参考复试课件java lecture5

Objectives 5-

Inheritance

Inheritance(super/sub-class,protected,constructor)

Overriding,FieldHidingandSuper

FinalClassesandMethods(security,optimization)

CastandInstanceof

AbstractMethodsandClasses

Polymorphism

Design(Isa/HasA,accesscontrol)

KeyConcepts 5-

Inheritance: Reuse of proven and debuggedsoftware

Polymorphism: handle existing and yet-to-be-specified,andaddnewcapabilities

RelationshipsamongClasses

ISA:sub/super-class

HASA:composition,anobjecthasoneormoreobjectsofotherclassesasmembers

Coordination: operations by public fields &methods

Inheritance 5-

Subclass/Superclass

Directsuperclass/indirectsuperclass

SingleInheritance/multipleinheritance

InheritanceHierarchy(tree-like/network-like)

InheritanceandInterfacesinJava

Inheritance:classsubclassextendssuperclass

Object:allclassesaresubclassofObject(root)

Interfaces:achievemanyoftheadvantagesofmultipleinheritance

RelationshipbetweenSuperclassandSubclassObjects

Subclass-object-is-a-superclass-object

Asuperclassobjectisnotasubclassobject

Useanexplicitcasttoconvertasuperclassreferencetoa

subclassreference(Justastypecoercion)

PhoneCard 返回1

无卡号电话卡

对应电话机型号

拨打电话

查询余额

剩余金额

继承None_Number_PhoneCard

继承 返回2

有卡号电话卡

获得电话机型号

继承 继承

Number_PhoneCard

卡号、密码、接入号码、接通

继承

登录交换机

返回3

失效日期

200卡

附加费

电话磁卡

使用地域

拨打电话 magCard 拨打电话

拨打电话

修改密码

IC_Card 拨打电话

IP_Card

D2

0

0

C

ard

Example(1) 5-

abstractclassPhoneCard

{ doublebalance;

abstractbooleanperformDial();doublegetBalance()

{ returnbalance; }

}

abstractPhoneCard

class

None_Number_PhoneCard

extends

{

StringphoneSetType;

StringgetSetType()

{returnphoneSetType;

}

}

返回

Example(3) 5-

doublebalance;

classmagCardextendsNone_Number_PhoneCard

{

StringusefulArea;booleanperformDial()

{

if(balance>0.9)

{

}

else

balance-=0.9;returntrue;

returnfslase;

}

} 返回

ProgramminginJava

Example(3) 5-

classIC_CardextendsNone_Number_PhoneCard

{

booleanperformDial()

{

if(balance>0.5)

{

}

else

balance-=0.9;returntrue;

returnfalse;

}

} 返回

ProgramminginJava

Example(3) 5-

classIP_CardextendsNumber_PhoneCard

{

DateexpireDate;booleanperformDial()

{

if(balance>0.3&&expireDate.after(newDate()))

{

}

else

balance-=0.3;returntrue;

returnfalse;

}

} 返回

ProgramminginJava

Example(3) 5-

classD200_CardextendsNumber_PhoneCard

doublebalance;//Number_PhoneCard

{

doublebalance;doubleadditoryFee;booleanperformDial()

{ if(balance>(0.5+additoryFee))

{ balance-=(0.5+additoryFee);returntrue; }

else

reurnfalse; }

}

返回

ProgramminginJava

Example(2) 5-

abstractclassNumber_PhoneCardextendsPhoneCard

{ longcardNumber;intpassword;

StringconnectNumber;booleanconnected;

booleanperformConnection(longcn,intpw)

{if(cn==cardNumber&&pw==password)

{ connected=true;returntrue; }

else

returnfalse; }

} 返回

ProgramminginJava

ObjectClass 5-

TheObjectclassdefinesandimplementsbehaviorthateveryclassintheJavasystemneeds.Itisthemostgeneralofallclasses.

