View source for Java/C3/Subclassing-and-Method-Overriding/English-timed


{| border=1
| <center>'''Time'''</center>
| <center>'''Narration'''</center>

|-
| 00:01
| Welcome to the '''spoken-tutorial''' on''' Subclassing''' and '''Method overriding.'''

|-
| 00:06
| In this tutorial, we will learn about:
* '''subclassing'''
* '''extends '''keyword and
* '''method overriding'''.

|-
| 00:15
|Here we are using:
* '''Ubuntu Linux '''version 12.04
* '''JDK '''1.7
* '''Eclipse '''4.3.1

|-
| 00:25
|To follow this tutorial, you must have knowledge of basics of '''Java''' and '''Eclipse IDE.'''

|-
| 00:32
| If not, for relevant '''Java '''tutorials, please visit our website.

|-
| 00:37
| First of all, we will learn what '''subclassing '''is.

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| 00:41
| It is a way to create a new '''class''' from an existing '''class.'''

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| 00:46
| The new '''class''' created is  '''subclass '''or''' derived class '''or''' child class.'''

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| 00:53
| The already existing '''class''' is called '''superclass '''or''' base class '''or''' parent class.'''

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| 01:00
| Now, let me show you how to create a '''subclass'''. I have already created a '''project''' named '''MyProject'''.

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| 01:10
| I have created a '''class ''' in it named '''Employee.'''

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| 01:15
| It has '''variables, name '''and '''email_address.'''

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| 01:19
| It also has the '''setter '''and '''getter''' '''method'''s for the '''class.'''

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| 01:24
| It has a '''method''' "getDetails()". This '''method '''returns  '''"name" '''and''' "email_address".'''

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| 01:31
| Now, let us come to the '''Manager class'''.

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| 01:35
| It has '''variables, name, email_address '''and '''department.'''

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| 01:40
| We can see that some '''variables''' are common to both '''Employee '''and '''Manager class.'''

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| 01:47
| '''name '''and''' email_address '''are present in the '''Employee class.''' We can see that it is also there in '''Manager class.'''

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| 01:57
| Thus, '''Manager class '''can be made a '''subclass''' of '''Employee class.'''

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| 02:03
| For that, we have to make some changes in the '''Manager class.'''

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| 02:08
| After '''public class Manager, '''type: '''extends Employee'''.

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| 02:14
| We use the '''extends''' keyword to create a '''subclass '''from an existing '''class.'''

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| 02:21
| Remove the duplicate '''variables '''common to both the '''class'''es.

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| 02:26
| So, remove '''name '''and''' email_address '''from '''Manager class.'''

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| 02:32
| Also remove '''setter '''and '''getter''' '''method'''s of the same.

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| 02:37
| In the '''class 'Manager', '''we will have only one variable '''department.'''

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| 02:43
| We also have the '''setter '''and '''getter method '''for '''department.'''

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| 02:59
| In this way, the ''''Manager' class '''inherits the members of ''''Employee' class.'''

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| 02:55
| This way of extending one '''class '''from another is called '''single inheritance.'''

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| 03:02
| I have also created another '''class '''named ''''TestEmployee'.'''

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| 03:08
| Inside the '''main''' method, we will create the '''object '''of  '''Manager''' class.

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| 03:14
| So, inside the '''main method''', type: '''Manager manager''' equal to '''new Manager''' parentheses. 

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| 03:23
| Next, we will call the '''setter''' methods of the '''Manager''' class.

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| 03:28
| So, type, '''manager dot setName''' within brackets and double quotes '''Nikkita Dinesh '''

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| 03:38
| Then type: '''manager''' dot '''setEmail''' within brackets and double quotes '''abc at gmail dot com'''. 

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| 03:49
| Then type, '''manager dot setDepartment''' within brackets and double quotes '''Accounts'''.

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| 03:57
| You can use any '''name, email address''' and '''department'''.

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| 04:02
| Now, let us call the '''getDetails()''' method using the '''Manager''' object.

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| 04:08
| So, type: '''System.out.println '''within brackets '''manager dot getDetails'''.

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| 04:17
| Now, let us '''save''' and '''run''' the program.

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| 04:21
| We can see that we get the output as ''':'''
'''Name: Nikkita Dinesh'''
'''Email: abc@gmail.com'''

|-
| 04:30
| Here, the object of the ''''Manager' class '''calls the '''getDetails()''' method.

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| 04:36
| Now, come to the '''Manager''' class.

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| 04:39
| We can see that there is no '''getDetails()''' method here.

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| 04:43
| But, we still got the output. This is because, the '''Manager class extends '''the '''Employee class.'''

