2023学年完整公开课版constructormethod

Constructor

MethodConstructormethodisakindofspecialmethod,whichhasthefollowingcharacteristics:

thenameoftheconstructormethodmustbethesameasthenameoftheclassname

theconstructormethodhasnoreturntype,evenincludingvoid

whentheconstructormethod

createsanobject,itusesthenewkeywordtocall

theconstructormethodinitializesanewobject

theconstructormethodwithoutparametersiscallednonparametricconstructormethod

therecanbemorethanoneconstructormethodinaclassProblemMusteveryclasswriteconstructormethod?Ifthereisnoconstructorinaclass,istherenoconstructorforthisclass?ConstructorMethodClasscouldnotwriteconstructormethod.Inthiscase,theclassimplicitlydeclaresanonparametricconstructormethodwhosemethodbodyisempty.Thisconstructormethodiscalledthedefaultconstructormethod.Thedefaultconstructormethodisautomaticallygeneratedonlywhentheconstructormethodisnotexplicitlydeclaredinthisclass.classWelcome{publicstaticvoidmain(String[]args){

System.out.println(“welcometoJava!”);}}

classWelcome{publicstaticvoidmain(String[]args){

System.out.println(“welcometoJava!”);}

publicWelcome(){}}beequaltoTheroleofconstructionmethodsJavausesthekeywordnewtocalltheconstructormethodtocreateobjectandinitializenewobject.

newconstructorname(actualparameter);Theprocessoftheobjectiscreatedisasfollows:(1)Thekeywordnewcreatesanobject,allocatesspacefortheobject,andassignstheinitialvalue(0value)tothemembervariable.(2)Calltheconstructormethodtoinitializethenewobject.(3)Theconstructormethodreturnsareferencetothenewobject,whichcanbeassignedtoavariableofthesametype.CallconstructormethodtocreateobjectclassCircle{ doubleradius; Circle(doubleradius){ this.radius=radius;} doublegetArea(){ returnthis.radius*this.radius*3.14;}}publicclassTest{ publicstaticvoidmain(String[]args){

Circlec1=newCircle(2.5);Circlec2=newCircle(5.0);}}Definemultipleconstructionmethods6publicclassCircle{ doubleradius; Circle(){this.radius=1.0;}Circle(doubleradius){ this.radius=radius;} doublegetArea(){ returnthis.radius*this.radius*3.14;}}publicclassTest{ publicstaticvoidmain(String[]args){

Circlec1=newCircle(2.5);//calltheconstructorthatrequiresadoubleparameter

Circlec2=newCircle();//calltheconstructormethodwithoutparameters}}Twoconstructionmethodsaredefinedinthecircleclass.

Theyhavedifferentparameters,whicharecalledconstructormethodoverloading.Usingobjectaccessthemembervariablesoftheobject:objectreference.membervariablenamemethodofcallingobject:objectreference.methodname(argument)Usingobjects1.radiusvariableisthemembervariableoftheCircleclass.Whenradiusisusedinastatement,itreferstotheradiusofthecurrentobject.2.Whatisthecurrentobject?WhocallsthegetAreamethodisthecurrentobjectofthegetAreamethod.

c1.getArea()c1isthecurrentobjectofgetAreamethod,andtheradiusofc1isusedinthestatementc2.getarea(),c2isthecurrentobjectofgetAreamethod,andtheradiusofc2isusedinthestatementpublicclassCircle{ doubleradius; Circle(doubleradius){this.radius=radius;} doublegetArea(){

returnthis.radius*this.radius*3.14;}}0x34AC00x34AB6c1radius：2.5Circleobjectc2radius：5Circleobjectthis.radiusiscurrentobjectpointstothepointertothepreviousobjectExercise1CreateahumanclasscalledPerson,therequirementsareasfollows:containstwomembervariables:name,ageconstructormethod1:withtwoparameters,assignvaluestonameandagerespectivelyconstructormethod2:with1parameter,onlynameisassignedmethod:voidspeak()Output:Helloeveryone,mynameisXXX.I'mXXyearsold.CreateatestclasscalledTestPerson,therequirementsareasfollows:createapersonobjectwithyourowninformationandcallitsspeakmethodExercise2CreateaclasscalledCar,therequirementsareasfollows:containthreemembervariables:name,priceandspeed,whichrepresentthebrand,priceandspeedofthecarrespectivelyconstructormethod:with3parameters,assignvaluesto3membervariablesrespectivelymethod1:voidspeedup(ints)isusedtoincreasethevehiclespeedtothespecifiedvaluemethod2:voidstop(),stopandreducethespeedto0CreateatestclasscalledTestCar,includingthemainmethod,whichimplementsthefollowingfunctions:createcarObjectc,thevaluesofthreepropertiesareself-definedacceleratecto100andoutputthespeedofcstopcandoutputthespeedofcExercise3CreateaclasscalledPoint,therequirementsareasfollows:containtwomembervariables:xandy,whichrepresenttheabscissaandordinateofthepointrespectivelyaconstructormethod:twoparameters,whichareusedtoassignthespecifiedinitialvaluestothemembervariablesxandymethod:voidmove(intx,inty)isusedtomovethepointtothecoordinatespecifiedbytheparameterCreateatestclassTestPoint,includingthemainmethod,whichimplementsthefollowingfunctions:createpointobjectp1withxcoordinateof2andycoordinateof3,andoutputitsxandycoordinatescreatepointobjectp2withxcoordinateof5andycoordinateof-1,andoutputitsxandycoordinatesmovep2to(0,0)andoutputitsxandycoordinatesExercise4PublicclassRectangle{//rectangleclass

Intwidth;//widthofrectangle

Intheight;//heightofrectangleRectangle(intwidth,intheight){ //supplement } voidmove(intx,inty){ //supplement } intgetArea(){ //supplement }}createatestclass,createaRectangle’sobject,outputitsareaThedefaultvalueofthemembervariablepublicclassTest{publicstaticvoidmain(String[]args){ Circlec=newcircle();//createCircleobjectc

System.out.println(c.getarea());//whatdoesthissentenceoutput?

