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

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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 it is 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.
02:49 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 their 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 ,match 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