“更好的”Java我学习C+的时候BjarneStroustrup建议，学习C+的一种方法是将它看作“更好的C语言”（参见参考资料）。在某些方面，如今的Java开发人员也可以将Scala看作是“更好的Java”，因为它提供了一种编写传统JavaPOJO的更简洁的方式。考虑清单11中显示的传统PersonPOJO：清单11.JavaPerson.java（原始POJO）publicclassJavaPersonpublicJavaPerson(StringfirstName,StringlastName,intage)this.firstName=firstName;this.lastName=lastName;this.age=age;publicStringgetFirstName()returnthis.firstName;publicvoidsetFirstName(Stringvalue)this.firstName=value;22publicStringgetLastName()returnthis.lastName;publicvoidsetLastName(Stringvalue)this.lastName=value;publicintgetAge()returnthis.age;publicvoidsetAge(intvalue)this.age=value;publicStringtoString()returnPerson:firstName+firstName+lastName:+lastName+age:+age+;privateStringfirstName;privateStringlastName;privateintage;现在考虑用Scala编写的对等物：清单12.person.scala（