JAVA网站制作之Declarations and Access Control (2)

比如模式、敏捷方法什么的，这些思想好，但是实施的人没有理解而且没有正确运用这些知识导致了开发周期的延长。比如说对象，通过getName（）方法不能获取对象的名字。accessObjective2
Declareclasses,innerclasses,methods,instancevariablesstatic,variablesandautomatic(methodlocal)variables,makingappropriateuseofallpermittedmodifiers(suchaspublicfinalstaticabstractandsoforth).Statethesignificanceofeachofthesemodifiersbothsinglyandincombinationandstatetheeffectofpackagerelationshipsondeclareditemsqualifiedbythesemodifiers.
1.Twotypesofvariables.
1.Membervariables
・Accessibleanywhereintheclass.
・Automaticallyinitializedbeforeinvokinganyconstructor.
・Staticvariablesareinitializedatclassloadtime.
・Canhavethesamenameastheclass.
2.Automaticvariables(methodlocal)
・Mustbeinitializedexplicitly.(compilerwillcatchitwhenusing,butdoesn’tcatchitifnousing)Objectreferencescanbeinitializedtonulltomakethecompilerhappy.
・Canhavethesamenameasamembervariable,resolutionisbasedonscope.
・Canonlybefinal.Notothermodifiers.

2.ModifiersareJavakeywordsthatprovideinformationtocompileraboutthenatureofthecode,dataandclasses.ThevisibilitymodifiersarepartoftheencapsulationmechanismforJava.Encapsulationallowsseparationoftheinterfacefromtheimplementationofmethods.


3.AccessmodifiersCpublic,protected,private
・Onlyappliedtoclasslevelvariables.(Methodvariablesarevisibleonlyinsidethemethod.)
・Canbeappliedtoclassitself(onlytoinnerclassesdeclaredatclasslevel,nosuchthingasprotectedorprivatetoplevelclass)
・Canbeappliedtomethodsandconstructors.
・Ifaclassisaccessible,itdoesn’tmean,themembersarealsoaccessible.Butiftheclassisnotaccessible,themembersarenotaccessible,eventhoughtheyaredeclaredpublic.
・Ifnoaccessmodifierisspecified,thentheaccessibilityisdefaultpackagevisibility.Allclassesinthesamepackagecanaccessthefeature.It’scalledasfriendlyaccess.ButfriendlyisnotaJavakeyword.SamedirectoryissamepackageinJava’sconsideration.
・Onlyoneouterclassperfilecanbedeclaredpublic.Ifyoudeclaremorethanoneclassinafiletobepublic,acompiletimeerrorwilloccur.
・‘private’meansonlytheclasscanaccessit,notevensub-classes.So,it’llcauseaccessdenialtoasub-class’sownvariable/method.
・Thesemodifiersdictate,whichclassescanaccessthefeatures.Aninstanceofaclasscanaccesstheprivatefeaturesofanotherinstanceofthesameclass.
・‘protected’meansallclassesinthesamepackage(likedefault)andsub-classesinanypackagecanaccessthefeatures.Butasubclassinanotherpackagecanaccesstheprotectedmembersinthesuper-classviaonlythereferencesofsubclassoritssubclasses.Asubclassinthesamepackagedoesn’thavethisrestriction.Thisensuresthatclassesfromotherpackagesareaccessingonlythemembersthatarepartoftheirinheritancehierarchy.
・Methodscannotbeoverriddentobemoreprivate.Onlythedirectionshowninfollowingfigureispermittedfromparentclassestosub-classes.

privateàfriendly(default)àprotectedàpublic

ParentclassesSub-classes

4.final
・finalclassescannotbesub-classed.
・finalvariablescannotbechanged.
・finalmethodscannotbeoverridden.Anymethodsinafinalclassareautomaticallyfinal.
・Methodargumentsmarkedfinalareread-only.Compilererror,iftryingtoassignvaluestofinalargumentsinsidethemethod.
・Finalvariablesthatarenotassignedavalueatthedeclarationandmethodargumentsthataremarkedfinalarecalledblankfinalvariables.Trytouseblankfinalvariableswillgivecompileerror.Theycanonlybeassignedavalueatmostonceinallconstructororinitializedblock.
・Staticfinalvariableshavetobeassignedatthedeclarationtimeorinstaticinitializedblock.
・Localvariablescanbedeclaredfinalaswell.

