polymorphism - ocw.udl.cat

Polymorphism

Josep Maria Ribo

[email protected]

February, 2010

J.M.Ribo ([email protected]) Polymorphism February, 2010 1 / 27

ndex

1 Types and classesTypesClasses

2 Notion of polymorphism

3 Abstract classes and interfacesAbstract classesInterfacesAbstract classes vs. Interfaces

4 Conversion and type checking

5 Reflection


Types and classes Types

Types

Type

A type characterizes a collection of objects or values in terms of theirbehaviour.This behaviour is modelled in terms of operations that can be performedon those objects/values.

Examples of types:int, float, char, String, Student, Comparable, Readable,MediaPlayer...

Operations associated to int: +,-,*, /, %

Operations associated to Student: setName(), getName(),enrolToSubject(), getEnrolledSubjects()


Types and classes Types

Types

In Java there exist 2 different kinds of types:

Primitive types: int, float, char, boolean,...Primitive types represent collection of valuesPrimitive types have operations associated that can be applied onthose values.Ex: int: +,-,*,/,%

Reference types:I ArraysI ClassesI Interfaces


Types and classes Classes

Classes

A class is a a reference type with a representation (i.e., a collection ofattributes) and an implementation of its operations in terms of theattributes that constitute its representation.

Objects are instances of classes

Operations are applied to objects.

The representation or the implementation associated to a class can beincomplete. If this is the case, we have abstract classes.


Notion of polymorphism

Notion of polymorphismConsider the following class:

pub l i c c l a s s T e s t P o l y g o n s {vo id showAreaAndPer imeter ( R e g u l a r P o l y g o n p ){

System . out . p r i n t l n ( ”Type o f p o l y g o n : ”+p . t o S t r i n g ( ) ) ;System . out . p r i n t l n ( ” Area=”+p . g e t A r e a ( ) ) ; // (3 )System . out . p r i n t l n ( ” P e r i m e t e r=”+p . g e t P e r i m e t e r ( ) ) ;System . out . p r i n t l n ( ” . . . . . . . . . . . . . . . . . . . . ” ) ;

}// . . . .

pub l i c s t a t i c vo id main ( S t r i n g [ ] a r g s )Square sq = new Square ( 1 2 . 0 ) ;

// sq i s a squa r e w i th edge l e n g t h =12.0

T r i a n g l e t r = new T r i a n g l e ( 6 . 0 ) ;// t r i s a t r i a n g l e w i th edge l e n g t h =6.0

showAreaAndPer imeter ( sq ) ; // (1 )showAreaAndPer imeter ( t r ) ; // (2 )

}// . . . .}J.M.Ribo ([email protected]) Polymorphism February, 2010 6 / 27



Clearly, the following output is expected:

Type of polygon: SquareArea=144.0Perimeter=48.0

........................Type of polygon: TriangleArea=7.8Perimeter=18.0

........................




In order to achieve this behaviour, the instruction:p.getArea() in (3) should get associated with:

Square.getArea() in the first call (with sq)

Triangle.getArea() in the second call (with (tr)).

That is:

The operation call (getArea() ) at line (1) should be associated withdifferent operation bodies according to the runtime type of the object p.




Polymorphism:

Polymorphism is the feature by which:

An object reference in a program code may refer to objects belongingto different classes at different moments throughout the execution ofthat programExample: The parameter p of showAreaAndPerimeter(RegularPolygon p)

refers on one occasion to a Square (1) and on another occasion to a Triangle (2)

AND

An operation call is associated to a different code according to theruntime type of the object on which that operation has been invokedExample: p.getArea() in (3) should get associated with:

I Square.getArea() in the first call (1)I Triangle.getArea() in the second call (2)


Abstract classes and interfaces Abstract classes

Abstract classes

In order to deal with polymorphism we use abstract classes andinterfaces

An abstract class is a class which cannot be instantiatedAbstract classes usually provide a partial implementation (e.g., attributes,some implemented methods).This partial implementation is shared and completed by teir subclasses



Abstract classesabs t rac t pub l i c c l a s s R e g u l a r P o l y g o n {

f i n a l protected double DEFAULTLENGTH =10. ;

protected double edgeLength ;

pub l i c R e g u l a r P o l y g o n ( ){t h i s . edgeLength = DEFAULTLENGTH ;

