Java/C3/Subclassing-and-Method-Overriding/English-timed

Time	Narration
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.
00:41	It is a way to create a new class from an existing class.
00:46	The new class created is subclass or derived class or child class.
00:53	The already existing class is called superclass or base class or parent class.
01:00	Now, let me show you how to create a subclass. I have already created a project named MyProject.
01:10	I have created a class in it named Employee.
01:15	It has variables, name and email_address.
01:19	It also has the setter and getter methods for the class.
01:24	It has a method getDetails(). This method returns name and email_address.
01:31	Now, let us come to the Manager class.
01:35	It has variables, name, email_address and department.
01:40	We can see that some variables are common to both Employee and Manager class.
01:47	name and email_address are present in the Employee class. We can see that its also there in Manager class.
01:57	Thus, Manager class can be made a subclass of Employee class.
02:03	For that, we have to make some changes in the Manager class.
02:08	After public class Manager, type extends Employee
02:14	We use the extends keyword to create a subclass from an existing class.
02:21	Remove the duplicate variables common to both the classes.
02:26	So, remove name and email_address from Manager class.
02:32	Also remove setter and getter methods of the same.
02:37	In the class Manager, we will have only one variable department.
02:43	We also have the setter and getter method for department.
02:59	In this way, the Manager class inherits the members of Employee class.
02:55	This way of extending one class from another is called single inheritance.
03:02	I have also created another class named TestEmployee.
03:08	Inside the main method, we will create the object of Manager class.
03:14	So inside the main method, type Manager manager equal to new Manager parentheses
03:23	Next, we will call the setter methods of the Manager class.
03:28	So type, manager dot setName within brackets and double quotes Nikkita Dinesh
03:38	Then type, manager dot setEmail within brackets and double quotes abc at gmail dot com
03:49	Then type, manager dot setDepartment within brackets and double quotes Accounts
03:57	You can use any name, email address and department.
04:02	Now, let us call the getDetails() method using the Manager object.
04:08	So, type System.out.println within brackets manager dot getDetails
04:17	Now, let us save and run the program.
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.
04:36	Now, come to the Manager class.
04:39	We can see that there is no getDetails() method here.
04:43	But, we still got the output. This is because, the Manager class extends the Employee class.
04:52	The Manager class automatically inherits the variables and methods of Employee class.
04:59	So it checks in the parent class which is Employee.
05:04	Let us come to the Employee class. It finds the getDetails() method here.
05:11	Note that we have not returned the department. As a result, it did not print the department in the output.
05:20	Now, let us change the getDetails method to private. Save the file.
05:27	We can see that we get a compilation error in the TestEmployee class.
05:34	It says The method getDetails() from the type Employee is not visible.
05:40	This means that getDetails() method cannot be accessed.
05:45	This is because we have declared getDetails() method as private.
05:52	A subclass does not inherit the private members of its superclass.
05:58	Subclass cannot directly access the private members of the superclass.
06:04	The superclass can have public or protected methods.
06:09	These methods can access its private fields.
06:13	The subclass can also access the private fields through these methods.
06:18	So, let us change it back to public.
06:21	Now, let us include the method getDetails in the Manager class.
06:27	This method will return name, email_address and department.
06:33	So type, public String getDetails parentheses.
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.
07:07	Note that now, we have the method getDetails in both Manager and Employee class.
07:15	The name, return type and argument list of the method is same in both the classes.
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.
07:33	Come back to the Manager class.
07:36	Before, the getDetails() method type @Override.
07:43	This is an override annotation. It indicates that a method is intended to override a method in superclass.
07:53	Now, let us see what annotation is.
07:57	Annotations: start with at sign character provide data about a program have no direct effect on the operation of the code.
08:10	If a method is annotated with @Override, compiler generates error if: the method does override a method declared in a superclass.
08:23	The method signature is different in its superclass.
08:28	Now, let us come back to the IDE. Come back to the Manager class.
08:34	The at sign character indicates the compiler that what follows is an annotation.
08:42	Here, it shows that the getDetails method is overridden.
08:48	Let us come to the TestEmployee class.
08:51	Save the file and run the program.
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.
09:11	But this time, it calls the method of Manager class itself.
09:16	In this way, we override the method of the parent class by the subclass.
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.”
09:40	Also create a class Bike which has a method run that prints “The Bike is running safely.”
09:48	The output should be “The Bike is running safely.”
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10:42	This is Arya Ratish from IIT Bombay signing off. Thanks for joining.

Contributors and Content Editors

Jyotisolanki, PoojaMoolya, Pratik kamble, Sandhya.np14