TIJ英文原版书籍阅读之旅——Chapter Seven:Reusing Classes

ReusingClasses

有两种常用方式实现类的重用，组件（在新类中创建存在类的对象）和继承。

Compositionsyntax

Everynon-PRimitiveobjecthasatoString()method,andit’scalledinspecialsituationswhenthecompilerwantsaStringbutithasanobject.

Inheritancesyntax

You’realwaysdoinginheritancewhenyoucreateaclass,becauseunlessyouexplicitlyinheritfromsomeotherclass,youimplicitlyinheritfromjava’sstandardrootclassObject.

Toallowforinheritance,asageneralrulemakeallfieldsprivateandallmethodspublic.Ofcourse,inparticularcasesyoumustadjustments,butthisisausefulguideline.

|_Initializingthebaseclass

Evenifyoudon’tcreateaconstructorforderived-class,thecompilerwillsynthesizeadefaultconstructorforyouthatcallsthebaseconstructor.

Ifyourclassdoesn’thavedefaultarguments,orifyouwanttocallabase-classconstructorthathasanargument,youmustexplicitlywritethecallstothebase-classconstructorusingthesuperkeyWordandtheappropriateargumentlist.Inaddition,thecalltothebase-classconstructormustbethefirstthingyoudointhederived-classconstructor.

Delegation

委托是重用类的第三种方式，介于继承和组件之间。在新建类中放置成员对象（像组件），同时暴露该对象的所有方法（像继承）。Youhavemorecontrolwithdelegationbecauseyoucanchoosetoprovideonlyasubsetofthemethodsinthememberobject.

Combiningcompositionandinherit

|_Guaranteeingpropercleanup

Notethatinyourcleanupmethod,youmustalsopayattentiontothecallingorderforthebase-classandmember-objectcleanupmethodsincaSEOnesubobjectdependsonanother.Ingeneral,youshouldfollowthesameformthatisimposedbyaC++compileronitsdestructors:Firstperformallofthecleanupworkspecifictoyourclass,inthereverseorderofcreation.

Therecanbemanycasesinwhichthecleanupissueisnotaproblem;youjustletthegarbagecollectordothework.Butwhenyoumustdoitexplicitly,diligenceandattentionarerequired,becausethere’snotmuchyoucanrelyonwhenitcomestogarbagecollection.Thegarbagecollectormightneverbecalled.Ifitis,itcanreclaimobjectsinanyorderitwants.Youcan’trelyongarbagecollectionforanythingbutmemoryreclamation.Ifyouwantcleanuptotakeplace,makeyourowncleanupmethodsanddon’tuseonfinalize().

Choosingcompositionvs.inheritance

Bothcompositionandinheritanceallowyoutoplacesubobjectinsideyournewclass(compositionexplicitlydoesthis-withinheritanceit’simplicit).

Theis-arelationshipisexpressedwithinheritance,andthehas-arelationshipisexpressedwithcomposition.

Upcasting

Inobject-orientedprogramming,themostlikelywaythatyou’llcreateandusecodeisbysimplypackagingdataandmethodstogetherintoaclass,andusingobjectsofthatclass.You’llalsouseexistingclassestobuildnewclasseswithcomposition.Lessfrequently,you’lluseinheritance.SoalthoughinheritancegetsalotofemphasiswhilelearningOOP,itdoesn’tmeanthatyoushoulduseiteverywhereyoupossiblycan.Onthecontrary,youshoulduseitsparingly,onlywhenit’sclearthatinheritanceisuseful.Oneoftheclearestwaystodeterminewhetheryoushouldusecompositionorinheritanceistoaskwhetheryou’lleverneedtoupcastfromyournewclasstothebaseclass.Ifyoumustupcast,theninheritanceisnecessary.ThePolymorphismprovidesoneofthemostcompellingreasonsforupcasting,butifyouremembertoask“DoIneedtoupcast?”you’llhavaagoodtoolfordecidingbetweencompositionandinheritance.

Thefinalkeyword

Therearethereplaceswherefinalcanbeused:fordata,methodsandclasses.

|_finaldata

Thatapieceofdataisconstantisusefulfortworeasons:

1.Itcanbeacompile-timeconstantthatwon’teverchange.

2.Itcanbeavalueinitializedatruntimethatyoudon’twantchanged.

Inthecaseofacompile-timeconstant,thecompilerisallowedto“fold”theconstantvalueintoanycalculationsinwhichit’sused;thatis,thecalculationcanbeperformedatcompiletime,eliminatingsomerun-timeoverhead.InJava,thesesortsofconstantsmustbeprimitivesandareexpressedwiththefinalkeyword.Avaluemustbegivenatthetimeofdefinitionofsuchaconstant.