}pub l i c R e g u l a r P o l y g o n ( double l e n ){

t h i s . edgeLength = l e n ;}pub l i c double getEdgeLength ( ){

re tu rn edgeLength ;}pub l i c vo id se tEdg eLe ngth ( double edgeLen ){

t h i s . edgeLength = edgeLen ;}abs t rac t pub l i c double g e t A r e a ( ) ;

abs t rac t pub l i c double g e t P e r i m e t e r ( ) ;

pub l i c S t r i n g t o S t r i n g ( ){re tu rn ”Edge l e n g t h=”+t h i s . edgeLength+

” Area=”+t h i s . g e t A r e a ()+” P e r i m e t e r=”+t h i s . g e t P e r i m e t e r ( ) ;

}}



pub l i c c l a s s Square extends R e g u l a r P o l y g o n {pub l i c Square ( ){

super ( ) ;}pub l i c Square ( double l e n ){

super ( l e n ) ;}pub l i c double g e t P e r i m e t e r ( ){

re tu rn t h i s . edgeLength ∗ 4 ;}pub l i c double g e t A r e a ( ){

re tu rn Math . pow ( t h i s . edgeLength , 2 ) ;}pub l i c S t r i n g t o S t r i n g ( ){

re tu rn ” Square : ”+super . t o S t r i n g ( ) ;

}}



pub l i c c l a s s T r i a n g l e extends R e g u l a r P o l y g o n {double h e i g h t ;pub l i c T r i a n g l e ( ){

super ( ) ;t h i s . h e i g h t = Math . s q r t (

Math . pow ( t h i s . edgeLength ,2)−Math . pow ( t h i s . edgeLength / 2 , 2 ) ) ;

}pub l i c T r i a n g l e ( double l e n ){

super ( l e n ) ;t h i s . h e i g h t = Math . s q r t (

Math . pow ( t h i s . edgeLength ,2)−Math . pow ( t h i s . edgeLength / 2 , 2 ) ) ;

}pub l i c double g e t H e i g h t ( ){

re tu rn t h i s . h e i g h t ;}



pub l i c double g e t P e r i m e t e r ( ){re tu rn t h i s . edgeLength ∗ 3 ;

}pub l i c double g e t A r e a ( ){

re tu rn t h i s . edgeLength ∗ t h i s . h e i g h t / 2 ;}pub l i c S t r i n g t o S t r i n g ( ){

re tu rn ” T r i a n g l e : ”+super . t o S t r i n g ()+” h e i g h t=”+t h i s . g e t H e i g h t ( ) ;

}}



getArea() and getPerimeter()The operations getArea() and getPerimeter() cannot beimplemented at RegularPolygon since we do not know how tocalculate the area or the perimeter of a polygon without knowingwhich type of polygon it is (square, triangle, circumference...).

toString()Notice the code of the operation toString(). It can calculate thearea and perimeter of a polygon by calling getArea() andgetPerimeter(). However, toString() does not know, at compilationtime, which type of polygon it is dealing with



Subclasses of RegularPolygon: getArea() getPerimeter()definition

The specific subclasses of RegularPolygon will implement theoperations (getArea() and getPerimeter()) with the appropriatecode (square: edgeLength*edgeLength; circumference:PI*radius*radius ...)

Subclasses of RegularPolygon: toString() redefinition

The specific subclasses of RegularPolygon redefine toString() inorder to generate a string that contains particular features of eachspecific type of regular polygon


Abstract classes and interfaces Interfaces

Interfaces

An interface is a reference type that defines the behaviour of the objectsthat have that type in terms of operation headers.

Interfaces are implemented by classes.

When a class implements an interface, it must declare and implement allthe methods whose header have been defined in the interface body

Interfaces cannot:

Define attributes or

Implement the methods defined by them.



Interface examplespub l i c i n t e r f a c e Readab le { // (1 )

pub l i c vo id r e a d ( ) ;}

pub l i c c l a s s Newspaper implements Readab le { // (2 )pub l i c vo id r e a d ( ){

. . . . .}

}

pub l i c c l a s s TestReader {pub l i c s t a t i c vo id readAndComment ( Readab le r ){ // (3 )

. . .r . r e a d ( ) ;. . .

