07-abstract and interfacessprenkle/cs209/pre_class/07-abstract_and_interfac… · • Abstract...

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Objec-ves• Javadocs• Inheritance

Ø Finalmethods,fields

• AbstractClasses• Interfaces

Sept26,2016 Sprenkle-CSCI209 1

JAVADOCS


“Documentation is a love letter that you write to your future self.” – Damian Conway

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Javadocs• Specialcomments,whichareusedtogenerateHTMLdocumenta-on

• Syntax:

• Putbeforeaclass,amethod,orafieldtodescribetherespec-veclass/method/field


/** * Comment */

Javadoc• CancontainHTMLsyntaxindescrip-on• Exampleblocktagstodescribeyourcode


@param <paramname> <description>@return <description> (includespecialcases)

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Examples


/** * A simple Java class that models a Chicken. The * state of the chicken is its name, height, and weight * * @author Sara Sprenkle */

/** * @return the height of the chicken, in centimeters */

/** * @param n the String representing the name of the chicken */

Expect these types of comments on all methods from now on

Tags always go last in Javadoc comment

Genera-ngJavadocs• Fromcommand-line:

javadoc [options] [packagenames] [sourcefiles] [@files]

• GeneratesHTMLfilesØ E.g.,Game’sJavadocs


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SummaryofInheritance• Removerepe--vecodebymodelingthe“is-a”hierarchyØ Move“commondenominator”codeuptheinheritancechain

• Don’tuseinheritanceunlessallinheritedmethodsmakesense

• Usepolymorphism

FINALKEYWORD


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Preven-ngInheritance• Some-mes,youdonotwantaclasstoderivefromoneofyourclasses

• Aclassthatcannotbeextendedisknownasafinal class

• Tomakeaclassfinal,simplyaddthekeywordfinal infrontoftheclassdefini-on:

• Exampleoffinal class:System

public final class Rooster extends Chicken { . . .

}


Finalmethods• Canmakeamethodfinal

Ø Anyclassderivedfromthisclasscannotoverridethefinal methods

• Bydefault,allmethodsinafinal classarefinal methods.

class Chicken {. . . public final String getName() { . . . }. . .

}

Why would we want to use final?What are possible benefits to us, the compiler, …?

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ABSTRACTCLASSES



AbstractClasses• Somemethodsdefined,othersnotdefined

Ø Par-alimplementa-on

• Classesinwhichnotallmethodsareimplementedareabstract classes Ø public abstract class ZooAnimal

• BlankmethodsarelabeledasabstractØ public abstract void exercise(Environment env);

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AbstractClasses• Anabstractclasscannotbeinstan-ated

Ø i.e.,can’tcreateanobjectofthatclassØ Butcanhaveaconstructor!

• Childclassofanabstractclasscanonlybeinstan-atedifitoverridesandimplementseveryabstractmethodofparentclassØ Ifchildclassdoesnotoverrideallabstractmethods,itisalsoabstract


AbstractClasses• static,private,andfinalmethodscannotbeabstractØ B/ccannotbeoverriddenbyachildclass

• final classcannotcontainabstractmethods

• AclasscanbeabstractevenifithasnoabstractmethodsØ Usewhenimplementa-onisincompleteandismeanttoserveasaparentclassforclass(es)thatcompletetheimplementa-on

• CanhavearrayofobjectsofabstractclassØ JVMwilldodynamicdispatchformethods

Why?

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Examplesofabstractclasses• Example1:

Ø java.net.SocketØ java.net.SSLSocket (abstract)

• Example2:Ø java.util.Calendar (abstract)Ø java.util.GregorianCalendar


Summary:DefiningAbstractClasses➨ Defineaclassasabstractwhenhavepar%alimplementa%on

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INTERFACES



Interfaces• Purespecifica-on,noimplementa-on

Ø Asetofrequirementsforclassestoconformto

• Classescanimplementoneormoreinterfaces

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ExampleofanInterface• WecancallArrays.sort(array)

• Arrays.sortsortsarraysofanyobjectclassthatimplementstheComparable interface

• ClassesthatimplementComparable mustprovideawaytodecideifoneobjectislessthan,greaterthan,orequaltoanotherobject


java.lang.Comparable

• Anyobjectthatis(inherits)Comparable musthaveamethodnamedcompareTo()