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| 04:52
| The '''Manager''' class automatically inherits the variables and '''method'''s of  '''Employee''' class.

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| 04:59
| So, it checks in the '''parent class '''which is '''Employee.'''

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| 05:04
| Let us come to the '''Employee''' class. It finds the '''getDetails()''' method here.

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| 05:11
| Note that we have not returned the '''department.''' As a result, it did not print the '''department '''in the output. 

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| 05:20
| Now, let us change the '''getDetails''' method  to '''private.''' '''Save''' the file.

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| 05:27
| We can see that we get a compilation error in the '''TestEmployee class.'''

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| 05:34
| It says "The method getDetails() from the type Employee is not visible".

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| 05:40
| This means that '''getDetails()''' method cannot be accessed.

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| 05:45
| This is because we have declared '''getDetails()''' method as 'private'.

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| 05:52
|  A '''subclass''' does not inherit the '''private''' members of its '''superclass'''.

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| 05:58
| '''Subclass '''cannot directly access the '''private''' members of the '''superclass.'''

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| 06:04
| The superclass can have '''public''' or '''protected''' methods.

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| 06:09
|  These '''method'''s can access their '''private''' '''field'''s.

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| 06:13
|  The '''subclass''' can also access the '''private''' '''field'''s through these '''method'''s.

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| 06:18
| So, let us change it back to '''public.'''

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| 06:21
| Now, let us include the method '''getDetails '''in the '''Manager''' class.

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| 06:27
| This '''method '''will return '''name, email_address '''and '''department.'''

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| 06:33
| So, type: '''public String getDetails''' parentheses.

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|  06:39
| 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.

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| 07:07
| Note that now, we have the '''method getDetails '''in both '''Manager''' and '''Employee  class.'''

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| 07:15
| The '''name, return type '''and '''argument list '''of the''' method '''is same in both the '''classes.'''

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| 07:22
| A''' method '''in the '''subclass '''is said to '''override '''the '''method '''in the '''parent class '''if:
'''name'''
'''return type'''
'''argument list'''
matches exactly.

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| 07:33
| Come back to the '''Manager class.'''

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| 07:36
| Before, the '''getDetails() method '''type '''@Override.'''

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| 07:43
| This is an '''override annotation.''' It indicates that a '''method''' is intended to '''override''' a '''method''' in '''superclass'''.

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| 07:53
| Now, let us see what '''annotation '''is.

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| 07:57
| '''Annotations:'''
start with '''at sign''' character
provide data about a program 
have no direct effect on the operation of the code.

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| 08:10
| If a '''method '''is '''annotated '''with '''@Override, compiler '''generates '''error '''if:  the''' method''' does '''override''' a '''method''' declared in a '''superclass'''. 
 
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| 08:23
| The '''method signature '''is different in its '''superclass.'''

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| 08:28
| Now, let us come back to the '''IDE.''' Come back to the '''Manager class.'''

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| 08:34
| The '''at sign''' character indicates the compiler that what follows is an '''annotation.'''

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| 08:42
| Here, it shows that the '''getDetails method '''is '''overridden.'''

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| 08:48
| Let us come to the '''TestEmployee class.'''

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| 08:51
| Save the file and run the program.

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| 08:55
| We get the '''output''' as follows.
'''Name: Nikkita Dinesh'''
'''Email: abc@gmail.com'''
'''Manager of Accounts'''

|-
| 09:05
| Here, the '''object '''of the '''Manager class '''calls the '''getDetails() method.'''

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| 09:11
| But this time, it calls the '''method''' of '''Manager class''' itself.

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| 09:16
| In this way, we override the '''method''' of the '''parent class '''by the '''subclass'''.

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| 09:23
| Let us summarize.
In this tutorial we have learnt about
'''Subclassing''' and
'''Method Overriding'''

|-
| 09:31
| As an '''assignment''' Create a '''class Vehicle '''which has a method '''run '''that prints “'''The Vehicle is running.”'''

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| 09:40
| Also create a class '''Bike '''which has a method '''run '''that prints “'''The Bike is running safely.”'''

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| 09:48
| The output should be “'''The Bike is running safely.”'''

|-
| 09:52
| To know more about the Spoken Tutorial Project
Watch the video available at the following link
It summarizes the Spoken Tutorial project
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|-
| 10:06
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|-
| 10:21
|Spoken Tutorial Project is a part of the Talk to a Teacher project.
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More information on this Mission is available at
http://spoken-tutorial.org/NMEICT- Intro

|-
| 10:42
| This is '''Arya Ratish '''from '''IIT Bombay '''signing off.
Thanks for joining.

|}