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.

Abstract Method

Example:

Highlight the method and abstract keyword

Note that this method is declared using the abstract keyword, with no current implementation.

Concrete Method:

Example:

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

Observe that this method is implemented completely.

Abstract Class:

Abstract methods can be added only to abstract classes.

Abstract Class - Example

It contains an abstract method called showDetails().

It can also contain variables and other concrete methods.

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.

Switch to Slide 10

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.

Concrete Class:

ExampleSlide 10

The class Employee illustrated here, is a concrete class.

All the methods inside this class including showDetails() are concrete.

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

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

Type

String name;

int age;

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);

So type public void showBasicDetails( ).

Within brackets type,

System.out.println within round brackets and within quotes type Name colon plus name semicolon

Also type

System.out.println within round brackets and within quotes Age colon plus age semicolon

public void showDetails( );

So type public void showDetails( ) semicolon

Point to the error

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

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.

So type

System.out.println within quotes Emp id colon plus empid semicolon

System.out.println within quotes Salary colon plus salary semicolon

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().

Click on new-> class and then type name as Demo.

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”);

Person p equals new Person.

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

Remove the line

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

Let's 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 ageE267 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.

• Hari is passed as the value of student name,

• 20 as the value of age.

• 12005 as the value of student Reg No. and

• A as the value of grade.

Now we can call the showBasicDetails() method using the object p2 as p2.showBasicDetails()

We can also invoke the showDetails() method using the object p2 as p2.showDetails()

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”
3. Also create asubclass Bike which again extends the Vehicle class and implements the run method that prints “Bike is running on 2 wheels”
4. Also create a Demo class containing the main method to verify the results.

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