inheritance polymorph is m

8/11/2019 Inheritance Polymorph is m

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Inheritance Visibility Overrides Polymorphism Abstract Classes Interfaces

MIN 545 - Lecture IVInheritance and Polymorphism

c2012 Aybar C. Acar

Rev. 1.1 (Build 20130327132700)

MIN 545 - Inheritance & Polymorphism 1of54


2/54


A Puzzle...

All HourlyEmployees are also Employees.

Some Employees are Salaried.Executives are always Salaried.HourlyEmployeesare never Salaried.Technical staff areSalaried employees that are not Executives.Some Hourly employees are Part-time.Technical staff

are eitherEngineers

,Technicians

orClerks

.Full-time hourly employees are Hourly Employees but not Part-time.

Employee

HourlyEmployee SalariedEmployee

TechnicalFullTime ExecutivePartTime

Engineer Technician Clerical



3/54


Inheritance

Class Inheritanceallows us to:

Create simple parent objects and to make them morespecialized.

Have specializations share the variables & methods of theircommon ancestors.

Write the code once, and re-use in many children! (Laziness)MIN 545 - Inheritance & Polymorphism 3of54


4/54


Deriving Classes in Java

Derived class,subclass,child classall mean the same thing.

Base class,superclass,parent classall mean the same thing.

Base Class:

p u bl i c c la s s E m pl o ye e

{

p r ot e ct e d S t ri n g n am e ;

p r ot e ct e d l on g i d Nu m be r ;

}

Child (derived class):

p u bl i c c la s s H o u rl y E mp l o ye e e x te n ds E m pl o ye e

{

p r iv a te f l oa t h o ur l y Ra t e ;p r iv a te i nt h o u rs W o rk e d ;

}

The HourlyEmployee objects have name and idNumber inherited from

Employee, but they also include new variables: hourlyRate and

hoursWorked.MIN 545 - Inheritance & Polymorphism 4of54


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Inheritance in UML

+ makeSound(): void+ eat(): void

# age : int- sound : String

Animal

+flyTo(Location):void+layEggs(): Egg[]

# airSpeed : int# eggSize: float

Bird

+sleep(int): void+giveBirth(): Mammal[]

# numTeeth : int

# hasFur: boolean

Mammal

In UML, inheritance relationships are shown with an arrow with aclosed, white arrowhead

The arrow is from the child to the parent and is called an Is-Arelationship

e.g. Bird is an Animal

Notice that the variables/methods inherited are not re-shown in thechild diagrams.


I h i Vi ibili O id P l hi Ab Cl I f


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Structure of a Derived Class

this super

Employee

HourlyEmployee

55.45

765

float hourlyRate

int hoursWorked3482942932

"Mehmet Sarcizmeli"

int idNumber

String name

Every child class is its

own type and also ofits parents type(s).

Creation of a derivedobject involves thecreation of its parent

as well.The child holds itsparents spec withinitself.

e.g. you have yourparents DNA.

Each derived objectrefers to its parentversion using the

reference superMIN 545 - Inheritance & Polymorphism 6of54

I h i Vi ibili O id P l hi Ab Cl I f


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Visibility in Inheritance

Remember the three classes of visibility, in order of permissiveness:

public : Public variables and methods are accessible byeveryone.

protected : Protected variables and methods are only accessible

by the class and its derived classes.

private : Private variables and methods are accessible only bythe class itself.

super and this are alwaysprivate.

super is even more restrictive than private as itcannot be returned in a method or assigned toanother reference. It can only be used directly.