ThereferenceofObjectclassobjectcaninsteadofanyobject

Objectoref=newPixel();oref=“SomeString”

返回

Field 5-

FieldInheritance:magCard

FieldHiding

WhatisFieldHiding?

—Afieldwiththesamenameasasuperclass’sfiledisdeclaredinsubclass

Notes

—Thehiddenfieldinthesuperclassstillexists

—The hidden field may be accessed by super, orreferencingasuperclass’sobject

—Whenafieldofanobjectisaccessed,thedeclaredtype

oftheobjectdetermineeithersuper-orsub-class’sfield

willbeused

Example 5-

PublicclassTestHiddenField

{publicstaticvoidmain(Stringargs[])

{D200_Cardmy200=newD200_Card();my200.balance=50.0;

System.out.println(“Supper:”+my200.getBalance());

0.0

if(my200.performDial())

System.out.println(“Subclass:”+my200.balance);

49.5

}

}

Method 5-

MethodInheritance:IC-Card

Overriding

—Replaceasuperclass’smethodimplementationbyanewoneinthesubclass

—Requirements

Theoldmethodandthenewmethodmusthavethesamesignatureandreturntype.(ifdifferent,sucharedefinitionisnotoverriding,butissimplyanexampleofoverloading)

Themethodmustbenon-static

Notes 5-

Supermaybeusedtorefertotheoverriddenmethodofthesuperclass

Thethrowsclausesmaybedifferent:theoverridingmethodhasfewerexceptiontypes(ornothrows)

Theoverridingmethodmayhavenewmodifiers,butmustprovidemoreaccesspermissions.Ifprotectedinsuperclass,thenprotectedorpublic,mustnotbeprivateinsubclass

DynamicMethodCall:whenamethodofanobjectiscalled,theactualtypeoftheobjectdetermine

whichimplementationwillbeused

Example(1) 5-

classSuperShow{

publicStringstr=“SuperStr”;publicvoidshow(){

System.out.println(“Super.show:”+str);}

}

classExtendShowextendsSuperShow

{publicStringstr=“ExtendStr”;publicvoidshow(){

System.out.println(“Extend.show”+str); }publicstaticvoidmain(String[]args){

ExtendShowext=newExtendShow();SuperShowsup=ext;

ProgramminginJava

Example(2) 5-

sup.show();//showofactualtype,i.e.ExtendShowext.show();System.out.println(“sup.str=”+sup.str);System.out.println(“ext.str=”+ext.str);

}

}

Twokindsofreference:actualtype(ext),supertype(sup)

Result:Extend.show:ExtendStr //actualtype:Extend

Extend.show:ExtendStr

sup.str=SuperStr//declaredtype:SuperShow

ext.str=ExtendStr//declaredtype:ExtendShow

Supper 5-

Function

Referencethecurrentobjectasaninstanceofthesuperclass,whenaccessfieldsandmethods

Use

Super.variable

Super.Method([paramlist])

Super([paramlist])

Note:super.methodalwayscallthesuperclass’s(overridden)method,notthesubclass’s(overriding)method.Sothemethodtobeusedforsuperisdeterminedbyreferencetype,

notactualtype

ConstructorsinSubclasses 5-

SelectandInvokeaSuperclass’sConstructor

Explicitcall

super()isthefirststatementofnewconstructor

Automaticcall

Ifsuper()isnotthefirststatement,calltheno-argconstructorofthesuperclass.Ifthereisnosuchconstructor,anexplicitcallofsuperclass’sconstructorwithparametersmustbecalled

classAttr{

Example(1) 5-

privateStringname;privateObjectvalue=null;

publicAttr(StringnameOf){name=nameOf; }publicAttr(StringnameOf,ObjectvalueOf){

name=nameOf; value=valueOf;}publicStringnameOf(){ returnname; }publicObjectvalueOf(){ returnvalue; }publicStringvalueOf(ObjectnewValue)

{ObjectoldVal=value;

value=newValue;returnoldVal; }

}

Example(2) 5-

classColorAttrextendsAttr{

privateScreenColormyColor;//thedecodedcolor

publicColorAttr(Stringname,Objectvalue)

