Difference between revisions of "Java/C3/Java-Interfaces/English"

Learning Objectives

• Creating an interface
• Creating an Implementation class and
• Usage of Interfaces

Software Requirements

• Ubuntu 12.04
• JDK 1.7 and
• Eclipse 4.3.1

Prerequisites

You should also have the knowledge of subclassing and Abstract classes in Java.

If not, for relevant Java tutorials, please visit the link shown.

What is an interface?

It defines the signatures of a set of methods, without the body.

It is declared using the interface keyword.

Implementing the interface

A class can implement more than one interface.

All the abstract methods defined in the interface or interfaces must be implemented in such classes.

Interface Vs Abstract Classes

Within interface there should not be any

• constructors
• concrete methods
• static methods and
• main method

But an abstract class can have all these inside it.

The variables in an interface should be static and final.

There are no such restrictions for an abstract class.

Interface:

Example:

Highlight the methods

It contains three abstract methods talk(), see() and move().

All such methods in an interface are implicitly public and abstract.

An interface can also contain constant variable declarations.

Here, the constant string value “Mammal” is assigned to the variable “type1”.

And “Reptiles” is assigned to the variable “type2”.

All constant values defined in an interface are implicitly public, static and final.

Implementation class:

Example:

So it must provide implementations for the methods talk(), see() and move().

Implementing multiple interfaces

Example:

Highlight the syntax and the usage of comma operator

As shown in the example, the class Human implements two interfaces Animal and Action.

Note that a comma operator used in the syntax is to identify the different interfaces.

Now this class should provide implementations to all the abstract methods in both Animal and Action interfaces.

Interface Usage - Example

The Animal class is an interface.

The Human and Snake classes are the two implementation classes.

The Human class provides its own different implementations for talk(), see() and move() methods.

And the Snake class provides its own different implementations for talk(), see() and move() methods.

Here we will create the necessary classes and interface to demonstrate the usage of interfaces.

Code is copied

Type the name as Animal and press Enter.

Note that the “interface” keyword is used for defining an interface.

Now type the code as displayed on the screen.

Type implements Animal

Now to make this an implementation class of Animal, type implements Animal.

This error indicates that, we should provide an implementations to the Animal interface.

System.out.println("I am a Human, I belong to the "+Animal.type1+" Family");

}

public void see(){

System.out.println("I CAN SEE ALL COLORS ");

}

public void move(){

System.out.println("I MOVE BY WALKING");

}

So type public void talk( )

Within curly brackets type

System.out.println within quotes I am a human and I belong to

Now we can use the value of the static, final variable type1 declared in the Animal interface

So type + Animal.type1+ within quotes family semicolon.

Let us now implement the see() method

So type public void see( )

Within curly brackets type

System.out.println within quotes I can see all colors semicolon

We should also define the move() method

So type public void move( )

Within curly brackets type

System.out.println within quotes I move by walking semicolon

Note that the error disappears, once all the methods are implemented.

Please create the snake class in your project and type the following code as displayed on the screen.

Now let us go through the code.

public void talk(){

System.out.println("I am a Snake, I belong to the "+Animal.type2+" Family");

}

public void see(){

System.out.println("I CAN SEE ONLY IN BLACK AND WHITE");

}

public void move(){

System.out.println("I MOVE BY CRAWLING");

}

Here the talk() method prints “I am a snake and I belong to

Then the value of Animal.type2 is to be printed and then “family”

Here the see() method prints “I can see only in black and white”

The move() method prints "I move by crawling"

Highlight the talk(), see() and move() methods of Snake class

And the Snake class has its own implementations of talk(), see() and move() methods.

Inside this class, we will have the main method,

So type main and then press ctrl+space to generate the main method.

Highlight the second line

Animal h=new Human();

h.talk()

h.see();

h.move();

In this line, we instantiate the Human class using the Animal interface.

This is represented as Animal h equals new Human();

Now we can invoke the different methods using this object as

h.talk()

h.see();

h.move();

Animal s =new Snake();

s.talk();

s.see();

s.move();

This is represented as Animal s equals new Snake()

Now we can invoke the different methods using this object as

s.talk();

s.see();

s.move();

I am a human and I belong to Mammal family

I can see all colors and

I move by walking

These are printed by the talk(), see() and move() methods invoked using the Human class object h.

I am a snake and I belong to Reptiles family

I can see only in black and white

and I move by crawling

These are printed by the talk(), see() and move() methods invoked using the Snake class object s.

Summary

• Creating an Interface
• Creating an implementation class and
• Usage of interfaces

Assignment

Create an interface Vehicle which contains the methods brake() and run().

Create another interface Fuel which contains the following methods.

fill(String type, int quantity)

pay(int quantity, int price)

Here brake method should print ”Car, Applies Power brake” and the run method should print ”Car is running on 4 wheels”.

For example: 10 Litres of Petrol

pay() method can be used to print the price to be paid.

For example: Pay Rs.640

Vehicle and Fuel.

Here brake method can print ”Bike, Applies hand brake” and the run method can print “Bike is running on 2 wheels”.

Finally, create a Demo class containing the main method to verify the results.

Please download and watch it.

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