I h it Vi ibilit O id P l m hi m Ab t t Cl I t f


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Exercise

Assume the following case:

class A {p ri va te i nt v A1 ;

p r ot e ct e d f lo a t v A2 ;

p ub li c S tr in g v A3 ;

p ub li c v oi d f A1 ( ) { S ys te m . o ut . p r in tl n ( v A1 ) };

p ro te ct ed int fA2 ( int m ) { r etu rn m * ( int ) vA2 };

p ri va te b oo le an fA 3 () { r et ur n ( vA 1 < vA 2) };

}

cla ss B e xt end s A {

pr iv at e int vB1 = 3;

p ub li c f lo at v B2 ;

}

class C {

p ri va te A vC 1 ;

p ri va te B vC 2 ;

}




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Exercise

+ fA1() : void# fA2(int) : int- fA3() : bool

- vA1 : int# vA2 : float+ vA3 : String

A

- vB1 : int+ vB2 : float

B

- vC1 : A- vC2 : B

C Example

Can we add the following methodto the definition of class C?

p ub li c f lo at f C4 ( )

{

r e tu r n v C1 . v A 2 ;

}

NO. Class C is not a child of

class A. Therefore it cannotaccess protected variables.




10/54


Exercise

+ fA1() : void

# fA2(int) : int- fA3() : bool


A


B

- vC1 : A- vC2 : B

C

Example

Can we add the following methodto the definition of class B?

p u bl i c S t ri n g f B1 ( ){

r e tu r n v A3 ;

}

YES. vA3 is an instance variable

of B inherited from parent classA. It is public so everyone canaccess it.

MIN 545 - Inheritance & Polymorphism 10 of 54



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Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c f lo at f B2 ( ){

r e tu r n v A2 ;

}

YES. vA2 is an instance variable

of B inherited from parent classA. It is protected and B is a childof A, so B can access it.




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Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c i nt f B3 ( ){

r e tu r n v A1 ;

}

NO.vA1 is an instance variable of

B inherited from parent class A.But, it is private and even if B isa child of A, B can not access it.




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y y p

Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c v oi d f C4 ( ){

v C 2 . f A 1 ( ) ;

}

YES. fA1 is a method of B

inherited from parent class A. Itis publicso it can be accessed byobjects of class C.




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Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c i nt f C5 ( i nt a ){

r e tu r n v C2 . f A 2 ( a );

}

NO.fA2 is a protected method

of B inherited from parent classA. Since C is not a child of A, itcannot access fA2.




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Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c i nt f B4 ( i nt a ){

r e tu r n f A2 ( a ) ;

}

YES. fA2 is a protectedmethod

of B inherited from parent classA. Since B is a child of A, it canaccess fA2.




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Exercise

+ fA1() : void# fA2(int) : int- fA3() : bool


A


B

- vC1 : A- vC2 : B

C Example


p u bl i c b o ol e an f B5 ( )

{

r e tu r n f A3 ( ) ;

}

NO.fA3 is a private method of

A. Although B is a child of A, itcan notaccess fA3.




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Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

ExampleCan we add the following methodto the definition of class B?

p ub li c B f B6 ( )

{

r e tu r n t hi s ;}

YES. this is a private variableof B. B can access and return itsown address.However, this is sort of useless...If you can call fB6() on someobject, it means you already haveits reference!




18/54

Exercise

+ fA1() : void



A


B

- vC1 : A- vC2 : B

C

Example


p ub li c A f B6 ( )

{r e tu r n s u pe r ;

}

NO. super is a private variableof B and normally this would be

fine. But super is an exceptionalcase in that B can use super butcan never give it away.




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Overriding Methods

A method is uniquely defined by its nameandparameter list.

This is called the methodssignature(or fingerprint)

A class can define two methods with the same name butdifferent parameters.

Remember, this is calledoverloading

Java will not allow two different methods with the samesignature in the same class.

However a child class can define a new method with the samesignature as that of its parent class.

Assume class A has:

p ub li c v oi d f A1 ( ) { S ys te m . ou t . p ri nt ln ( v A1 ) ; }

Child class B can define:

p ub li c v oi d f A1 ( ) { S ys te m . ou t . p ri nt ln ( v A2 * 2 ); }

This is calledoverriding.




