Java/C3/Subclassing-and-Method-Overriding/English-timed
From Script | Spoken-Tutorial
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| 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. |
