Difference between revisions of "Java/C3/Abstract-Classes/English"

Learning Objectives

• Abstract Methods and Concrete Methods
• Abstract Classes and Concrete Classes and
• How to use Abstract Classes

Software Requirements

• Ubuntu 12.04
• JDK 1.7 and
• Eclipse 4.3.1

Prerequisites

You should also have the knowledge of subclassing in Java.

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

What is an Abstract Method?

It is declared using abstract keyword

There should not be opening and closing parenthesis for this method.

Inside this project we will create the necessary classes to demonstrate the usage of Abstract class.

Type

String name;

int age;

Type the name of the class as Person and press Enter.

Now we will add the fields to represent the name and age of the Person.

Type String name semicolon.

Also type int age semicolon.

Delete the super keyword from the generated code.

public void showBasicDetails( )

{

System.out.println("Name:"+name);

System.out.println("Age:"+age);

A Concrete method is completely implemented within the curly brackets.

We will add a concrete method to this class to print the name and age.

Type the following code as displayed on the screen.

This method showBasicDetails() illustrated here is an example of concrete method.

Observe that this method is implemented completely.

public void showDetails( );

Point to the error

So type public void showDetails( ) semicolon.

An error comes up, since we have not yet added the abstract keyword.

Type the keyword abstract

Highlight in the video

This is because, abstract methods can be added only to abstract classes.

So now add the keyword abstract to the Person class to make it an abstract class.

The class Person illustrated here is an abstract class.

It contains an abstract method called showDetails().

Abstract Class - Example

Here, the Person class is an abstract class.

The Employee class and the Student class are subclasses of the Person class.

These subclasses can provide their own different implementations.

For example:

ShowDetails() Method in the Employee class prints the Employee ID and the Salary, where as

ShowDetails() Method in the Student class prints the Student Reg No and the Grade.

Type extends Person

Now to make this a subclass of Person class, type extends Person.

It indicates that, we should provide an implementation to the abstract method showDetails( ).

We will do it a little later.

String empid semicolon;

int salary;

So type String empid semicolon

and int salary semicolon;

This constructor can initialise the values of name, age, empid and salary.

public void showDetails()

{

System.out.println("Emp id"+empid);

System.out.println("Salary:"+salary);

}

So type public void showDetails( ).

Inside this method, we need to print the employee details.

Type the following code as displayed on the screen. (5.28 from 2.20)

Note that the error disappears, once the showDetails() method is implemented.

This constructor can be used to initialize the values for name, age, regno and grade.

public void showDetails()

{

System.out.println("Student regno:"+regno);

System.out.println("Student grade:"+grade);

}

It prints the values of Student Reg. Number and grade

Highlight showDetails() method of student class

And the Student class has its own implementation of showDetails().

Inside this class we will have the main method,

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

Person p =new Person(“John”);

Point to the error

Remove the line

Person p equals new Person.

Within brackets and double quotes type John and put a semicolon.

Now we can see an error.

This is because the Person class is abstract and it cannot be instantiated.

Lets remove this line.

Person p1=

new Employee("John",40,"E267",10000);

p1.showBasicDetails();

p1.showDetails();

Highlight the code line by line

Person p2=

new Student("Hari",20,"12005",'A');

p2.showBasicDetails();

p2.showDetails();

Now let us instantiate the Person class using the Employee class as Person p1 equals new Employee.

In the first line, we are passing the values of different arguments.

John is passed as Employee name,

40 as the value of age E267 as the value of Employee ID and

10000 as the value of Employee salary.

Now we can invoke the concrete method in the Person class as p1.showBasicDetails()

We can also call the showDetails() method using the object p1 as p1.showDetails()

Similarly instantiate the Person class using the Student class.

This is represented as Person p2 equals new Student.

Now we are passing the values of different arguments.

We can invoke the showBasicDetails() method and showDetails() method using the object as shown.

So right click on the class Demo and then select Run as -> Java Application

These are printed by showBasicDetails() method.

Other Employee details like employee ID and salary are printed by the showDetails() method.

Similarly the basic student details like name and age are printed by showBasicDetails() method.

Other details of the student like Student reg no and grade are printed by the showDetails() method.

Summary

• Abstract Methods and Concrete Methods
• Abstract Classes and Concrete Classes and
• How to create and use Abstract Classes

Assignment

1. Create an abstract class Vehicle which contains an abstract method run()
2. Create a subclass Car which extends the Vehicle class and implements the run method that prints

“Car is running on 4 wheels”

1. Also create a subclass Bike which again extends the Vehicle class and implements the run method that prints

“Bike is running on 2 wheels”

1. Also create a Demo class containing the main method to verify the results.

Please download and watch it.

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