20/54

Example

p ub li c c la ss P er so n {


p u bl i c v oi d g e tN a me ( )

{

r e tu r n n am e ;

}

}

p ub li c c la ss D oc to r e xt en ds P er so n {

@Override


{

r et ur n " D r . " + n am e ;

}

}

Theoverride annotation(@Override) is optional but recommended

The compiler will warn you if your override is not OK

(misspelled name &c).MIN 545 - Inheritance & Polymorphism 20 of 54



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Object: The Mother of All Classes

Java has a predefined class named Object

Every class in Java is a child of the Object class

Even if a class does not extend another class, it is assumed toextend Object.

Two important methods provided by Object:public String toString()Gives a string representation of the object.

System.out.printXXfamily of functions use this if given an

arbitrary object to print.

The default is to a string of the form

< Classname> @ < Address>

public boolean equals(Object other)

The default is to check whether they have the same address.

http://docs.oracle.com/javase/7/docs/api/java/lang/Object.htmlMIN 545 - Inheritance & Polymorphism 21 of 54

http://docs.oracle.com/javase/7/docs/api/java/lang/Object.htmlhttp://docs.oracle.com/javase/7/docs/api/java/lang/Object.html


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Using superfor Overrides

If a method is overridden by the child class, you can use super

to access the parent version.

Example



p u bl i c v oi d g e tN a me ( ){

r e tu r n n am e ;

}

}

p ub li c c la ss D oc to r e xt en ds P er so n {

@Override


{

r et ur n " D r . " + s up er . g e t Na me ( ) ;

}

}




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superin Constructors

The construction of every object requires construction of the parent

object as well.This is usually done automatically by Java, if the parent has adefault constructorWhat if it doesnt?

The child must define a constructor and call the parentconstructor explicitly.

Example



p ub li c P er so n ( St ri ng n am e ) { t hi s . na me = n am e ; }

}p ub li c c la ss D oc to r e xt en ds P er so n {

p ub li c D oc to r ( S tr in g n am e ) { s up er ( n a me ) ; }

}

Like this, super must be the first thing called in a constructor!MIN 545 - Inheritance & Polymorphism 23 of 54



24/54

Case Study: Aircraft

Aircraft

Helicopter Airplane Airport

Airliner MilitaryPlane

B747 F16

Airbase




25/54

Case Study: Aircraft Class

p ub li c c la ss A ir cr af t {

// E ve ry c ra ft h as a u ni qu e r eg is tr at io n ID ( e. g. TC - A CA )

p ub li c s ta ti c f in al i nt M AX _A LT = 5 00 00 ;

p r iv a te S t ri n g r e gI D ;

p r ot e ct e d i nt a l ti t ud e ;

p ub li c A ir cr af t ( St ri ng id ) { r eg ID = id ; }

p ub li c S tr in g g et Re gI D () { r et ur n r eg ID ; }

p ub li c i nt g et A lt it u de ( ) { r et ur n a lt it ud e ; }

p ub li c v oi d s en d Ra di o Ms g ( S t ri ng m sg ) {

S ys te m . ou t . p ri nt f ( " %s t o B as e : % s %n " , r eg ID , m sg ) ;

}

p ub li c b oo le an t ak eO ff ( i nt a lt ) {

if ( alt > MA X_ AL T ) {s en dR ad io Ms g (" Can t go to " + alt + " m ." );

r e tu r n f a ls e ;

}

s e n d R a d io M s g ( " T a k i ng o f f ! " ) ;

a lt it ud e = a lt ;

r e tu r n t ru e ;

}}MIN 545 - Inheritance & Polymorphism 25 of 54



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Case Study: Helicopter Class

p u bl i c c la s s H e l ic o p te r e x te n ds A i rc r af t

{

p u bl i c H e li c o pt e r ( S t r in g r eg I D )

{

s u p e r ( r e g I D ) ;

}

p ub li c b oo le an t ak eO ff ( i nt a lt )

{

S y s t em . o u t . p r i n t l n ( " S t a r t i n g r o t o r . . . " ) ;

r e t u rn s u p er . t a k e Of f ( a l t ) ;

}

}




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Late Binding

Consider the following method invocation:

obj.doIt();

At some point, this invocation is bound to the definition of themethod that it invokes:

At compile time,At initial run,During runtime.

