JAVAL Interfaces

8/4/2019 JAVAL Interfaces

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Java InterfacesJava Interfaces

Similar to a class definitionSimilar to a class definition

Used mainly for function prototypesUsed mainly for function prototypes Abstract method declarationsAbstract method declarations

Public and abstract by default!Public and abstract by default!

Can also include:Can also include:

constants (public static finals)constants (public static finals)


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InterfacesInterfaces

Using interface you can fully abstract a classUsing interface you can fully abstract a class

interface from its implementation.interface from its implementation.

Using interface, you can specify what a classUsing interface, you can specify what a class

must do, but not how it does them.must do, but not how it does them.

Interfaces are syntactically similar to classes, butInterfaces are syntactically similar to classes, but

they lack instance variables, and their methodsthey lack instance variables, and their methods

are declared without any body.are declared without any body.


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Java does not support the inheritance of multipleJava does not support the inheritance of multiple

super classes into a single sub class.super classes into a single sub class.

You can only specify one super class for anyYou can only specify one super class for any

subclass that you create.subclass that you create. To realize multiple inheritance in Java,To realize multiple inheritance in Java,

interfaces are used.interfaces are used.


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One class can implement any number ofOne class can implement any number of

interfaces.interfaces.

To implement an interface, a class must createTo implement an interface, a class must create

the complete set of methods defined by thethe complete set of methods defined by the

interface.interface.

Each class is free to determine the details of itsEach class is free to determine the details of its

own implementation.own implementation.


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//define an integer stack interface//define an integer stack interface

InterfaceInterface IntStackIntStack {{// store an item// store an item

voidvoidpush(intpush(int item);item);

//retrieve an item//retrieve an item

intintpop();pop();

}}


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Access specifier of an interface is eitherAccess specifier of an interface is either

publicpublic or not specified.or not specified.

If access specifier is not specified, it isIf access specifier is not specified, it is

considered to be default, which isconsidered to be default, which ispackagepackage..

Each class that implements an interfaceEach class that implements an interface

must implement all of the methods.must implement all of the methods.


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Partial ImplementationPartial Implementation

If a class includes an interface but does not fullyIf a class includes an interface but does not fully

implement the methods defined by that interface,implement the methods defined by that interface,then that class must be declared asthen that class must be declared as abstractabstract..

abstract class MyStack implements IntStack{

abstract void push(int item);

int pop() {

System.out.println(in pop..);

}

}


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classclass FixedStackFixedStack implementsimplements IntStackIntStack {{

privateprivate intint stckstck[];[];

privateprivate intint tostos;;

public voidpublic voidpush(intpush(int item){item){

System.out.println(System.out.println(InInpushpush););

}}

publicpublic intintpop(){pop(){

System.out.println(System.out.println(InInpoppop););

}}

}}


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When you implement an interface method, itWhen you implement an interface method, it

must be declared asmust be declared aspublicpublic..

Interface reference variables can be used toInterface reference variables can be used to

refer to objects of class type that implement therefer to objects of class type that implement the

interface.interface.

An interface reference variable only has theAn interface reference variable only has the

knowledge of the methods declared by itsknowledge of the methods declared by itsinterface declaration.interface declaration.


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Advantage of InterfacesAdvantage of Interfaces

(over inheritance)(over inheritance)

Any class can implements the interfaceAny class can implements the interface

(i.e., provides the contract(i.e., provides the contracts functionality)s functionality)can be usedcan be used

Not constrained by sub classingNot constrained by sub classing


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Interface InheritanceInterface Inheritance

A commitment to implement a contractA commitment to implement a contract

No implementation is inheritedNo implementation is inherited

Disadvantage:Disadvantage:

No code sharingNo code sharing

Advantage:Advantage:

No code commitmentNo code commitment Freedom to implement any way you wantFreedom to implement any way you want


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Variables in InterfacesVariables in Interfaces

Variables can be declared inside of interfaceVariables can be declared inside of interface

declarations.declarations.

They are implicitlyThey are implicitly finalfinal andand staticstatic, meaning, meaning

they cannot be changed by the implementingthey cannot be changed by the implementing

class.class. They must also be initialized with a constantThey must also be initialized with a constant

value.value.

This concept is used to import shared constantsThis concept is used to import shared constantsinto multiple classes by declaring an interfaceinto multiple classes by declaring an interfacethat contains variables initialized to a desiredthat contains variables initialized to a desired

value.value.


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Extending InterfacesExtending Interfaces

One interface can inherit another by the use ofOne interface can inherit another by the use of

the keywordthe keyword extendsextends.. The net result is just the union of all theThe net result is just the union of all the

method specificationsmethod specifications

When a class implements an interface thatWhen a class implements an interface that

inherits another interface, it must provideinherits another interface, it must provide

implementations for all methods defined withinimplementations for all methods defined withinthe interface inheritance chain.the interface inheritance chain.


