Difference between revisions of "Java/C2/Instance-fields/English-timed"

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Revision as of 14:42, 9 July 2014

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 usingstud1 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 initialize the variables in the Student class


04:04 We can see that here the fields have no modifier or the 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 int roll no=50.


04:48 Next line private 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 to the TestStudent class.


06:43 Here, we will create one more object of the Student class.


06:50 So type next lineStudent space stud2 equal to new space Student , opening and closing brackets semicolon.
07:06 We will now initialize both the objects in theTestStudent class.


07:18 Next line type stud1 dot selectroll_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 selectroll_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 roll_no is 30, 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 Test Employee 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.
12:34 To know more about the Spoken Tutorial Project
12:37 Watch the video available at http://spoken-tutorial.org/What_is_a_Spoken_Tutorial
12:40 It summarizes the Spoken Tutorial project
12:43 If you do not have good bandwidth, you can download and watch it


12:47 The Spoken Tutorial Project Team
12:49 Conducts workshops using spoken tutorials
12:52 Gives certificates for those who pass an online test
12:56 For more details, please write to contact@spoken-tutorial.org


13:01 Spoken Tutorial Project is a part of the Talk to a Teacher project
13:05 It is supported by the National Mission on Education through ICT, MHRD, Government of India
13:11 More information on this Mission is available at
 http://spoken-tutorial.org/NMEICT-Intro 


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