当final被用于对象引用上时，对象的引用不能被更改，即该引用不能指向其他对象，但是对象本身可以被修改。这个规则对隶属于对象的数组引用同样适用。

在定义时不赋初值的final变量必须在使用之前被初始化（比如在构造体中初始化，staticfinal不能在构造体中初始化，因为static不依赖于对象而存在），否则编译报错。You’reforcedtoperformassignmentstofinalseitherwithanexpressionatthepointofdefinitionofthefieldorineveryconstructor.Thatwayit’sguaranteedthatthefinalfieldisalwaysinitializedbeforeuse.

|_finalarguments

参数被final修饰时可读不可写。Thisfeatureisprimarilyusedtopassdatatoanonymousinnerclasses.

|_finalmethods

用final修饰的方法不能被重写。

|_finalandprivate

Anyprivatemethodsinaclassareimplicitlyfinal.Becauseyoucan’taccessaprivatemethod,youcan’toverrideit.Youcanaddthefinalspecifiertoaprivatemethod,butitdoesn’tgivethatmethodanyextrameaning.

“Overriding”canonlyoccurifsomethingispartofthebase-classinterface.Thatis,youmustbeabletoupcastanobjecttoitsbasetypeandcallthesamemethod.Ifamethodisprivate,itisn’tpartofthebase-classinterface.Itisjustsomecodethat’shiddenawayinsidetheclass,anditjusthappenstohavathename,butifyoucreateapublic,protected,orpackage-accessmethodwiththesamenameinthederivedclass,there’snoconnectiontothemethodthatmighthappentohavethatnameinthebaseclass.Youhaven’toverriddenthemethod;you’vejustcreatedanewmethod.Sinceaprivatemethodisunreachableandeffectivelyinvisible,itdoesn’tfactorintoanythingexceptforthecodeorganizationoftheclassforwhichitwasdefined.

|_finalclasses

Whenyousaythatanentireclassisfinal(byprecedingitsdefinitionwiththefinalkeyword),youstatethatyoudon’twanttoinheritfromthisclassorallowanyoneelsetodoso.Inotherwords,forsomereasonthedesignofyourclassissuchthatthereisneveraneedtomakeanychanges,orforsafetyorsecurityreasonsyoudon’twantsubclassing.

Notethatthefieldofafinalclasscanbefinalornot,asyouchoose.Thesamerulesapplytofinalforfieldsregardlessofwhethertheclassisdefinedasfinal.However,becauseitpreventsinheritance,allmethodsinafinalclassareimplicitlyfinal,sincethere’snowaytooverridethem.Youcanaddthefinalspecifiertoamethodinafinalclass,butitdoesn’taddanymeaning.

Initializationandclassloading

InJava,thecompiledcodeforeachclassexistsinitsownseparatefile.Thatfileisn’tloadeduntilthecodeisneeded.Ingeneral,youcansaythat“classcodeisloadedatthepointoffirstuse.”Thisisusuallywhenthefirstobjectofthatclassisconstructed,butloadingalsooccurswhenastaticfieldorstaticmethodisaccessed(Theconstructorisalsoastaticmethodseventhoughthestatickeywordisnotexplicit.Sotobeprecise,aclassisfirstloadedwhenanyoneofitsstaticmembersisaccessed.).

|_Initializationwithinheritance

第一次运行主类的main方法时，装载器找到该类对应的class文件，在装载的过程中如果发现它有父类，则继续装载其父类，父类还有父类，继续装载。接下来，在根类开始执行静态初始化，接下来是其派生类，等等。这是重要的，因为派生类静态初始化可能依赖父类成员被合理的初始化。

Atthispoint,thenecessaryclasseshavaallbeenloadedsotheobjectcanbecreated.First,alltheprimitivesinthisobjectaresettotheirdefaultvaluesandtheobjectreferencesaresettonull-thishappensinonefellswoopbysettingthememoryintheobjecttobinaryzero.Thenthebase-classconstructorwillbecalled.Inthiscasethecallisautomatic,butyoucanalsospecifythebase-classconstructorcallbyusingsuper.Thebaseclassconstructiongoesthroughthesameprocessinthesameorderasthederived-classconstructor.Afterthebase-classconstructorcompletes,theinstancevariablesareinitializedintextualorder.Finally,therestofthebodyoftheconstructorisexecuted.

Summary

Bothinheritanceandcompositionallowyoutocreateanewtypefromexistingtypes.Compositionreusesexistingtypesaspartoftheunderlyingimplementationofthenewtype,andinheritancereusestheinterface.

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