毕业论文外文翻译Effectjava.docx

毕业论文外文翻译Effectjava.docx

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毕业论文外文翻译Effectjava

外文资料

《Effectjava》

1.AlwaysOverrideHashCodeWhenYouOverrideEquals

AcommonsourceofbugsisthefailuretooverridethehashCodemethod.YoumustoverridehashCodeineveryclassthatoverridesequals.FailuretodosowillresultinaviolationofthegeneralcontractforObject.hashCode,whichwillpreventyourclassfromfunctioningproperlyinconjunctionwithallhash-basedcollections,includingHashMap,HashSet,andHashtable.

Hereisthecontract,copiedfromtheObjectspecification[JavaSE6]:

（1）Wheneveritisinvokedonthesameobjectmorethanonceduringanexecutionofanapplication,thehashCodemethodmustconsistentlyreturnthesameinteger,providednoinformationusedinequalscomparisonsontheobjectismodified.Thisintegerneednotremainconsistentfromoneexecutionofanapplicationtoanotherexecutionofthesameapplication.

（2）Iftwoobjectsareequalaccordingtotheequals（Object）method,thencallingthehashCodemethodoneachofthetwoobjectsmustproducethesameintegerresult.

（3）Itisnotrequiredthatiftwoobjectsareunequalaccordingtotheequals（Object）method,thencallingthehashCodemethodoneachofthetwoobjectsmustproducedistinctintegerresults.However,theprogrammershouldbeawarethatproducingdistinctintegerresultsforunequalobjectsmayimprovetheperformanceofhashtables.

ThekeyprovisionthatisviolatedwhenyoufailtooverridehashCodeisthesecondone:

equalobjectsmusthaveequalhashcodes.Twodistinctinstancesmaybelogicallyequalaccordingtoaclass’sequalsmethod,buttoObject’shashCodemethod,they’rejusttwoobjectswithnothingmuchincommon.ThereforeObject’shashCodemethodreturnstwo

seeminglyrandomnumbersinsteadoftwoequalnumbersasrequiredbythecontract.

2.AlwaysOverridetoString

Whilejava.lang.ObjectprovidesanimplementationofthetoStringmethod,thestringthatitreturnsisgenerallynotwhattheuserofyourclasswantstosee.Itconsistsoftheclassnamefollowedbyan“at”sign（@）andtheunsignedhexadecimalrepresentationofthehashcode.ThegeneralcontractfortoStringsaysthatthereturnedstringshouldbe“aconcisebutinformativerepresentationthatiseasyforapersontoread”[JavaSE6].Whileitcouldbearguedthat“PhoneNumber@163b91”isconciseandeasytoread,itisn’tveryinformativewhencomparedto“（707）867-5309.”ThetoStringcontractgoesontosay,“Itisrecommendedthatallsubclassesoverridethismethod.”Goodadvice,indeed!

Whileitisn’tasimportantasobeyingtheequalsandhashCodecontracts,providingagoodtoStringimplementationmakesyourclassmuchmorepleasanttouse.ThetoStringmethodisautomaticallyinvokedwhenanobjectispassedtoprintln,printf,thestringconcatenationoperator,orassert,orprintedbyadebugger.

Ifyou’veprovidedagoodtoStringmethodforPhoneNumber,generatinga

usefuldiagnosticmessageisaseasyasthis:

System.out.println（"Failedtoconnect:

"+phoneNumber）;

ProgrammerswillgeneratediagnosticmessagesinthisfashionwhetherornotyouoverridetoString,butthemessageswon’tbeusefulunlessyoudo.ThebenefitsofprovidingagoodtoStringmethodextendbeyondinstancesoftheclasstoobjectscontainingreferencestotheseinstances,especiallycollections.Whichwouldyouratherseewhenprintingamap,“Jenny=PhoneNumber@163b91”or“Jenny=（707）867-5309”?

Whenpractical,thetoStringmethodshouldreturnalloftheinterestinginformationcontainedintheobject,asinthephonenumberexamplejustshown.Itisimpracticalifthe

objectislargeorifitcontainsstatethatisnotconducivetostringrepresentation.Underthesecircumstances,toStringshouldreturnasummarysuchas“Manhattanwhitepages（1487536listings）”or“Thread[main,5,main]”.Ideally,thestringshouldbeself-explanatory.（TheThreadexampleflunksthistest.）

Oneimportantdecisionyou’llhavetomakewhenimplementingatoStringmethodiswhethertospecifytheformatofthereturnvalueinthedocumentation.Itisrecommendedthatyoudothisforvalueclasses,suchasphonenumbersormatrices.Theadvantageofspecifyingtheformatisthatitservesasastandard,unambiguous,human-readablerepresentationoftheobject.Thisrepresentationcanbeusedforinputandoutputandinpersistenthuman-readabledataobjects,suchasXMLdocuments.Ifyouspecifytheformat,it’susuallyagoodideatoprovideamatchingstaticfactoryorconstructorsoprogrammerscaneasilytranslatebackandforthbetweentheobjectanditsstringrepresentation.ThisapproachistakenbymanyvalueclassesintheJavaplatformlibraries,includingBigInteger,BigDecimal,andmostoftheboxedprimitiveclasses.