• Returns:Ø Returnanega-veintegerifthethisobjectislessthantheobjectpassedasaparameter

Ø Returnaposi-veintegerifthethisobjectisgreaterthantheobjectpassedasaparameter

Ø Returna0ifthetwoobjectsareequal

public interface Comparable {int compareTo(Object other);

}

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ComparableInterfaceAPI/Javadoc• SpecifieswhatthecompareTo()methodshoulddo

• SayswhichJavalibraryclassesimplementComparable

http://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html


Implemen-nganInterface• Intheclassdefini-on,specifythattheclasswillimplement thespecificinterface

• Provideadefini-onforallmethodsspecifiedininterface

public class Chicken implements Comparable

How to determine Chicken order?

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Comparable Chickens Oneway:orderbyheight

What if otherObject is not a Chicken?

public class Chicken implements Comparable { . . . public int compareTo(Object otherObject) { Chicken other = (Chicken)otherObject; if (height < other.getHeight() )

return –1;if (height > other.getHeight())

return 1;return 0;// simpler: return height-other.getHeight()

}}


InterfaceSummary• Containonlyobject(notclass)methods• Allmethodsarepublic

Ø Impliedifnotexplicit

• Fieldsareconstantsthatarestaticandfinal

• Aclasscanimplementmul-pleinterfacesØ Separatedbycommasindefini-on

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Tes-ngforInterfaces

• Usetheinstanceof operatortoseeifanobjectimplementsaninterfaceØ e.g.,todetermineifanobjectcanbecomparedtoanotherobjectusingtheComparableinterface

if (obj instanceof Comparable) { // runs if obj is an object variable of a class// that implements the Comparable interface

}else {

// runs if it does not implement the interface }


InterfaceObjectVariables• Canuseanobjectvariabletorefertoanobjectofanyclassthatimplementsaninterface

• Usingthisobjectvariable,canonlyaccesstheinterface’smethods

• Forexample…

public void aMethod(Object o) {…if (obj instanceof Comparable) {

Comparable comp = (Comparable) obj; boolean res = comp.compareTo(obj2);

}}

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InterfaceDefini-ons

• Interfacemethodsarepublic bydefaultØ Donotneedtospecifymethodsaspublic

public interface Comparable {int compareTo(Object other);

}


InterfaceDefini-onsandInheritance• Canextendinterfaces

Ø Allowsachainofinterfacesthatgofromgeneraltomorespecific

• Forexample,defineaninterfaceforanobjectthatiscapableofmoving:

public interface Movable {void move(double x, double y);

}

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InterfaceDefini-onsandInheritance• ApoweredvehicleisalsoMovable

Ø MustalsohaveamilesPerGallon() method,whichwillreturnitsgasmileage

public interface Powered extends Movable {double milesPerGallon();

}


ConstantsinanInterface• Ifavariableisspecifiedinaninterface,itisautoma-callyaconstant:Ø public static final variable

• AnobjectthatimplementsPoweredinterfacehasaconstantSPEED_LIMITdefined

public interface Powered extends Movable {double milesPerGallon();double SPEED_LIMIT = 95;

}

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InterfaceDefini-onsandInheritance• PoweredinterfaceextendsMovableinterface• AnobjectthatimplementsPoweredinterfacemustsa-sfyallrequirementsofthatinterfaceaswellastheparentinterface.Ø APoweredobjectmusthaveamilesPerGallon()andmove() method


Mul-pleInterfaces• Aclasscanimplementmul-pleinterfaces

Ø Mustfulfilltherequirementsofeachinterface

• ButNOTpossiblewithinheritanceØ Aclasscanonlyextend(orinheritfrom)oneclass

public final class String implementsSerializable, Comparable, CharSequence { …

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BenefitsofInterfaces• ??

UsinganInterfaceorAbstractClass


When should we use an interface or an abstract class?

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AbstractClassesandInterfaces• ImportantstructuresinJava

Ø Makecodeeasiertochange

• Willreturnto/applytheseideasthroughoutthecourse

• ConceptsareusedinmanylanguagesbesidesJava


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