If this binding occurred at compile time, then that line of codewould call the same method every time

However, Java defers method binding until run time this iscalleddynamic bindingorlate binding




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Polymorphism

Polymorphism: having many forms

Apolymorphic referenceis a reference that may refer todifferent objects.

e.g. Every object in Java is of type Object.

Object o may refer to any object of the class Object.

The method invoked through a polymorphic reference canchange from one invocation to the next.

All object references in Java are potentially polymorphic.

Any object can be refered to as its own type or any of itsparent types.Java decides which method to invokeat the last possible step.




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Example

P er so n p1 = n ew P er so n ( " Al i " );P er so n p2 = n ew D oc to r ( " Ba nu " ) ;

S y s t e m . o u t . p r i n t l n ( p 1 . g e t N a m e ( ) ) ;

S y s t e m . o u t . p r i n t l n ( p 2 . g e t N a m e ( ) ) ;

Output:

AliDr. Banu

From Javas perspective, at first, both p1and p2are Person

objects.When getName() needs to be invoked, Java binds theinvocation.

Realizes at that point that p2 is a Doctor




30/54

The instanceofoperator

Type membership can be checked using the instanceof

operator.Expression format:

< object reference> instanceof < class name>

Returns true if the object belongs to the class, false if not.

Example

P er so n p1 = n ew P er so n ( " Al i " );

P er so n p2 = n ew D oc to r ( " Ba nu " ) ;

p 1 i ns t an ce of P er so n ; / / r e tu r ns t ru e ;p 2 i ns t an ce of P er so n ; / / r e tu r ns t ru e ;

p 1 i ns t an ce of D oc to r ; / / r e tu r ns f a ls e ;

p 2 i ns t an ce of D oc to r ; / / r e tu r ns t ru e ;

p 1 i ns t an ce of O bj ec t ; / / r e tu r ns t ru e ;

p 2 i ns t an ce of S tr in g ; / / r e tu r ns f a ls e ;




31/54

Case Study: Airport Class

p u bl i c c la s s A i rp o rt

{

p r iv a te i nt r u n wa y L en g t h ;p r iv a te S t ri n g c od e ;

p ub li c A ir po rt ( i nt r wL en , S tr in g c od e )

{

r un w ay L en g th = r wL en ;

t hi s . c od e = c od e ;}

p u bl i c i nt g e t Ru n w ay L e ng t h ( )

{

r e tu r n r u n wa y L en g t h ;

}

p u bl i c b o ol e an g e t Pe r m is s i on ( A i rc r af t a )

{

i f ( a i n st an c eo f M i li t ar y Pl an e )


r e tu r n t ru e ;

}

}MIN 545 - Inheritance & Polymorphism 31 of 54



32/54

Case Study: Airplane Class

p u bl i c c la s s A i rp l an e e x te n ds A i rc r af t

{

p r iv a te i nt m i nR u nw a y ;

p r iv a te A i rp o rt m y Ai r p or t ;

p ub li c A ir pl an e ( S tr in g r eg ID , i nt m in Ru nw ay )

{

s u p e r ( r e g I D ) ;

t hi s . m i n R un w ay = m i nR u nw a y ;

}

p u bl i c v oi d s e tA i r po r t ( A i r po r t a )

{ m yAirport = a ;

}



C S d A l Cl ( )


33/54

Case Study: Airplane Class (cont.)