{super(name,value);decodeColor(); }publicColorAttr(Stringname)

{this(name,”transparent”);}

publicColorAttr(Stringname,ScreenColorvalue)

{super(name,value.toString());myColor=value; }

publicObjectvalueOf(ObjectnewValue){

//dothesuperclass’svalueOfworkfirst

Objectretval=super.valueOf(newValue);decodeColor();returnretval; }

/**SetvaluetoScreenColor,notdescription*/

NotesonConstructors 5-

“this()”cancallaconstructorintheclassitself

Subclassneednottohavethesameconstructorsignaturesassuperclass

Defaultconstructorofsubclassstartsbycallingtheno-argconstructorofthesuperclass

Ifthesuperclasshasnotanynon-argconstructor,subclassmustprovideatleastoneconstructor

Ifasuperclass’sconstructorcallsamethod,themethodwouldbemostlyoverriddeninsubclass

InitializingOrdersinSubclass 5-

Initializebydefault(0,\u0000,false,null),andcallthesubclass’sconstructor(i.e.thefollowing3steps)

Callthesuperclass’sconstructor

Initializebydeclarationassignment

Executethesubclass’sconstructorbody

Example(1) 5-

classYextendsX{

protectedintyMask=0xff00;publicY(){

fullMask|=yMask;

}

}

? How is a Y object to becreated?

ProgramminginJava

Example(2) 5-

Step Action xMask yMask fullMask

0 Default

0

0

0

1 CallYconstructor

0

0

0

2 CallXconstructor

0

0

0

3 InitializeXfields

0x00ff

0

0

4 ExecuteXConstructor

0x00ff

0

0x00ff

5 InitializeYfields

0x00ff

0xff00

0x00ff

6 ExecuteYconstructor

0x00ff

0xff00

0xffff

Example(3) 5-

classYextendsX{

protectedintyMask=0xff00;publicY(){

fullMask|=yMask;

}

}

classX{

protectedintxMask=0x00ff;protectedintfullMask;publicX(){

fullMask=mask(xMask);

}

publicintmask(intorig)

{return(orig&fullMask);

}

}

ProgramminginJava

ProgramminginJava

Example(4) 5-

Step Action xMask yMask fullMask

0 Default

0

0

0

1 CallYconstructor

0

0

0

2 CallXconstructor

0

0

0

3 InitializeXfields

0x00ff

0

0

4 ExecuteXConstructor

0x00ff

0

0

5 InitializeYfields

0x00ff

0xff00

0

6 ExecuteYconstructor

0x00ff

0xff00

0xff00

Example(5) 5-

classYextendsX{

protectedintyMask=0xff00;publicY(){

fullMask|=yMask;

}

publicintmask(intorig)

{return(orig|fullMask);

}

}

ProgramminginJava

Example(6) 5-

Step Action xMask yMask fullMask

0 Default

0

0

0

1 CallYconstructor

0

0

0

2 CallXconstructor

0

0

0

3 InitializeXfields

0x00ff

0

0

4 ExecuteXConstructor

0x00ff

0

0x00ff

5 InitializeYfields

0x00ff

0xff00

0x00ff

6 ExecuteYconstructor

0x00ff

0xff00

0xffff

FinalMethod&FinalClass 5-

FinalMethods

—Cannotbere-implemented(overridden)

—Thefinalversionofthemethod

—Staticandprivatemethodsareimplicitlyfinal

FinalClasses:PreventingInheritance

—Cannotbeextended(inherited)

—AllMethodsinafinalclassareimplicitlyfinal

Security 5-

Security:‘final’improvesthesecurity

Behavioroffinalmethodisdetermined,unlessitcallsnon-finalmethods

Theassociatedfieldsoffinalmethodshouldbeprivate,otherwisesubclassesmaychangethemethodbehaviorthroughchangingthesefields

