15 interface, Study notes of Computer Fundamentals

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Amity School of Engineering

B.Tech., CSE(5th^ Semester)

Java Programming

Topic: Interface

ANIL SAROLIYA

What is Interface?

• Except, How a class does? Interface explains, What a class does?
• In many ways, Interface is very similar to abstract class but it marks with

interface keyword instead of abstract class keyword.

public interface InterfaceName

/ Constant declarations / / only abstract Method signatures /

• Unlike abstract class which can also contain non-abstract methods, interface contain ONLY abstract methods and constants (will be clear in next slide).
• Inside the interface body, its not essential to write abstract keyword(before method name). - Such keyword is implicitly (by default) attached to the respective members.

 Interface can be used as an alternative to fulfill the requirement of multiple inheritance in Java_._

Syntax: Defining an interface

Implementing Interfaces

• Interfaces are used like super-classes who properties are inherited

by classes. This is achieved by creating a class that implements the given interface as follows:

• Note: Interfaces cannot be instantiated—they can only be

implemented by classes(as above) or extended by other interfaces(as below).:

class ClassName implements InterfaceName [, InterfaceName2, …]

// Body of Class

interface NewInterfaceName extends InterfaceName1[,InterfaceName2,.....] { / New Constant declarations / / New abstract Method signatures / }

public class WorkingDog extends Dog implements Worker { private int hoursOfTraining; public void setHoursOfTraining(int hrs) { hoursOfTraining = hrs; } public int getHoursOfTraining() { return hoursOfTraining; } public void work() { speak(); System.out.println("I am a dog who works"); System.out.println("I have " + hoursOfTraining + " hours of professional training!"); } }

Example: The Animal and Dog Classes and a Worker Interface

public abstract class Animal { private String nameOfAnimal; public abstract void speak(); public String getAnimalName() { return nameOfAnimal; } public void setAnimalName(String name) { nameOfAnimal = name; } }

public class Dog extends Animal { public void speak() { System.out.println("Woof!"); } } public interface Worker { public void work(); }

//by default abstract

Animal Worker

Dog

WorkingDog

extends

extends

implements

Output: DemoWorkingDogs Class

OUTPUT: Simon, the Border Collie says Woof! Woof! I am a dog who works I have 40 hours of professional training!

Sophie, the German Shepherd says Woof! Woof! I am a dog who works I have 300 hours of professional training!

• As we all know, an interface is basically a kind of class—it contains

methods and variables, but they have to be only abstract

methods and public, static, final fields/variables.

• Therefore, it is the responsibility of the class that implements an

interface to supply the code for methods.

• In java, a class can implement any number of interfaces, but

cannot extend more than one class at a time. Means no Multiple

Inheritance in Java.

• Therefore, interfaces are considered as an informal way of

realizing multiple inheritance in Java.

Interfaces: An informal way of realizing multiple inheritance

public class DemoDetectiveWorkingDogs { public static void main(String[] args) { DetectiveWorkingDogs aSheepHerder = new DetectiveWorkingDog(); aSheepHerder. setAnimalName ("Simon, the Border Collie"); //access from Animal class aSheepHerder. setHoursOfTraining (40);//access from WorkingDog class

System.out.println(aSheepHerder.getAnimalName() + " says "); aSheepHerder. speak() ;//access from Dog class, declared in Animal class aSheepHerder.work() ;//access from DetectiveWorkingDog class, declared in Worker interface aSheepHerder. speak() ;//access from Dog class, declared in Animal class aSheepHerder. detect() ;//access from DetectiveWorkingDog class, declared in Detective interface System.out.println(); } }

Use of the previous slide classes

Output: DemoWorkingDogs Class

OUTPUT:

Simon, the Border Collie says Woof! Woof! I am a dog who works I have 40 hours of professional training! Woof! Woof! I am also trained for detective work, to help the Police.

Animal Worker

Dog

DetectiveWorkingDog

extends

extends

implements

Detective

implements Informal Multiple Inheritance

DemoDetectiveWorkingDogs

Used by

Output: PizzaDemo Class

OUTPUT: Welcome to Antonio's Pizzeria We are having a special offer: a 12 inch pizza with four ingredients or a 16 inch pizza with one ingredient for only $11. With tax, that is only $12.

Interface Abstract Class

 Multiple inheritance possible; a

class can inherit any number of

interfaces.

 Multiple inheritance not possible; a class

can inherit only one class.

 implements keyword is used to

inherit an interface.

 extends keyword is used to inherit a class.

 By default, all methods in an

interface are public and abstract ;

no need to tag it as public and

abstract explicitly.

 Methods have to be tagged as public and

abstract or both, if required.

 All methods of an interface need

to be overridden.

 Only abstract methods need to be

overridden.

 All variables declared in an

interface are by default public ,

static , or final.

 Variables, if required, have to be declared

as public , static , or final.

 Methods in an interface cannot be

static

 Non-abstract methods can be static.

Interface versus Abstract Class