5.abstract
・Canbeappliedtoclassesandmethods.
・Oppositeoffinal,abstractmustbesub-classed.
・Aclassshouldbedeclaredabstract,
1.ifithasanyabstractmethods.
2.ifitdoesn’tprovideimplementationtoanyoftheabstractmethodsitinherited
3.ifitdoesn’tprovideimplementationtoanyofthemethodsinaninterfacethatitsaysimplementing.
・Justterminatetheabstractmethodsignaturewitha‘;’,curlybraceswillgiveacompilererror.
・Aclasscanbeabstractevenifitdoesn’thaveanyabstractmethods.
・Abstractmethodsmaynotbestatic,final,private,native,synchonized.
・Aclassthatisabstractmaynotbeinstantiated(ie,youmaynotcallitsconstructor,butinsubclass’sconstructor,super()works)

6.static
・Canbeappliedtonestedclasses,methods,variables,freefloatingcode-block(staticinitializer)
・staticmeansoneperclass,notoneforeachobjectnomatterhowmanyinstanceofaclassmight
exist.Thismeansthatyoucanusethemwithoutcreatinganinstanceofaclass.
・Staticvariablesareinitializedatclassloadtime.Aclasshasonlyonecopyofthesevariables.
・Staticmethodscanaccessonlystaticvariables.(Theyhavenothis)
・Accessbyclassnameisarecommendedwaytoaccessstaticmethods/variables.
・Staticmethodsmaynotbeoverriddentobenon-static.
・Non-staticmethodsmaynotbeoverriddentobestatic.
・Localvariablescannotbedeclaredasstatic.
・Actually,staticmethodsarenotparticipatingintheusualoverridingmechanismofinvokingthemethodsbasedontheclassoftheobjectatruntime.Staticmethodbindingisdoneatcompiletime,sothemethodtobeinvokedisdeterminedbythetypeofreferencevariableratherthantheactualtypeoftheobjectitholdsatruntime.

publicclassStaticOverridingTest{
publicstaticvoidmain(Strings[]){
Childc=newChild();
c.doStuff();//ThiswillinvokeChild.doStuff()
Parentp=newParent();
p.doStuff();//ThiswillinvokeParent.doStuff()
p=c;
p.doStuff();//ThiswillinvokeParent.doStuff(),ratherthanChild.doStuff()
}
}

classParent{
staticintx=100;
publicstaticvoiddoStuff(){
System.out.println("InParent..doStuff");
System.out.println(x);
}
}

classChildextendsParent{
staticintx=200;
publicstaticvoiddoStuff(){
System.out.println("InChild..doStuff");
System.out.println(x);
}
}


7.native
・Canbeappliedtomethodsonly.(staticmethodsalso)
・Writteninanon-Javalanguage,compiledforasinglemachinetargettype.
・JavaclassesuselotofnativemethodsforperformanceandforaccessinghardwareJavaisnotawareof.
・Nativemethodsignatureshouldbeterminatedbya‘;’,curlybraceswillprovideacompilererror.
・nativedoesn’taffectaccessqualifiers.Nativemethodscanbeprivate.
・abstractcanappearwithnativedeclaration.Thisforcestheentireclasstobeabstract.(obviously)
・Canpass/returnJavaobjectsfromnativemethods.
・System.loadLibraryisusedinstaticinitializercodetoloadnativelibraries.Ifthelibraryisnotloadedwhenthestaticmethodiscalled,anUnsatisfiedLinkErroristhrown.