{ if ( m yA ir po rt == n ul l ) {

s e n d R a d i oM s g ( " N o a i r p or t ! " ) ;


}

s e n d R a d io M s g ( " S t a r t in g e n g i ne . " ) ;

s e n dR a d io M s g ( " M o vi n g t o r u nw a y . " );

i f ( ! m y A ir p or t . g e t P e r mi s s io n ( t h i s ) ) {

s e n dR a d io M s g ( " No p e rm i s si o n t o t ak e - o f f ! " ) ;


}

i f ( m y Ai rp o rt . g e t R un w ay L en g th ( ) < m in R un wa y ) {s e n dR a d io M s g ( " R u nw a y t oo s h or t ! " ) ;


}


}}



C S d Mili Pl Cl


34/54

Case Study: MilitaryPlane Class

p u bl i c c la s s M i l it a r yP l a ne e x te n ds A i rp l an e

{

p ub li c M i li t ar y Pl an e ( S t ri ng r eg ID , i nt r l )

{

s u pe r ( r eg ID , r l );

}

}



C S d Ai li Cl


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Case Study: Airliner Class

p u bl i c c la s s A i rl i ne r e x te n ds A i rp l an e

{p ub li c A ir li ne r ( S tr in g id , S tr in g M od el , i nt rl )

{

su per ( Mod el + " -" + id , rl );

}


{s e n dR a d io M s g ( " T u r ni n g o n s e at b el t s ig n . " ) ;

i f ( s u pe r . t a k e Of f ( a l t ) )

{

s e n dR a d io M s g ( " T u rn i ng o ff s e at b el t s ig n . " ) ;

r e tu r n t ru e ;

}

else


}

}



C St d B 747


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Case Study: B-747

p ub li c c la ss B 74 7 e xt en ds A ir li ne r

{

p u bl i c B 74 7 ( S t r in g i d )

{

s u pe r ( id , " B 7 47 " , 2 0 00 ) ;

}


{


}

}



C St d F 16


37/54

Case Study: F-16

p ub li c c la ss F 16 e xt en ds M i li t ar y Pl an e

{

p ub li c F 16 ( S t ri ng i d)

{

s up er ( " F16 - " + id , 1 00 0) ;

}


{


}

}



M ltiple Inheritance


38/54

Multiple Inheritance

Sometimes an class will naturally be a subclass of two different

classes.Example

The V-22 Osprey, flies like a plane, lands like a helicopter.

Airplane Helicopter

Osprey

Some programming languages allow this.Multiple Inheritance

In Java, multiple inheritance is not allowed.A class is allowed to extend only one class.Why?



The Problem with Multiple Inheritance


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The Problem with Multiple Inheritance

+ land()

Aircraft

+ land()Airplane

+ land()Helicopter

Osprey

land() ?

Assume that the following:Aircraft class has a method called land(), so all its childrenmust have it.

Airplane and Helicopter have their own overrides ofland()

Osprey has no implementation ofland()

What happens when someone invokes land() on an Ospreyobject?



The Diamond Problem


40/54

The Diamond Problem

+ land()

Aircraft

+ land()Airplane

+ land()Helicopter

Osprey

land() ?

This is called theDiamond ProblemThe Deadly Diamond of Death

It is the major reason why some programming languages

eschew multiple inheritance.In Java, the diamond problem would be very common.

All classes descend from Object.The Object methods (e.g. toString()) would frequentlycause problems.



Abstract Methods and Classes


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Abstract Methods and Classes

Anabstract methodis a method prototype without the

implementation.It is the promiseof a method.

They are declared just like normal methods but with thekeyword abstract and no body. e.g.

p ub li c a bs tr ac t v oi d f oo ( i nt x );

Classes that declare abstract methods cannot be instantiated.

What if somebody actually calls the abstract method?These are calledAbstract Classes

The child of an abstract classmust

override the abstractmethod or also be abstract itself.

