Java/C2/Instance-fields/English-timed

Time	Narration
00:02	Welcome to the Spoken Tutorial on Instance fields in Java.
00:06	In this tutorial, we will learn:
00:08	* About instance fields
00:10	* To access the instance fields of a class
00:13	* Modifiers for instance fields
00:15	* And, why instance fields are called so?
00:18	Here we are using:
00:20	* Ubuntu version 11.10
00:22	* jdk 1.6
00:24	* And Eclipse IDE 3.7.0
00:27	To follow this tutorial, you must know
00:30	how to create a class in Java using Eclipse.
00:33	You must also know how to create an object for the class.
00:38	If not, for relevant tutorials, please visit our website which is as shown. http://www.spoken-tutorial.org
00:43	We know that objects store their individual states in fields.
00:48	These fields are declared without the static keyword.
00:51	We will learn about static fields in the coming tutorials.
00:55	Non-static fields are also known as instance variables or instance fields.
01:01	Let us go back to the Student class we had already created.
01:09	We can see that here roll_no and name are the instance fields of this class.
01:15	Now, we will learn how to access these fields.
01:18	For that, let us open the TestStudent class which we had already created.
01:27	We can remove the statement for creating the second object.
01:33	We will also remove the println statements.
01:41	Now we will access the fields, roll_no and name, of the student class using stud1 and the dot operator.
01:49	So, for that, type: System dot out dot println within brackets and double quotes The roll number is then plus stud1 dot from the option provided select ' roll_no' press Enter then semicolon.
02:15	Next line, type: System dot out dot println within brackets and double quotes The name is plus stud1 dot select 'name' press Enter then semicolon.
02:39	Now, save and run the file TestStudent.java. So press Ctrl, S and Ctrl, F11.
02:48	We get the output as:
02:51	The roll number is 0.
02:53	The name is null.
03:00	This is because, we have not initialized the variables to any value.
03:05	In Java, the fields cannot have random values.
03:09	After the memory is allocated for the object, the fields are initialized to null or zero.
03:15	This work is done by the constructor.
03:18	We will learn about constructor in the coming tutorials.
03:21	Now, we will initialize the fields explicitly and see the output.
03:27	So, type: int roll_no equal to 50 next line String name equal to within double quotes Raju.
03:42	Now, save and run the file. Press Ctrl, S and Ctrl, F11.
03:50	We get the output as expected; The roll number is 50.
03:54	The name is Raju.
03:56	This is because we have explicitly initialized the variables in the Student class.
04:04	We can see that here the fields have no modifier or default modifier.
04:10	Recall modifiers, we had discussed in Creating Classes.
04:14	We can access the fields because both Student.java and TestStudent.java are in the same package.
04:22	We can see that, here, they are in the same default package.
04:30	We will learn about packages in the later tutorials.
04:34	We will now change the modifier to private.
04:37	So, before the field declarations, type: private. So, type: private space int roll_no=50;.
04:48	Next line, private space String name = Raju;.
04:53	Now save the file Student.java.
05:00	We can see that we get errors in TestStudent.java.
05:05	Hover the mouse over the error symbol.
05:08	It says The field Student dot roll number is not visible.
05:12	And The field Student dot name is not visible.
05:16	This is because private fields can be accessed only within its own class.
05:23	You can try accessing roll_no and name from the Student class itself.
05:27	You will find that you can access them without any error.
05:32	Now let us change the modifier to protected.
05:52	Now Save the file and Run the program.
06:00	We see the output on the console. The Roll no is 50 The name is Raju.
06:07	This is because protected fields can be accessed within the same package.
06:17	Now let us see why instance fields are called so?
06:22	Instance fields are called so because their values are unique to each instance of a class.
06:29	In other words each object of a class will have unique values.
06:34	Let us go back to the TestStudent class.
06:43	Here, we will create one more object of the Student class.
06:50	So, type: next line, Student space stud2 equal to new space Student opening and closing brackets semicolon.
07:06	We will now initialize both the objects in the TestStudent class.
07:18	So, next line, type: stud1 dot select roll_no press Enter equal to 20 semicolon.
07:32	Next line, type: stud1 dot select name press Enter equal to within double quotes Ramu semicolon press Enter.
07:54	Thus we have initialized the fields for the first object.
07:58	Now, we will initialize the fields for the second object.
08:02	So, type: stud2 dot select roll_no equal to 30 semicolon.
08:15	Next line, stud2 dot select name equal to within double quotes Shyamu semicolon press Enter.
08:34	Now, after the println statements, type: System dot out dot println within brackets and double quotes The roll number is plus stud2 dot select roll_no and semicolon.
09:03	System dot out dot println within brackets and double quotes The name is, plus stud2 dot select name and semicolon.
09:28	Now, save and run the file. Press Ctrl, s and Ctrl, F11.
09:38	We get the output as follows. The roll_no is 20, The name is Ramu The roll_no is 30, The name is shyamu.
09:47	Here, both stud1 and stud2 are referring to two different objects.
09:52	This means that the two objects have unique values.
09:56	We can see that here.
09:57	The first object has the values 20 and Ramu.
10:02	The second object has the values 30 and Shyamu.
10:09	Now, let us create one more object.
10:13	So type Student space stud3 equal to new space Student within brackets opening and closing brackets semicolon.
10:36	We will now print the values of the third object.
10:44	So, type: System dot out dot println within brackets and double quotes The roll_no is plus stud3 dot select roll_no semicolon.
11:09	next line, type: System dot out dot println within brackets and double quotes The name is plus stud3 dot name semicolon.
11:29	Now, save and run the file. So press Ctrl, S and Ctrl, F11 .
11:36	We can see that the third object contains the values 50 and Raju.
11:46	This is because we had explicitly initialized the fields of the Student class to 50 and Raju.
11:54	Now, try de-initializing the fields and see the output for the third object.
12:02	So in this tutorial, we learnt:
12:05	* About instance fields.
12:07	* Accessing the fields using dot operator.
12:11	For self assessment,
12:13	Create an object emp2 in the TestEmployee class already created.
12:18	Then initialize the values of the two objects using dot operator.
12:23	Use 55 and Priya as values for first object.
12:27	Use 45 and Sandeep as values for second object.
12:31	Display the values for both the objects in the output.
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13:09	Thus we have come to the end of this tutorial.
13:22	This is Arya Ratish from IIT Bombay, signing off. Thanks for joining.

Contributors and Content Editors

Arya Ratish, Gaurav, PoojaMoolya, Sandhya.np14, Sneha