8.transient
・Canbeappliedtoclasslevelvariablesonly.(Localvariablescannotbedeclaredtransient)
・Variablesmarkedtransientareneverserialized.(Staticvariablesarenotserializedanyway.)
・Notstoredaspartofobject’spersistentstate,i.e.notwrittenoutduringserialization.
・Canbeusedforsecurity.

9.synchronized
・Canbeappliedtomethodsorpartsofmethodsonly.
・Usedtocontrolaccesstocriticalcodeinmulti-threadedprograms.

10.volatile
・Canbeappliedtovariablesonly.
・Canbeappliedtostaticvariables.
・Cannotbeappliedtofinalvariables.
・Declaringavariablevolatileindicatesthatitmightbemodifiedasynchronously,sothatallthreadswillgetthecorrectvalueofthevariable.
・Usedinmulti-processorenvironments.

ModifierClassInnerclasses(Exceptlocalandanonymousclasses)VariableMethodConstructorFreefloatingCodeblock
publicYYY(notlocal)YYN
protectedNYY(notlocal)YYN
(friendly)NoaccessmodifierYY(OKforall)YYYN
privateNYY(notlocal)YYN
finalYY(Exceptanonymousclasses)YYNN
abstractYY(Exceptanonymousclasses)NYNN
staticNYY(notlocal)YNY(staticinitializer)
nativeNNNYNN
transientNNY(notlocal)NNN
synchronizedNNNYNY(partofmethod,alsoneedtospecifyanobjectonwhichalockshouldbeobtained)
volatileNNYNNN

Objective3
Foragivenclass,determineifadefaultconstructorwillbecreatedandifsostatetheprototypeofthatconstructor.

ConstructorsandSub-classing
・Constructorsarenotinheritedasnormalmethods,theyhavetobedefinedintheclassitself.
・Constructorshavesamenamewiththeclass,noreturntype.Amethodwithaclassname,butwithareturntypeisjustamethodbycompiler.Expecttrickquestionsusingthis.
・Ifyoudefinenoconstructorsatall,thenthecompilerprovidesadefaultconstructorwithnoargumentsandsamemodifierwithclass.
・Wecan’tcompileasub-classiftheimmediatesuper-classdoesn’thaveanoargumentdefaultconstructor,andsub-classconstructorsarenotcallingsuperorthisexplicitly(andexpectthecompilertoinsertanimplicitsuper()call)
・Aconstructorcancallotheroverloadedconstructorsby‘this(arguments)’.Ifyouusethis,itmustbethefirststatementintheconstructor.
・Aconstructorcan’tcallthesameconstructorfromwithin.Compilerwillsay‘recursiveconstructorinvocation’
・Aconstructorcancalltheparentclassconstructorexplicitlybyusing‘super(arguments)’.Ifyoudothis,itmustbefirstthestatementintheconstructor.
・Obviously,wecan’tuseboththisandsuperinthesameconstructor.Ifcompilerseesathisorsuper,itwon’tinsertadefaultcalltosuper().
・Constructorbodycanhaveanemptyreturnstatement.
・Onlymodifiersthataconstructorcanhavearetheaccessibilitymodifiers.

・Initializersareusedininitializationofobjectsandclassesandtodefineconstantsininterfaces.Theseinitializersare:
1.StaticandInstancevariableinitializerexpressions.
Literalsandmethodcallstoinitializevariables.Staticvariablescanbeinitialized
onlybystaticmethodcalls.
Cannotpassonthecheckedexceptions.Mustcatchandhandlethem.
2.Staticinitializerblocks.
Usedtoinitializestaticvariablesandloadnativelibraries.
Cannotpassonthecheckedexceptions.Mustcatchandhandlethem.
3.Instanceinitializerblocks.
Usedtofactoroutcodethatiscommontoalltheconstructors.
Alsousefulwithanonymousclassessincetheycannothaveconstructors.
Allconstructorsmustdeclaretheuncaughtcheckedexceptions,ifany.
InstanceInitializersinanonymousclassescanthrowanyexception.(?)
・Inalltheinitializers,forwardreferencingofvariablesisnotallowed.Forwardreferencingofmethodsisallowed.
・Orderofcodeexecution(whencreatinganobject)isabittricky.
1.staticvariablesinitialization.
2.staticinitializerblockexecution.(intheorderofdeclaration,ifmultipleblocksfound)
3.constructorheader(superorthisCimplicitorexplicit)
4.instancevariablesinitialization/instanceinitializerblock(s)execution
5.restofthecodeintheconstructor