System.out.println(c.radius);//whatdoesthissentenceoutput?}}publicclassCircle{ doubleradius;

Circle(){}Circle(doubleradius){this.radius=radius;} doublegetArea(){returnthis.radius*this.radius*3.14;}}ThedefaultvalueofthemembervariableWheninstantiatinganobject,themembervariablewillbeautomaticallyassignedinitialvalue:Theinitialvalueofthemembervariableofthereferencetypeisnull.Theinitialvalueofanumericmembervariableis0.Theinitialvalueofthemembervariableofbooleantypeisfalse.Theinitialvalueofthemembervariableofchartypeis'\u0000'.LocalvariableshavenodefaultinitialvaluepublicclassTest{publicstaticvoidmain(String[]args){intx;//xhasnodefaultvalueStringy;//yhasnodefaultvalue

System.out.println("xis"+x);//compilationerror

System.out.println(“yis"+y);//compilationerror}}classCircle{doubleradius;Circle(doubleradius){this.radius=radius;}..}publicclassTest{publicstaticvoidmain(String[]args){Circlec1=newCircle(4);

c1.radius=-5;//Isthisassignmentmeaningful?

Circlec2=newCircle(-3);//Isitmeaningfultocreatesuchacircle?}}Whendesigningaclass,themembervariablesshouldnotbeexposedtotheoutsideworld.Thenusetheassignmentstatementtoassignvaluestothemembervariables.classCircle{

privatedoubleradius;Circle(doubleradius){this.radius=radius;}..}Step1:definethemembervariableasprivatepublicclassTest{publicstaticvoidmain(String[]args){ Circlec1=newCircle(4);

C1.radius=-5;//compilationerror

System.Out.Print(C1.Radius);//compilationerror

}}membersdefinedasprivatecannotbeaccessedoutsidetheclass(read/write)classCircle{privatedoubleradius;publicCircle(doubleradius){if(radius>=0){this.radius=radius;}}publicdoublegetRadius(){returnradius;}publicvoidsetRadius(doubleradius){if(radius>=0){this.radius=radius;}}}Circlec1=newCircle(5);

c1.Setradius(7);//callthesetmethodtoassignvaluestothemembervariablesoftheobject

doubler=c1.getRadius();//callthegetmethodtogetthevalueofamembervariableoftheobjectStep2:Providepublicgetandsetmethodsformembervariablestoaccessvariablevalues.Inthesetmethod,conditionalstatementscanbeusedtoensurethevalidityoftheassignment.GetandsetmethodnamingclassA{privateintabc;publicintgetAbc(){returnabc;}publicvoidsetAbc(intnewAbc){abc=newAbc;}}1.Thegetmethodisnamedget+membervariablename(thefirstletterofthemembervariablenameischangedtouppercase),andthereturntypeisthetypeofthemembervariable.Noparametersarerequired.Themethodreturnsthevalueofthemembervariable.2.Thesetmethodisnamedasset+membervariablename.Thefirstletterofthemembervariablenameischangedtouppercase.Thereturntypeisvoidand1parameter.Thetypeisconsistentwiththetypeofthemembervariabletobemodified.Themethodassignsthevalueofthemembervariableastheparametervalue.ReflectionIfamembervariableisnotallowedtobemodifiedoutsidetheclass,whatmeasuresshouldbetakenwhenwritingtheclass?Ifthevalueofamembervariableisnotallowedtobeaccessed(readorwrite)outsidetheclass,whatmeasuresshouldbetakenwhenwritingtheclass?PracticeChangeallmembervariablesofperson,car,point,rectangleclassesintoprivate,andwritegetandsetmethodsforthem.ObjectmemorymodelJavalanguagedividesdatatypesintotwocategories:Primitivedatatype:byte,short,int,long,float,double,boolean,charReferencetype:class,array,interfacePrimitivedatatypevariablesstoredatavaluesinti=10;intj=12;10i:j:12ObjectmemorymodelThereferencevariablecontainsthereferenceaddressoftheobjectorarray

Personp=newPerson("ZhangSan",18);0x34AC0p:age：name："ZhangSan"Personobject,memoryaddress0x34ac0Object'sinstancemembervariables(nonstaticvariables)arestoredintheobject'sspace18Replicationofbasicdatatypevariablesinti=2;intj=i;i2j2CopyingofreferencetypevariablesPersonp1=newPerson("ZhangSan",18);Personp2=newPerson("LiSi",20);p1=p2；p1:0x34AB6p2:age:18name：Personobject,memoryaddress0x34ac0"ZhangSan"age:20name：Personobject,memoryaddress0x34ab6"LiSi"Ifanobjecthasnoreferencetoit,itbecomesuseless.Itwillbegarbagecollected,Javavirtualmachineautomaticallyreclaimsuselessobjectsandfreespace.0x34AC00x34AB6Uselessobject,garbageclassA{intx,y;A(inta,intb){x=a;y=b;