Objec&ves• Inheritance• Polymorphism

Ø Dispatch

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Inheritance• Buildnewclassesbasedonexis&ngclasses

Ø Allowscodereuse• Startwithaclass(parentorsuperclass)• CreateanotherclassthatextendsorspecializestheclassØ Calledthechild,subclassorderivedclassØ Useextends keywordtomakeasubclass

Examples?

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Childclass• Inheritsallofparentclass’smethodsandfields

Ø Noteonprivatefields:allareinherited,justcan’taccess

• CanalsooverridemethodsØ Usethesamenameandparameters,butimplementa&onisdifferent

• Addsmethodsorfieldsforaddi/onalfunc/onality• Usesuperobjecttocallparent’smethod

Ø Evenifchildclassredefinesparentclass’smethod

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InheritanceRules• Constructorsarenotinherited

Ø Forexample:wewillhavetodefineRooster( String name, int height, double weight ) eventhoughsimilarconstructorinChicken

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Rooster class• Couldwriteclassfromscratch,but…• Aroosterisachicken

Ø Butitaddssomethingto(orspecializes)whatachickenis/does

• Classicmarkofinheritance:isarela&onship• Roosterischildclass• Chickenisparentclass

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AccessModifiers• public

Ø Anyclasscanaccess• private

Ø Nootherclasscanaccess(includingchildclasses)• Mustuseparentclass’spublicaccessor/mutatormethods

• protectedØ ChildclassescanaccessØ MembersofpackagecanaccessØ Otherclassescannotaccess

AccessModes

Accessibleto MemberVisibilitypublic protected package private

Definingclass Yes Yes Yes YesClassinsamepackage

Yes Yes Yes No

Subclassindifferentpackage

Yes Yes No No

Non-subclassdifferentpackage

Yes No No No

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Default (if none specified)

protected• Accessibletosubclassesandmembersofpackage• Can’tkeepencapsula&on“pure”

Ø Don’twantotherstoaccessfieldsdirectlyØ Maybreakcodeifyouchangeyourimplementa&on

• Assump&on?Ø Someoneextendingyourclasswithprotectedaccessknowswhattheyaredoing

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AccessModifiers• Ifyou'reuncertainwhichtouse(protected,package,orprivate),usethemostrestric/veØ Changingtolessrestric&velateràeasyØ Changingtomorerestric&veàmaybreakcodethatusesyourclasses

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Rooster classpublic class Rooster extends Chicken {

public Rooster( String name, int height, double weight) {// all instance fields inherited// from super classthis.name = name;this.height = height;this.weight = weight;is_female = false;

}

// new functionalitypublic void crow() {… }…

By default calls default super constructor with

no parameters

extends means that Rooster is a child of Chicken

(not one of the examples posted online)

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Rooster classpublic class Rooster extends Chicken {

public Rooster( String name, int height, double weight) {

super(name, height, weight, false);}

// new functionalitypublic void crow() { … }

…}

Call to super constructor must be first line in constructor

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ConstructorChaining• Constructorautoma-callycallsconstructorofparentclassifnotdoneexplicitlyØ super();

• Whatifparentclassdoesnothaveaconstructorwithnoparameters?Ø Compila&onerrorØ Forceschildclassestocallaconstructorwithparameters

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OverridingandNewMethodspublic class Rooster extends Chicken {

…

// overrides superclass; greater gains@Overridepublic void feed() {

weight += .5;height += 2;

}

// new functionalitypublic void crow() {

System.out.println("Cocka-Doodle-Doo!");}

}

Same method signature as parent class

Specializes the class

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InheritanceTree• java.lang.Object

Ø Chicken• Rooster

• Callparentclass’sconstructorfirstØ Knowyouhavefieldsofparentclassbeforeimplemen&ngconstructorforyourclass

Object

Chicken

Rooster

1

2

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InheritanceTree• java.lang.Object

Ø Chicken• Rooster

• Nofinalize()chainingØ Shouldcallsuper.finalize() insideoffinalizemethod

Object

Chicken

Rooster

1

2

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ShadowingParentClassFields• ChildclasshasfieldwithsamenameasparentclassØ Youprobablyshouldn’tbedoingthis!Ø Butcouldhappen

• Example:moreprecisionforaconstant

field // this class's fieldthis.field // this class's fieldsuper.field // super class's field

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Mul&pleInheritance• InPython,itispossibleforaclasstoinherit(orextend)morethanoneparentclassØ Childclasshasthefieldsfrombothparentclasses

• ThisisNOTpossibleinJava.Ø Aclassmayextend(orinheritfrom)onlyoneclass

POLYMORPHISM&DISPATCH

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Polymorphism• Polymorphismistheabilityforanobjecttovarybehaviorbasedonitstype

• Youcanuseachildclassobjectwhenevertheprogramexpectsanobjectoftheparentclass

• Objectvariablesarepolymorphic• AChicken objectvariablecanrefertoanobjectofclassChicken, Rooster, Hen,oranyclassthatinheritsfromChicken

Chicken[] chickens = new Chicken[3];chickens[0] = momma;chickens[1] = foghorn;chickens[2] = baby;

We can guess the actual typesBut compiler can’t

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Compiler’sBehavior

• Weknowchickens[1]isprobablyaRooster,buttocompiler,it’saChicken sochickens[1].crow(); willnotcompile

Chicken[] chickens = new Chicken[3];chickens[0] = momma;chickens[1] = foghorn;chickens[2] = baby;

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Compiler’sBehavior• WhenwerefertoaRooster objectthroughaRoosterobjectvariable,compilerseesitasaRoosterobject

• IfwerefertoaRoosterobjectthroughaChickenobjectvariable,compilerseesitasaChickenobject.

• WecannotassignaparentclassobjecttoaderivedclassobjectvariableØ Ex:RoosterisaChicken,butaChickenisnotnecessarilyaRooster

Rooster r = chicken;

à Object variable determines how compiler sees object.

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Polymorphism

Chicken[] chickens = new Chicken[3];chickens[0] = momma;chickens[1] = foghorn;chickens[2] = baby;

chickens[1].feed();

CompilesbecauseChickenhasafeedmethod.But,whichfeedmethodiscalled–

Chicken’sorRooster’s?

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Dynamicvs.Sta&cDispatch• Dynamicdispatchisnotnecessarilyapropertyofobject-orientedprogrammingingeneral

• SomeOOPlanguagesusestaCcdispatchØ Typeoftheobjectvariableusedtocallthemethoddetermineswhichversiongetsrun

• Theprimarydifferenceiswhendecisiononwhichmethodtocallismade…Ø Sta&cdispatch(C#)decidesatcompile&meØ Dynamicdispatch(Java,Python)decidesatrun&me

• DynamicdispatchisslowØ  Inmidtolate90s,ac&veresearchonhowtodecrease&me

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InheritanceRules:AccessModifiers

• Why?• Whatwouldhappenifamethodintheparentclassispublic butthechildclass’smethodisprivate?

Access modifiers in child classes• Can make access to child class less restrictive but

not more restrictive