ReusingClasses

有兩種常用方式實現類的重用，組件（在新類中創建存在類的對象）和繼承。

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

委托是重用類的第三種方式，介于繼承和組件之間。在新建類中放置成員對象（像組件），同時暴露該對象的所有方法（像繼承）。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.

當final被用于對象引用上時，對象的引用不能被更改，即該引用不能指向其他對象，但是對象本身可以被修改。這個規則對隸屬于對象的數組引用同樣適用。

在定義時不賦初值的final變量必須在使用之前被初始化（比如在構造體中初始化，staticfinal不能在構造體中初始化，因為static不依賴于對象而存在），否則編譯報錯。You’reforcedtoperformassignmentstofinalseitherwithanexpressionatthepointofdefinitionofthefieldorineveryconstructor.Thatwayit’sguaranteedthatthefinalfieldisalwaysinitializedbeforeuse.

|_finalarguments

參數被final修飾時可讀不可寫。Thisfeatureisprimarilyusedtopassdatatoanonymousinnerclasses.

|_finalmethods

用final修飾的方法不能被重寫。

|_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

第一次運行主類的main方法時，裝載器找到該類對應的class文件，在裝載的過程中如果發現它有父類，則繼續裝載其父類，父類還有父類，繼續裝載。接下來，在根類開始執行靜態初始化，接下來是其派生類，等等。這是重要的，因為派生類靜態初始化可能依賴父類成員被合理的初始化。

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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