Abstract classes are declared using the abstract keyword.

p ub li c a bs tr ac t c la ss H ed e { .. . }



Motivating Problem


42/54

Motivating Problem

Lets say you create the following design:

Ellipse Rectangle Triangle

+ Shape(double,double)+ getWidth() : double+ getHeight() : double

+ getArea() : double

- double width- double height

Shape

height

width

The problem is getArea()

How do you calculate the area of an undefined shape?If only we could leave it for later...



Solution: Abstract Parent Class


43/54

Solution: Abstract Parent Class

Make shape abstractThe child classes will have to deal with calculating the area.

p ub li c a bs tr ac t S ha pe {

p r iv a te d o ub l e w i dt h ;

p r iv a te d o ub l e h e ig h t ;

p ub li c S ha pe ( d o ub le w , d ou bl e h )

{ width = w ; height = h ; }

p ub li c d ou bl e g et Wi dt h ( ) { r e tu rn w id th ; }

p ub li c d ou bl e g et He ig h t () { r e tu rn h ei gh t ; }

/ * T he i m pl em e nt a ti o n of g et Ar ea is l ef t to c hi ld c la ss es */

p u bl i c a b st r ac t d o ub l e g e tA r ea ( ) ;

}



Solution (cont ): Child Classes


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Solution (cont.): Child Classes

p ub li c c la ss R ec t an gl e {

p ub li c R ec ta ng le ( d ou bl e w , d ou bl e h ) { s up er ( w ,h ); }p ub li c d ou bl e g et Ar ea ( ) {

r et ur n g et Wi dt h ( ) * g et H ei gh t ( );

}

}

p ub li c c la ss T ri an gl e {

p ub li c T ri an gl e ( d ou bl e w , d ou bl e h ) { s up er ( w ,h ); }p ub li c d ou bl e g et Ar ea ( ) {

r et ur n 0 .5 * g et Wi dt h ( ) * g et He i gh t ( );

}

}

p ub li c c la ss E ll ip se {

p ub li c E ll ip se ( d ou bl e w , d ou bl e h ) { s up er ( w ,h ); }

p ub li c d ou bl e g et Ar ea ( ) {

r et ur n 0 .2 5 * M at h . PI * g et Wi dt h () * g et He ig ht ( );

}

}



Rules of the Game


45/54

Rules of the Game

You cannot instantiate an abstract class even if it has a

constructor and no abstract methodse.g. Foo f = new Foo() is not allowed if Foo is abstract.Foo is abstract; cannot be instantiated

Concrete (non-abstract) classes cannot declare abstractmethods.

MyClass is not abstract and does not override abstractmethod foo() in MyClass

Concrete classes cannot not override abstract methods of theirparents.

MyChild is not abstract and does not override abstractmethod foo() in MyParent

Abstract methods cannot be private

illegal combination of modifiers: abstract andprivate



Interfaces


46/54

Interfaces

Interfacesare a special form similar to abstract classes that

embody a property or ability of objects:Cannot be instantiated.Cannot have variables.Can only have public abstractmethods.

They are declared using the interface keyword:

Example

p ub li c i n te rf ac e C ol or fu l {

p u bl i c C o lo r g e tC o lo r ( ) ;

p ub li c v oi d s et Co lo r ( C ol or c ) ;

}

Notice that you do not need to declare individual methodsabstract.It is assumed since all methods of an interface are abstract.



Use of Interfaces


47/54

Use of Interfaces

Interfaces sometimes embody the adjectives in a requirements

specification.e.g. ...some kinds types fabric will be colorful.Typically interface names end in -ableor-er

e.g. ConnectionHandler, Comparable, Iterator

Classesimplementthe interfaces:

Example

c la ss C o tt o nF a br i c e xt en ds F ab ri c i m pl em en t s C ol or fu l {

C o lo r c o lo r ;

//...

p ub li c C ol or g et Co lo r ( ) { r e tu rn c ol or }

p ub li c v oi d s et Co lo r ( Co lo r c ) { c ol or = c ;}

}

A concrete class that implements an interface must implementall its methods.



