Java/C3/Static-Variables/English-timed

Time	Narration
00:01	Welcome to the Spoken Tutorial on Static Variables.
00:05	In this tutorial we will learn about: What are static variables Creating static variables and How to use static variables
00:17	Here we are using Ubuntu 12.04 JDK 1.7 and Eclipse 4.3.1
00:27	To follow this tutorial, you must have knowledge of basics of Java and Eclipse IDE.
00:35	You should also have the knowledge of classes, objects and instance variables in Java.
00:43	If not, for relevant Java tutorials, please visit the link shown.
00:49	A static variable is a variable which is associated with the entire class.
00:55	It is also called a class variable. It is declared using the static keyword.
01:02	We have seen about static variable briefly in the earlier tutorial.
01:08	In this tutorial we will see it in detail. Let us illustrate the usage of static variables with an example.
01:16	Consider a class created to represent student enrollments in an organization.
01:23	This class contains the Name, Id , Branch and Total Count of students enrolled.
01:30	Now let us visualize what happens when student enrollments are done.
01:36	By default, Total Count is 0. The Name of the first student is ADIL. Id is IT101. Branch is IT.
01:48	Now the Total Count is updated as 1.
01:52	Similarly when the second student AMAL gets enrolled, Total Count is updated to 2.
01:59	When the third student CAROL gets enrolled, Total Count is updated to 3.
02:06	Now we can identify that the variable Total Count is common to all the objects and has a single value.
02:14	So the variable Total Count can be represented as a static variable.
02:20	We can also see that the variables Name, Id and Branch have their own copies for each object.
02:28	Also they have specific values for each object. So these variables can be treated as instance variables.
02:38	Now let us explore the differences between instance variables and static variables.
02:45	Every object has its own copies for instance variables.
02:50	Id, Name, Branch are examples of instance variables.
02:55	Static variables in a class share a single copy with all objects in that class.
03:02	Total Count is an example of static variable.
03:06	When a class is loaded a static variable occupies a single fixed memory location whereas Instance variables of each object occupy separate memory locations.
03:19	Now let us look at the sample code representing the student enrollment class.
03:25	The name of the class is StudentEnroll. The instance variables are declared as id, name and branch.
03:35	The variable count is declared as static since it is common to the whole class.
03:41	Now we will switch to Eclipse and create a new project called StaticVariableDemo.
03:48	Inside this project we will create the necessary classes to demonstrate the usage of Static variables.
03:56	So right click on src folder and click on new -> class, and type the name of the class as StudentEnroll and Press Enter.
04:07	Now type the code which was shown in the earlier slide.
04:12	Now click on source -> and select Generate Constructor using Fields. Delete the super keyword from the generated code.
04:21	This constructor can initialize the values of the id, name and branch fields.
04:28	We also need to increment the value of the variable count by one every time an object is created.
04:36	So inside the constructor type, count ++ semicolon.
04:42	Now we will add a method showData( ) to this class to print the values of all the variables.
04:50	So type public void showData( ) within brackets type the following code to print the values of id, name, branch and total number of students enrolled.
05:04	Now right click on the default package click on new -> class and then type name as Demo.
05:12	Inside this class we will have the main method.
05:16	So type main and then press ctrl+space to generate the main method.
05:22	Now we need to print the Student Enrollment data.
05:27	We will create a few objects of StudentEnroll class to represent student enrollments.
05:34	So type the following code. StudentEnroll s1 equals new StudentEnroll
05:41	Now we can pass the values of different arguments. Within brackets type IT101 as id, ADIL as name and IT as branch.
05:54	Now let us invoke the showData method to print the enrollment details. So type s1.showData( ).
06:03	Now run the Demo program.
06:06	We can see that the values of the instance variables corresponding to s1 get printed.
06:13	Also note that the value of the number of student enrollments is 1.
06:19	This is because we have created only 1 object.
06:23	Now type the following code to create one more object s2.
06:28	The showData method can be called once again using s2.
06:33	Again run the Demo Program.
06:36	We can see that the values of the instance variables corresponding to s2 get printed.
06:43	Also note that the value of number of student enrollments is updated to 2 for both s1 and s2.
06:51	Now create one more object s3.
06:54	Now Let us Invoke the showData method again using s3.
07:00	Run the Demo Program again.
07:02	We can see that the values of the instance variables corresponding to s3 get printed.
07:09	Also note that the value of number of student enrollments is now updated to 3 in all the cases.
07:17	Now we can understand that the value of the number of student enrollments is common to all the objects.
07:25	Come back to slides. The static modifier, is also used along with the final modifier.
07:33	It is done to define a constant which is common to the entire class.
07:38	Conventionally, the names of such constant variables are spelled in uppercase letters.
07:44	If the name is composed of more than one word, the words are separated by an underscore.
07:51	As an Example, if all the students are getting enrolled to the same organisation say IITB.
07:59	It can be represented by using a constant static variable say ORG_NAME.
08:06	Usually we declare such constants with a public visibility.
08:11	A Static variable can be accessed directly from another class by using its own class name. For Example: StudentEnroll.ORG_NAME;
08:24	Now come back to eclipse.
08:27	Open the StudentEnroll class and type the variable declaration as public static final String ORG_NAME = “IITB”;
08:39	Now go to the Demo class and type the following code
08:45	Here you can see that ORG_NAME is accessed by using its class name StudentEnroll.
08:53	Now run the Demo Program again.
08:56	We can see that the ORGANISATION name is getting printed as IITB.
09:01	Let us summarize. In this tutorial we have learnt about What is a static variable and when it is used How to create and invoke the static variables
09:15	As an Assignment. Design a class CarService to represent a Car Service Station.
09:22	This class should contain variables to represent the following details.
09:27	Name of the Service Station. Car make, model and reg. no - which are in for service. No. of Cars in for Service.
09:37	Identify the instance variables and static variables.
09:41	Declare them using suitable keywords.
09:45	Define a constructor to initialize the values for Car make, model and regno.
09:51	Define a method show( ) to print the values of all the variables.
09:57	Also create a Demo class containing the main method to verify the results. i.e Create a few objects of CarService.
10:06	Invoke the show( ) method using these objects.
10:10	Also access the static variables directly using the class name.
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10:46	This script has been contributed by: Department of Information Technology, Amal Jyothi College of Engineering. This is Priya from IIT Bombay signing off. Thanks for joining.

Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14