Finalclassescannotbeextended(alsoadefect)

Usenon-finalclasswithmethodsallfinal,sothattheclasscanbeextended,butthemethodcannotbere-implemented

OptimizationandEfficiency 5-

Optimization:‘Final’simplifiestheoptimization

Somepublicstepsofmethodcallareomitted,e.g.Matchformal-actualpara.;Pushreturnaddr.tostack

Inlinethecode:Removingcallstofinalmethods,andreplacingthemwiththeexpandedcodeoftheirdefinitionsateachmethodcalllocation

Efficiency

‘Final’makessometypescheckedatcompiletime,typesfornon-finalreferencearecheckedatruntime

Dynamicbinding(suchasdynamicallyselectamethodimplementationfromseveralones)hasmoreoverhead

thanstaticbinding

Notation

Cast 5-

<type><var>=(<type>)<var>

Convertanobjectfromoneclasstoanother

Justastypecoercion(bymeansofcasts)

CastingUp(Widening)

Toassignasubclassobjecttoasuperclassvariable

Safecasting

CastingDown(Narrowing)

Toassignasuperclassobjecttoasubclassvariable

Unsafecasting(notalwaysvalid)

Ifanobjecthasbeenassignedtoareferenceofoneofitssuperclasses,it’sacceptabletocastthatobjectbacktoitsowntype

Employee:john;classManagerextendsEmployee,…

Managerboss=(Manager)john;

Example 5-

ClassA{

publicvoidf(){};staticvoidg(Ai){

i.f();} A

}

ClassBextendsA{

publicstaticvoidmain(String[]args){ BBb=newB();

A.g(b);

}

} 返回

TheInstanceofOperator 5-

TheOperator

IinstanceofC

IisaninstanceofclassC

Example

if(johninstanceofManager)

{Managerboss=(Manager)john;}

Note

nullinstanceofTypeisfalseforanyType

NotesonCast 5-

Castscanonlybeusedwithinaninheritancehierarchy

instanceofshouldbeusedtocheckhierarchybeforecastingfromaparenttoachild,otherwiseaclassCastExceptionoccurs

Performingacastisnotusuallyagoodidea,becausethedynamicbindingoftenautomaticallylocatesthecorrectmethod

AbstractMethod

Amethodhasnotbeenspecified(signatureonly)

Abstract methods must be implemented in non-abstractsubclasses

AbstractClass:oneormoreabstractmethods

Abstractclassesmayhaveconcretedataandmethods

Example

Notes

Noinstanceofabstractsuperclassescanbeinstantiated

Superclass’smethodsmaybeoverriddenbyabstractmethods(changeconcretetoabstract)

Theabstractmethodgivestheclassdesignerconsiderablepoweroverhowsubclasseswillbe

implementedinaclasshierarchy

Example(1) 5-

abstractclassGraphicObject

{intx,y;

...

voidmoveTo(intnewX,intnewY)

{...}

abstractvoiddraw();}

classCircleextendsGraphicObject

{voiddraw(){...}

}

…

Example(2) 5-

abstractclassA

{abstractvoidcallme();voidmetoo()

{System.out.println(“A’smetoo”)}

}

classBextendsA

{voidcallme()

{System.out.println(“M’scallme”)}

}

Aa=newB();a.callme();/*B’scallme*/a.metoo();

Methods

Overloading(methodswithdifferentsignatures)

Overriding(inheritancewithredefinition)

Types

Subclass-object-is-a-superclass-object,soanobjectcanbereferencedbythesupertypes

Aclass’sconstructorusuallyinvokesasuperclass’sconstructor,becausecreatinganobjectalsoleadstocreatinganotherobjectofthesuperclass

AvariableoftypeObjectmayrefertoanyobject

Example 5-

GraphicObjectg=newCircle();g.draw();

CastingUp

Circle.draw()

在C++中，用虚函数实现动态绑定；

在JAVA中，缺省情况就是动态绑定；否则可用final定义

Notes 5-

Polymorphis