ThedisadvantageofspecifyingtheformatofthetoStringreturnvalueisthatonceyou’vespecifiedit,you’restuckwithitforlife,assumingyourclassiswidelyused.Programmerswillwritecodetoparsetherepresentation,togenerateit,andtoembeditintopersistentdata.Ifyouchangetherepresentationinafuturerelease,you’llbreaktheircodeanddata,andtheywillyowl.Byfailingtospecifyaformat,youpreservetheflexibilitytoaddinformationorimprovetheformatinasubsequentrelease.

Whetherornotyoudecidetospecifytheformat,youshouldclearlydocumentyourintentions.Ifyouspecifytheformat,youshoulddosoprecisely.Whetherornotyouspecifytheformat,provideprogrammaticaccesstoalloftheinformationcontainedinthevaluereturnedbytoString.Forexample,thePhoneNumberclassshouldcontainaccessorsfortheareacode,prefix,andlinenumber.Ifyoufailtodothis,youforceprogrammerswhoneedthisinformationtoparsethestring.Besidesreducingperformanceandmakingunnecessaryworkforprogrammers,thisprocessiserror-proneandresultsinfragilesystemsthatbreakif

youchangetheformat.Byfailingtoprovideaccessors,youturnthestringformatintoadefactoAPI,evenifyou’vespecifiedthatit’ssubjecttochange.

3.MinimizetheAccessibilityofClassesandMembers

Thesinglemostimportantfactorthatdistinguishesawell-designedmodulefroma

poorlydesignedoneisthedegreetowhichthemodulehidesitsinternaldataandotherimplementationdetailsfromothermodules.Awell-designedmodulehidesallofitsimplementationdetails,cleanlyseparatingitsAPIfromitsimplementation.ModulesthencommunicateonlythroughtheirAPIsandareoblivioustoeachothers’innerworkings.Thisconcept,knownasinformationhidingorencapsulation,isoneofthefundamentaltenetsofsoftwaredesign[Parnas72].

Informationhidingisimportantformanyreasons,mostofwhichstemfromthefactthatitdecouplesthemodulesthatcompriseasystem,allowingthemtobedeveloped,tested,optimized,used,understood,andmodifiedinisolation.Thisspeedsupsystemdevelopmentbecausemodulescanbedevelopedinparallel.Iteasestheburdenofmaintenancebecausemodulescanbeunderstoodmorequicklyanddebuggedwithlittlefearofharmingothermodules.Whileinformationhidingdoesnot,inandofitself,causegoodperformance,itenableseffectiveperformancetuning:

onceasystemiscompleteandprofilinghasdeterminedwhichmodulesarecausingperformanceproblems（Item55）,thosemodulescanbeoptimizedwithoutaffectingthecorrectnessofothermodules.Informationhidingincreasessoftwarereusebecausemodulesthataren’ttightlycoupledoftenproveusefulinothercontextsbesidestheonesforwhichtheyweredeveloped.

Finally,informationhidingdecreasestheriskinbuildinglargesystems,because

individualmodulesmayprovesuccessfulevenifthesystemdoesnot.Javahasmanyfacilitiestoaidininformationhiding.Theaccesscontrolmechanism[JLS,6.6]specifiestheaccessibilityofclasses,interfaces,andmembers.Theaccessibilityofanentityisdeterminedbythelocationofitsdeclarationandbywhich,ifany,oftheaccessmodifiers（private,protected,andpublic）ispresentonthedeclaration.Properuseofthesemodifiersisessentialtoinformationhiding.

Theruleofthumbissimple:

makeeachclassormemberasinaccessibleaspossible.Inotherwords,usethelowestpossibleaccesslevelconsistentwiththeproperfunctioningofthesoftwarethatyouarewriting.Fortop-level（non-nested）classesandinterfaces,thereareonlytwopossibleaccesslevels:

package-privateandpublic.Ifyoudeclareatop-levelclassorinterfacewiththepublicmodifier,itwillbepublic;otherwise,itwillbepackage-private.Ifatop-levelclassorinterfacecanbemadepackage-private,itshouldbe.Bymakingitpackage-private,youmakeitpartoftheimplementationratherthantheexportedAPI,andyoucanmodifyit,replaceit,oreliminateitinasubsequentreleasewithoutfearofharmingexistingclients.Ifyoumakeitpublic,youareobligatedtosupportitforevertomaintaincompatibility.

Ifapackage-privatetop-levelclass（orinterface）isusedbyonlyoneclass,considermakingthetop-levelclassaprivatenestedclassofthesoleclassthatusesit（Item22）.Thisreducesitsaccessibilityfromalltheclassesinitspackagetotheoneclassthatusesit.Butitisfarmoreimportanttoreducetheaccessibilityofagratuitouslypublicclassthanofapackage-privatetop-levelclass:

thepublicclassispartofthepackage’sAPI,whilethepackage-privatetop-levelclassisalreadypartofitsimplementation.

Aftercarefullydesigningyourclass’spublicAPI,yourreflexshouldbetomakeallothermembersprivate.Onlyifanotherclassinthesamepackagereallyneedstoaccessamembershouldyouremovetheprivatemodifier,makingthememberpackage-private.Ifyoufindyourselfdoingthisoften,youshouldreexaminethedesignofyo