Difference between revisions of "Java/C3/Subclassing-and-Method-Overriding/English"

• subclassing
• extends keyword and
• method overriding

Software Requirements

• Ubuntu Linux version 12.04
• JDK 1.7
• Eclipse 4.3.1

Prerequisites

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Subclassing

• It is a way to create a new class from an existing class.

• The new class created is called subclass or derived class or child class.

• The already existing class is called superclass or base class or parent class.

public class Employee {

public String name="";

public String email_address;

public void setName(String newName) {

name=newName;

}

public String getName() {

return name;

}

public void setEmail(String newEmail) {

email_address=newEmail;

}

public String getEmail() {

return email_address;

}

public String getDetails()

{

return("Name: " + getName() + "\n" + "Email: " + getEmail());

}

}

This method returns the name and email_address.

public class Manager {

public String name="";

public String email_address;

public String department="";

public String name="";

public String email_address;

name and email_address are there in Employee class.

We can see that its also there in Manager class.

extends Employee

So, remove name and email_address from Manager class.

Also remove setter and getter methods of the same.

public void setDepartment(String newDepartment) {

department=newDepartment;

}

public String getDepartment() {

return department;

}

public class TestEmployee {

public static void main(String[] args) {

Manager manager = new Manager();

So inside the main method, type

Manager manager equal to new Manager parentheses

manager.setName("Nikkita Dinesh");

So type,

manager dot setName within brackets and double quotes Nikkita Dinesh

manager.setEmail("abc@gmail.com");

manager dot setEmail within brackets and double quotes abc at gmail dot com

manager.setDepartment(“Accounts”);

manager dot setDepartment within brackets and double quotes Accounts

You can use any name, email address and department.

System.out.println(manager.getDetails());

So, type

System.out.println within brackets manager dot getDetails

Name: Nikkita Dinesh

Email: abc@gmail.com

Name: Nikkita Dinesh

Email: abc@gmail.com

This is because, the Manager class extends the Employee class.

The Manager class automatically inherits the variables and methods of the Employee class.

Highlight the return statement.

Let us come to the Employee class.

It finds the getDetails() method here.

Note that we have not returned the department.

As a result, it did not print the department in the output.

Save the file.

Private members in a superclass

• A subclass does not inherit the private members of its superclass.
• Subclass cannot directly access the private members of the superclass.
• However, the superclass can have public or protected methods.
• These methods can access its private fields.
• Then the subclass can also access the private fields through these methods.

public String getDetails()

{

return ( "Name: " + getName() + "\n" + "Email: " +getEmail()+"\n"

+"Manager of: " + getDepartment());

}

So type,

public String getDetails parentheses.

Inside the method, type

return within brackets Name plus getName() plus slash n plus Email plus getEmail() plus slash n plus Manager of plus getDepartment() semicolon

Save the file.

public String getDetails()

Method overriding

• name
• return type
• argument list

matches exactly.

This is an override annotation.

It indicates that a method is intended to override a method in superclass.

Now, let us see what annotation is.

Annotation

• start with at sign character(@)
• provide data about a program
• have no direct effect on the operation of the code.

@Override Annotation

• the method does override a method declared in a superclass.
• the method signature is different in its superclass.

Highlight the annotation.

Come back to the Manager class.

The at sign character indicates the compiler that what follows is an annotation.

Here, it shows that the getDetails method is overridden.

Name: Nikkita Dinesh

Email:abc@gmail.com

Manager of:Accounts

Name: Nikkita Dinesh

Email: abc@gmail.com

Manager of Accounts

But this time, it calls the method of Manager class itself.

In this way, we override the method of the parent class by the subclass.

Summary

In this tutorial we have learnt about

• Subclassing and
• Overriding

Assignment

Create a class Vehicle which has a method run that prints “The Vehicle is running.”

Also create a class Bike which has a method run that prints

“The Bike is running safely.”

The output should be “The Bike is running safely.”

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Acknowledgement

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This is Arya Ratish from IIT Bombay signing off.

Thanks for joining.