Objective4
Statethelegalreturntypesforanymethodgiventhedeclarationsofallrelatedmethodsinthisorparentclasses.
Seemynote_6foroverloeadingandoverridden.
Examples:
Q1.
Assumewehavethefollowingcodeinthefile/abc/def/Q.java:
//File:/abc/def/Q.java:
packagedef;

publicclassQ{
privateintprivateVar;
intpackageVar;
protectedintprotectedVar;
publicintpublicVar;
}
andthiscodeisin/abc/Sub.java:
//File:/abc/Sub.java:
publicclassSubextendsTester{
}
andthiscodeisin/abc/Tester.java:
//File:/abc/Tester.java:
importdef.Q;

publicclassTesterextendsQ{
Qq=newQ();
Subsub=newSub();

publicvoidsomeMethod(){
//First,trytorefertoqsmemebers.
q.privateVar=1;//compilererror
q.packageVar=2;//compilererror
q.protectedVar=3;//compilererror
q.publicVar=4;//fine

//Next,trytorefertothisobjectsmembers
//suppliedbyclassQ.
privateVar=5;//compilererror
packageVar=6;//compilererror
protectedVar=7;//fine
publicVar=8;//fine

//Next,letstrytoaccessthemembersof
//anotherinstanceofTester.
Testert=newTester();
t.privateVar=9;//compilererror
t.packageVar=10;//compilererror
t.protectedVar=11;//fine
t.publicVar=12;//fine

//Finally,trytorefertothemembersina
//subclassofTester.
sub.privateVar=13;//compilererror
sub.packageVar=14;//compilererror
sub.protectedVar=15;//fine
sub.publicVar=16;//fine
}
}
Q2
Determinetheresultofattemptingtocompileandrunthefollowingcode:
classA{
publicA(){
System.out.println("AAA");
}
{
System.out.println("456");
}
}

publicclassBextendsA{
B(){
this(12);
System.out.println("BBB");
}
B(intx){
System.out.println("CCC");
}
{
System.out.println("123");
}
publicstaticvoidmain(String[]args){
newB();
}
}

Theoutputis:
456
AAA
123
CCC
BBB
Q3
Determinetheresultofattemptingtocompileandrunthefollowingcode:
classA{
publicintAvar;
publicA(){
System.out.println("AAA");
doSomething();
}
publicvoiddoSomething(){
Avar=1111;
System.out.println("A.doSomething()");
}
}

publicclassBextendsA{
publicintBvar=2222;
publicB(){
System.out.println("BBB");
doSomething();
System.out.println("Avar="+Avar);
}
publicvoiddoSomething(){
System.out.println("Bvar="+Bvar);
}
publicstaticvoidmain(String[]args){
newB();
}
}

Theoutputis:
AAA
Bvar=0
BBB
Bvar=2222
Avar=0
Q4
Aclassisnotabstractifitimplementsallsuperinterfaceseitherdirectlyorinherited
fromsuperclass.

eg.classa{voidmethod(){}}
interfaceab{voidmethod();}
classabcextendsaimplementsab{}//PERFECTLYLEGALsincesuperclasshas
implementedtheinterfacemethodsignature.


认真的记，感觉很紧张根本就没有时间和能力，来对技术知识点进行思考。这样课下就只能对知识进行简单的理解，其实简单的理解就是记忆课堂上讲的知识点，