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interface Ainterface A

{{void test();void test();

}}

interface Binterface B extendsextendsAA

{{

void test1();void test1();

}}


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class inter implements B {class inter implements B {

public void test(){public void test(){

System.out.println("inSystem.out.println("in test");test");

}}

public void test1(){public void test1(){

System.out.println("inSystem.out.println("in test1");test1");

}}

public static void main(Stringpublic static void main(String argsargs[]){[]){

inter a = new inter();inter a = new inter();

a.test();a.test();

a.test1();a.test1();

}}


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Interfaces vs. Abstract ClassesInterfaces vs. Abstract Classes

Use Abstract Classes when there is someUse Abstract Classes when there is some

implementation to shareimplementation to share In C++, an abstract class with only pureIn C++, an abstract class with only pure

virtual functions and no implementationvirtual functions and no implementationbehaves as an interfacebehaves as an interface


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Why PolymorphismWhy Polymorphism

Many implementations and commonMany implementations and common

InterfaceInterface


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Interfaces in the Java LibraryInterfaces in the Java Library

ComparableComparable::

publicpublic intint compareTo(ObjectcompareTo(Object x)x) Like CLike Css strcmpstrcmp, returns:, returns:

Negative, if this < xNegative, if this < x Zero if this.equals(x)Zero if this.equals(x)

Positive, if this > xPositive, if this > x

You decide how the ordering worksYou decide how the ordering works

Used throughout the libraryUsed throughout the library


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Employee {

Empid

fixed

Virtual Calyearlysal(){return 12*fixed;}}

Adhoc: public Employee {

Wageperday

No of days worked

Calyearlysal(){

return Employee::calyearlysal+(wagesperday*noofdays);}}

Permanent emp :public employee{

Yearlybonus

calyearlysalary(){Return employee::calyearlysal+yearlybonus;}}


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Int Caltotalsal(emp * e[3])

{

totsal=0;

For(i=0;icalyearlysal();}

Return totsal;

}

Main()

{Employee *e[3];

e[0]=new employee(1,200);

e[1]=new adhoc(1,200,22,300);

e[2]= new permanent(1,200,4000);

}


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What If the classes cannot be related

by inheritance hierarchy??


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public interface Measurable{

double getMeasure();}


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Object Class

All other classes are subclasses of Object classThe reference variable of type Object can refer to an object of

any other class.

It defines the following methods

Object clone()

Boolean equals(Object obj);

Class getClass();

Int hashcode()

String toString();

Etc.


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ExampleExamplePrint (object x)

{

System.out.print(x);

x.display();

}

Main(){

Account a=new account(2,1000);

student s=new student(1,abc);

print(a);

Print(s);

}


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Comparing FractionsComparing Fractions

( 0, 0)

a c

b d

ad bc

b d

> >

compareTo( ) should return ad - bc


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Interfaces in the Java LibraryInterfaces in the Java Library

CloneableCloneable

For copying objectsFor copying objects

SerializableSerializable

For automatic object storage and retrievalFor automatic object storage and retrieval CollectionCollection

Basic contract for collectionsBasic contract for collections


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Abstract classesAbstract classes

Suppose it is required to create a super classSuppose it is required to create a super class

that only defines a generalized form that will bethat only defines a generalized form that will beshared by all of its subshared by all of its sub--classes, leaving it toclasses, leaving it to

each subclass to fill in the details.each subclass to fill in the details.

Such a class determines the nature of theSuch a class determines the nature of the

methods that the subclasses must implement.methods that the subclasses must implement.

This can be done by creating abstract classesThis can be done by creating abstract classes

containing abstract methods.containing abstract methods.


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An abstract class cannot be directly instantiatedAn abstract class cannot be directly instantiated

with thewith the newnew operator.operator.

Any subclass of an abstract class must eitherAny subclass of an abstract class must either

implement all of the abstract methods in theimplement all of the abstract methods in thesuper class or be itself declared abstract.super class or be itself declared abstract.


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abstract class Figure{abstract class Figure{

abstract double area();abstract double area();

void exist(){void exist(){

System.out.println(System.out.println(FigureFigure existexist); }); }

}}

class Rectangleclass Rectangle extendsextends Figure {Figure {

double area() {double area() {

System.out.println(System.out.println(AreaArea CalculateCalculateherehere););

}}

}}


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Abstract classes can consist of methodsAbstract classes can consist of methods

that are not abstract.that are not abstract.

A subA sub--class that does not implementclass that does not implement allall

the abstract methods of its super class,the abstract methods of its super class,

will be abstract.will be abstract.

Abstract classes can be used to createAbstract classes can be used to create

object references.object references.


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abstractabstract class Figure{class Figure{

double dim1;double dim1;double dim2;double dim2;

Figure(double a, double b){Figure(double a, double b){

dim1 = a;dim1 = a;

dim2 = b;dim2 = b;

}}

abstractabstract double area();double area();

}}


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class Rectangleclass Rectangle extendsextends Figure {Figure {

Rectangle(double a, double b) {Rectangle(double a, double b) {

super(a,b)super(a,b)

}}

double area() {double area() {System.out.println(System.out.println(InsideInside rectanglerectangle););

return (dim1 * dim2);return (dim1 * dim2);

}}

}}


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classclassAbstractDemoAbstractDemo {{

public static void main(Stringpublic static void main(String argsargs[])[])

{{

//Figure f = new Figure(10,10);//Figure f = new Figure(10,10);

Rectangle r = new Rectangle(9,6);Rectangle r = new Rectangle(9,6);

FigureFigure figreffigref;;

//reference, no object created//reference, no object created

figreffigref = r;= r;

System.out.println(System.out.println(AreaArea isis+figref.area+figref.area());());

}}

}}

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JAVAL Interfaces