}pub l i c s t a t i c vo id main ( S t r i n g [ ] a r g s ){

Newspaper nwp = new NewsPaper ( . . . ) ;readAndComment ( nwp ) ; // (4 )

}}J.M.Ribo ([email protected]) Polymorphism February, 2010 18 / 27


(1): An interface called Readable with a method read() has been createdThis interface provides the behaviour of reading. Any class that hasto offer this behaviour will implement the Readable interface

(2): The class Newspaper offers the reading behaviour. Hence, itimplements the Readable interface and implements the methodread() according to its specific meaning for the Newspaper classOther classes such as Book, Magazine, CD, DVD... could alsoimplement the Readable interface

(3): An operation ReadAndComment is defined with a parameter of theinterface ReadableAny object which is an instance of a class that implements Readablecan act as a parameter of this operation

(4): readAndComment(..) is called with a Newspaper object



Interface examples (2)

pub l i c i n t e r f a c e P l a y e r {pub l i c vo id p l a y ( ) ;pub l i c vo id s t o p ( ) ;pub l i c vo id pause ( ) ;pub l i c vo id f f ( ) ;pub l i c vo id rew ( ) ;pub l i c vo id s e t V o l ( i n t v o l ) ;

}

pub l i c c l a s s DVDPlayer implements P l a y e r {pub l i c vo id p l a y ( ) { . . . . }pub l i c vo id s t o p ( ) { . . . . }pub l i c vo id pause ( ) { . . . . }pub l i c vo id f f ( ) { . . . . }pub l i c vo id rew ( ) { . . . . }pub l i c vo id s e t V o l ( i n t v o l ) { . . . . }

}



Interfaces

If a class A implements an interface I then A is not a subclass of I. Itmerely implements all the methods defined in IThat is, the class A adheres to the behaviour defined by the interface I

A class may implement several interfaces. This means that A providesthe behaviour defined by ALL those implemented interfacesThis is the way provided by Java to deal with multiple inheritance

There can be hierarchies of interfaces


Abstract classes and interfaces Abstract classes vs. Interfaces

Abstract classes vs. Interfaces

Abstract classes may have animplementation (they usuallyhave one!!!), although it may beincomplete.

Abstract classes are part of aclass hierarchy

Abstract classes must havenon-abstract subclasses thatimplement the operationsdeclared (but not implemented)in them

Interfaces cannot have eitherimplementated operations orattributes . Usually, they onlyhave operation headers

Classes implement interfaces,do not extend themTherefore, classes are notsubclasses of an interface

Classes that implement aninterface declare that they haveimplemented all the operationsdefined in that interface

A class may extend just anotherclass. However, a class mayimplement several interfaces


Conversion and type checking

Downcast

Downcast

a reference type can be converted to a more particular reference type (i.e.,downcasted)

Example:

L i n k e d L i s t <Regu la rPo lygon > l r p = new L i n k e d L i s t <Regu la rPo lygon >;Square sq = new Square ( 1 9 . 0 ) ;

l r p . a d d F i r s t ( sq ) ; // (1 )

Square p = ( Square ) l r p . g e t F i r s t ( ) ; // (2 )

The operation LinkedList<RegularPolygon>.getFirst() returns aRegularPolygon. However, since the RegularPolygon that has beenadded at the begining of the list is a Square (1), it is clear thatgetFirst will get a Square.For this reason, it is possible to downcast lpr.getFirst() to Square,as it is done in (2)



A downcast will throw an exception in the case that an object isdowncasted to the wrong type:

The following code would have led to a ClassCastException :

t r y {L i n k e d L i s t <Regu la rPo lygon > l r p = new L i n k e d L i s t <Regu la rPo lygon >;Square sq = new Square ( 1 9 . 0 ) ;

l r p . a d d F i r s t ( sq ) ;

T r i a n g l e p = ( T r i a n g l e ) l r p . g e t F i r s t ( ) ;}catch ( C l a s s C a s t E x c e p t i o n e ) { . . . . t h i s code w i l l be e x e c u t e d . . . }



Type checking

The runtime type of an object can be checked:

T r i a n g l e p ;

i f ( l r p . g e t F i r s t ( ) i n s t anceo f T r i a n g l e )p = ( T r i a n g l e ) l r p . g e t F i r s t ( ) ;


Reflection

Reflection

The Java class Class<T> is a special class such that any other class canbe seen as an instance of itThat is, a class can be seen as an object instance of Class<T>

In other words, Class<T> is a metaclass (a class, whose instances areclasses themselves)Two operations of Class<T>:

static Class<?> forName(String className)

Returns the Class object associated with the class or interface withthe given string name

boolean isInstance(Object obj)

Determines if the specified Object is an instance of this Class


Reflection

Reflection

Example:

c l a s s A {}

pub l i c c l a s s R e f l e c t i o n E x a m p l e {pub l i c s t a t i c vo id main ( S t r i n g [ ] a r g s ){

t r y {C l a s s c l s = C l a s s . forName ( ”A” ) ;i f ( c l s . i s I n s t a n c e (new A ( ) ) . . . .

}catch ( Throwable e ) {

System . e r r . p r i n t l n ( e ) ;}

}}


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