Interfaces and Multiple Inheritance


48/54

p

Since interfaces do not have state or concrete methods, they

do not cause the diamond problem.

Therefore a class can extend one class and implement one ormoreinterfaces

Example

c la s s E m pl o ye e e x te n ds P e rs o n i m pl e m en t s T ax pa ye r , E m pl o y ab l e

c la s s C o mp a ny e x te n ds O r g an i z at i o n i m p le m e nt s T ax pa ye r , E m pl o ye r

c la s s C h ar i ty e x te n ds O r g an i z at i o n i m p le m e nt s E m pl o ye r

c la s s I n te r n e x te n ds P e rs o n i m p le m e nt s E m pl o y ab l e

Therefore Java uses them to simulatemultiple inheritance

Other languages have similar constructs calledMixins



Abstract Classes and Interfaces in UML


49/54

+ Organization(String)+ getName() : String...

# name : String

Organization

+ Charity(String, String)+ getCause() : String..

# cause : StringCharity

+ Company(String)+ getRevenue() : double..

# revenue : double# numEmployees : int

Company

+ Person(String)+ getName() : String

# name : String# age : int# idnumber : long

Person

+ Employee(String)+ getSalary() : int..

# salary : double# ssn

Employee

+ Intern(String)+ getSchool() : String..

# school : StringIntern

+ getNumEmployees() : int

Employer

+ calculateTax() : double

Taxpayer

+ getIDNumber() : long

Employable

Abstract classes and interfaces are marked above their names.

The implementsrelationships are shown with dashed arrows.



Solution to the Osprey Problem


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p y

The problem with the Osprey is that it is a plane in allrespects, except it can take off and land like a helicopter.

Helicopters can do Vertical Take Off and Landing (VTOL) onlyAirplanes can do Convential Take-Off and Landing (CTOL)onlyThe Osprey can do both

So if the Osprey becomes the child of Airplane but also hasVTOL, we solve the problem.

We might encapsulate these into two interfaces:

i nt er fa ce V TO L {

p u bl i c v oi d V T ak e Of f ( ) ;

p u bl i c v oi d V L an d ( ) ;

}

i nt er fa ce C TO L {

p u bl i c v oi d T a ke O ff ( ) ;

p ub li c v oi d L an d ( );

}



Solution to the Osprey Problem (cont.)


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p y ( )

Aircraft

Airplane

Helicopter

Osprey

VTOL

CTOL

The Osprey now inherits its CTOL behaviour from Airplane

It gets the VTOL behviour by implementing VTOL separately.



Polymorphism With Interfaces


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You can have references to interfaces.Can refer to any object implementing that class.

Hence, interface references are polymorphic:

Example

V TO L v [] = ne w V TO L [2 ];

v [ 0] = n ew H e li co p te r ( );

v [1] = n ew O sp re y () ;

CTOL c = ( CTOL ) v [1];

However, CTOL c = (CTOL) v[0]will cause a ClassCastException since v[0] is a

Helicopterand does not implement CTOL.

When referring to an object through an interface reference,you can only call methods defined by that interface.



Interface Hierarchies


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Interfaces can inherit from other interfaces, just like classes.

The child interface inherits all the abstract methods of theparent.

A class implementing the child interface must also implementthe parent interface.

Example

p ub li c i n te rf ac e W al ke r {

p ub li c v oi d w al k ( );

}

p ub li c i n te rf ac e R un ne r e xt en ds W al ke r {p ub li c v oi d r un ( );

}

Anything that implements Runner must have both the walk() andrun() methods.



Summary


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Concrete Class Abstract Class Interface

Normal Methods YES YES NOAbstract Methods NO YES YES

Variables YES YES NOStatics YES YES NO

Constants YES YES YESInstantiation YES NO NO


inheritance polymorph is m

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