Difference between revisions of "Java/C3/Using-final-keyword/English"

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Revision as of 11:54, 8 May 2014

Title of script: USING FINAL KEYWORD

Author: Trupti Rajesh Kini

Keywords:


Visual Clue
Narration


Slide 1 Welcome to the spoken-tutorial on Using final keyword.



Slide 2 In this tutorial we will learn :

When to invoke final keyword.

final keyword in various contexts:

  • variable,
  • method,
  • class


Slide 3

Software Requirements


Here we are using
  • Ubuntu Linux version 12.04
  • JDK 1.7
  • Eclipse 4.3.1


Slide 4

Prerequisites


To follow this tutorial, you must have,

Knowledge of basics of Java and Eclipse IDE.

Knowledge of Subclassing and Method overriding .

If not, for relevant Java tutorials, please visit our website.



Slide 5

final keyword

First of all, we will learn what final keyword is.

The final keyword in java is used to restrict the user.



What is final variable?

Slide 6

Now, let us learn what is final variable?

If you make any variable as final, you cannot change the value of final variable.


i.e It will be constant.

Go to the Eclipse IDE >> Click on MyProject I have already created a project named MyProject in Subclassing and Method overriding tutorial.


Point to Employee class Let us come to the Employee class.
Point to the variable 'name' Come to the variable name.
Type final keyword Type final keyword before the variable name.
Save & run the program.
Point to the error It gives a compilation error,

cannot assign a value to final variable name

name=newName;

Slide 7 This is because,

final variable if declared and not initialised then,

It can be initialised by the constructor only.



For that, lets create a constructor.

We have already learnt what is a constructor.

We know that constructor has the same name as class name.
Point to the constructor Employee Type Employee,

Parentheses,

Open curly brackets,

Inside we will declare name as Sneha.

Point to the method setName Let us comment the method setName by enclosing in /* */.
Point to TestEmployee class Go to TestEmployee class.

Go to the main method and comment instance for the method manager.setName("Nikkita Dinesh");

Save and run the program.
Point to the error We get a compilation error,

cannot assign a value to final variable name

name="Sneha";

Slide 8 This is because,

You can only initialize a final variable once.

Come back to the Employee class.
Point to the final variable name Remove the blank initialized in the declaration of name.
Save and run the program.
Point to output Great!!!

We got output as ,

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

Now let us learn about final static variables.
Point to the Employee class For that let us go to the Employee class.
Point to the variable name Let us go to the variable name.

Let us add keyword static before final keyword.

Save & run the program.
Point to the error We get a compilation error,

cannot assign a value to final variable name

name="Sneha";

Slide 9 This is because,

static final variables cannot be assigned value in constructor; they must be assigned a value with their declaration OR

they must be declared ina static block.

Slide 10 static variables are shared among all the objects of a class.

Creating a new object would change the static variable.

This is not allowed if the static variable is final.

Point to the constructor Let us change the constructor name to static to create a static block.
Point to the output Save and run the program.

We will get the output,

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts



Now lets use final variable as parameter to the method.
Point to class Employee Let us go to the class Employee
Point to the method setEmail Let us go to the method setEmail.

Add final keyword before String newEmail.

Save and run the program.

Point to the output We will get the output,

Name: Sneha

Email: abc@gmail.com

Manager of: Accounts

Type newEmail="kinitrupti@gmail.com"; Now lets type inside the method,

newEmail="kinitrupti@gmail.com";

Save & run the prorgam.

Point to the error We will get a compilation error,

final parameter newEmail may not be assigned

Slide 11 This is because,

Value of final variable as parameter to a method cannot be changed by that method.

Now lets learn about final method.
Point to the newEmail initialization Lets remove the newEmail initialization.
Slide 12 If you make any method as final, you cannot override it.
Point to the Employee class Come to the Employee class.
Point to the method getDetails Come to the method getDetails.
Add final keyword before getDetails.
Save and runAfter saving and running i dnt get compilation error. I get exception that final method cannot be overridden. Is it right?
What if final method is private?

Slide 13

What if final method is private?

Since private methods are inaccessible, they are implicitly final in Java.

So adding final specifier to a private method doesn’t add any value.

Point to Employee class Let us try this out.

Come to the Employee class.

Let us create a constructor again as Employee.

Add final before the constructor.

Point to the compilation error Save and run the program.

We get a compilation error,

modifier final not allowed here

Slide 14 This is because,

constructor cannot be final since constructors are not inherited.



Now lets learn about final class.



Point to the Employee class Come to the Employee class.

Add final keyword before the class name.

Save and run the program.



Does not give a compilation error instead throws an exception during execution. Is it right? Try this out with the codes i have attached.
Slide 15 This is because,

If you make any class as final, you cannot extend it.

Slide 7

Summary


In this tutorial we have learnt:
  • When to invoke final keyword.
  • final keyword in various contexts:
    • variable,
    • method,
    • class


Slide 8

Assignment


Assignment
  • Repeat the same as shown in the tutorial for the Bike and Vehicle class.



Slide 9

About Spoken Tutorial Project


Watch the video available at the following link
  • It summarizes the Spoken Tutorial project
  • If you do not have good bandwidth, you can download and watch it


Slide 10

About Spoken Tutorial workshops


The Spoken Tutorial Project Team
  • Conducts workshops using spoken tutorials
  • Gives certificates for those who pass an online test
  • For more details, please write to contact at spoken hyphen tutorial dot org


Slide 11

Acknowledgement


Spoken Tutorial Project is a part of the Talk to a Teacher project
  • It is supported by the National Mission on Education through ICT, MHRD, Government of India
  • More information on this Mission is available at

This is Trupti Kini from IIT Bombay signing off.

Thanks for joining.



Contributors and Content Editors

Nancyvarkey, Trupti

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