Java/C3/Polymorphism/English-timed

Time	Narration
00:01	Welcome to the spoken-tutorial on Polymorphism in Java.
00:06	In this tutorial we will learn : About Polymorphism in Java Run-time polymorphism Virtual Method Invocation and Compile-time polymorphism
00:19	Here we are using Ubuntu Linux Version 12.04 JDK Version 1.7 and Eclipse 4.3.1
00:31	To follow this tutorial, you must have basic knowledge of Java and Eclipse IDE.
00:37	You must have knowledge of Subclassing & Method overriding and overloading
00:43	If not, for relevant Java tutorials, please visit our website.
00:48	Polymorphism is an ability of an object to take on many forms.
00:54	The major advantages of Polymorphism are: 1. Reduction of complexity & 2. Code re-usability
01:03	In Java, there are two types of polymorphism: Compile-time and Run-time polymorphism.
01:11	Compile-time polymorphism is essentially refered as Method overloading. It is also called Static Binding.
01:20	Run-time polymorphism is essentially referred as Method overriding. It is also called Dynamic Binding.
01:29	We have already learnt Run-time polymorphism i.e. Method overriding.
01:35	Let us switch to Eclipse IDE. I have already created a project named MyProject in the previous tutorial.
01:44	Let us take the code files of Using final keyword tutorial.
01:49	Employee class is the parent class.
01:52	Manager class is the subclass.
01:55	Manager class contains an additional variable department.
02:01	Manager class method getDetails() overrides the Employee class method getDetails().
02:08	We are calling the getDetails() method by Manager class object i.e. Manager.
02:16	In order to print the details, type system.out.println Details of Manager Class.
02:28	Save and run the program. So we can see department variable value in the output.
02:37	Therefore subclass method is invoked at runtime.
02:42	Method invocation is determined by the JVM, not compiler.
02:48	Therefore it is known as Runtime polymorphism or method overriding.
02:55	We learnt what is Run time polymorphism.
02:58	Now let us learn Virtual Method Invocation.
03:03	Come to Employee class in Eclipse IDE.
03:07	Remove the static and final keywords for variable name.
03:13	Uncomment the method setName.
03:16	Remove the static block. Save the file.
03:21	Come to TestEmployee class. Uncomment the value instance, manager.setName(“Nikkita Dinesh”);
03:31	We uncommented this instance, as we have uncommented the method setName() in Empolyee class.
03:38	Now, let's instantiate Employee object emp1 for Employee class reference.
03:46	Type, Employee emp1 = new Employee open and close parenthesis semicolon
03:57	Let's initialize the value for setEmail and setName for Employee class.
04:03	Type, emp1.setName("Jayesh"); emp1.setEmail("pqr@gmail.com");
04:16	In order to print the employee details type, System.out.println("Details of Employee class:" emp1.getDetails()) semicolon
04:37	Let us instantiate Manager object emp2 for Employee class reference i.e. type Employee emp2 = new Manager open and close parenthesis semicolon
04:54	We are able to do this because any Java object that pass more than one IS-A test, is polymorphic.
05:04	In Java, all objects are polymorphic, since any object will pass the IS-A test for their own type and for the class Object.
05:16	A Manager IS-A Employee A Manager IS-A Manager A Manager IS-A Object
05:23	Only possible way to access an object is through a reference variable.
05:29	Reference variables like emp1, emp2 and manager.
05:36	Here, we instantiated two Manager objects: One which references Employee class. And other which references Manager class.
05:47	Let’s initialize the values for setEmail, setName and setDepartment using emp2 object.
05:55	Type, emp2.setName("Ankita"); emp2.setEmail(“xyz@gmail.com”); emp2.setDepartment(“IT”);
06:14	We see that there is an error, The method setDepartment(String) is undefined for the type Employee
06:23	This is because, setDepartment method does not exist for Employee class.
06:30	So, remove the line, emp2.setDepartment("IT");
06:37	In order to print the details, type, System.out.println("Details of Manager class:" emp2.getDetails()) semicolon
06:55	Save and Run the program.
06:58	Here in the output. We get the Manager of: as blank.
07:04	This is because we have not initialized department in Manager class using emp2.
07:12	For demo purpose, let the default department be IT.
07:17	So, go to Manager class and initialize the value for department.
07:25	Save and run the program.
07:28	We get the output: Employee object referring Employee class,
07:34	Manager object referring Employee class & Manager object referring Manager class,
07:42	Here we see that the getDetails() method of Manager class is called by emp2.
07:49	But when emp2 tried calling setDepartment, we got an error.
07:54	The reason for that is as follows: The compiler sees the getDetails() method in the Employee class during emp2.getDetails().
08:05	So, it does not throw an error and validates the code.
08:10	At run time, however, the JVM invokes getDetails() in the Manager class as getDetails() of Manager class overrides getDetails() of Employee class.
08:24	So, we get the output as per getDetails() of Manager class. But the compiler does not see the setDepartment method in the Employee class.
08:36	Therefore, it raises an error in case of setDepartment call by emp2.
08:43	Here, Employee method getDetails() is invoked for Employee class.
08:49	The compiler references Employee class for getDetails() during emp1.getDetails().
08:57	At run time, the JVM invokes getDetails() in the Employee class. So we get the output as per getDetails() of Employee class.
09:08	Thus the JVM calls the appropriate method for the object that is referred to in each variable.
09:16	This behavior is referred to as Virtual Method Invocation.
09:21	The methods are referred to as Virtual Methods.
09:26	All methods in Java behave in this manner.
09:31	We successfully learnt what is Virtual Method Invocation.
09:36	We have already learnt Compile-time polymorphism i.e. method overloading.
09:42	Let us quickly know Compile time polymorphism in brief.
09:47	In Compile time polymorphism, class can have more than one method.
09:53	The methods have same name but with different number of arguments.
09:59	Compiler is able to figure out the method call at compile-time. That’s the reason it is known as compile time polymorphism.
10:09	So, let us summarize.
10:11	In this tutorial we learnt: What is Polymorphism in Java? Run-time polymorphism Virtual Method Invocation and Compile-time polymorphism
10:23	As an assignment, Override methods for Vehicle and Bike class which we used in previous tutorials.
10:32	The video available at the following link summarizes the Spoken Tutorial project. Please watch it.
10:40	The Spoken Tutorial Project Team conducts workshops and gives certificates to those who pass an online test.For more details, please write to us.
10:51	Spoken Tutorial Project is supported by NMEICT, MHRD, Government of India.More information on this Mission is available at this link.
11:03	Thats it for the tutorial. This is Trupti Kini from IIT Bombay signing off. Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14