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

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| 06:00
 
| 06:00
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|  We see the '''output''' on the console. The '''Roll no is 50'''   the '''name is Raju'''.
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|  We see the output on the console. '''The Roll no is 50''' '''The name is Raju'''.
  
 
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| 06:22  
 
| 06:22  
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|  '''Instance fields''' are called so because their values are unique to each '''''instance''''' of a class.
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|  '''Instance fields''' are called so because their values are unique to each '''instance''' of a class.
  
 
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|-
 
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| 06:50
 
| 06:50
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|  So type next line'''Student''' space '''stud2''' equal to '''new''' space '''Student''' , opening and closing brackets semicolon.
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|  So, type: next line '''Student''' space '''stud2''' equal to '''new''' space '''Student''' opening and closing brackets semicolon.
  
 
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| 07:06
 
| 07:06
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|  We will now initialize both the objects in the'''TestStudent class'''.
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|  We will now initialize both the objects in the '''TestStudent class'''.
  
 
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| 07:18
 
| 07:18
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|  Next line type '''stud1''' ''dot '' select'''roll_no'''  press ''' enter''' equal to '''20 '''  ''semicolon.''
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|  Next line, type: '''stud1''' dot select '''roll_no'''  press ''' Enter''' equal to '''20 ''' semicolon.  
  
 
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|-
 
| 07:32
 
| 07:32
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|  Next line type '''stud1''' ''dot'' select  '''name''' press '''enter''' equal to within double quotes '''Ramu '''''semicolon'' press '''enter'''.
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|  Next line, type: '''stud1''' dot select  '''name''' press '''Enter''' equal to within double quotes '''Ramu ''' semicolon press '''Enter'''.
  
 
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|-
 
| 07:54
 
| 07:54
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|  Thus we have initialized the fields for the first object.
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|  Thus we have initialized the '''fields''' for the first object.
  
 
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|-
 
| 07:58
 
| 07:58
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|  Now, we will initialize the fields for the second object.
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|  Now, we will initialize the '''fields''' for the second object.
  
 
|-
 
|-
 
| 08:02
 
| 08:02
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|  So type '''stud2''' ''dot'' select'''roll_no''' equal to '''30 '''  ''semicolon''.
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|  So, type: '''stud2''' dot select '''roll_no''' equal to '''30 '''semicolon.
  
 
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|-
 
|08:15  
 
|08:15  
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| Next line   '''stud2''' ''dot'' select '''name''' equal to within double quotes''' Shyamu '''''semicolon'''''  press''' enter'''.
+
| Next line, '''stud2''' dot select '''name''' equal to within double quotes''' Shyamu ''' semicolon''' press''' Enter'''.
  
 
|-
 
|-
 
| 08:34
 
| 08:34
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|  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''.  
+
|  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.  
  
 
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|-
 
| 09:03
 
| 09:03
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|  '''System''' ''dot'' '''out''' ''dot'' '''println''' within brackets and double quotes '''The name is''', ''plus '''''stud2 '''''dot select '''''name''''' ''and   ''semicolon''.
+
|  '''System''' dot '''out''' dot '''println''' within brackets and double quotes '''The name is''', plus '''stud2 ''' dot select '''name''' and semicolon.
  
 
|-
 
|-
 
| 09:28
 
| 09:28
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|  Now, '''save''' and''' run '''the file. Press ''' Ctrl,s''' and '''Ctrl, F11'''   
+
|  Now, '''save''' and''' run '''the file. Press ''' Ctrl, s''' and '''Ctrl, F11'''.  
  
 
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|-
 
| 09:38
 
| 09:38
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|   We get the output as follows.  The roll_no is '''20'', The name is''' Ramu''' roll_no is '''30''',   name is''' shyamu'''.
+
| 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
 
| 09:47
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|  Here both '''stud1''' and '''stud2''' are referring to two different '''objects'''.
+
|  Here, both '''stud1''' and '''stud2''' are referring to two different '''objects'''.
  
 
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| 10:36
 
| 10:36
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|  We will now, print the values  of the third '''object'''  
+
|  We will now print the values  of the third '''object'''.
  
 
|-
 
|-
 
| 10:44
 
| 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''.
+
| 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
 
| 11:09
−
|next line type  '''System''' ''dot'' '''out''' ''dot'' '''println''' within brackets and double quotes '''The  name is''',  ''plus '''''stud3 '''''dot   ''''' name'''''    ''semicolon''.
+
|next line, type: '''System''' dot '''out''' dot '''println''' within brackets and double quotes '''The  name is''' plus '''stud3 ''' dot''' name''' semicolon.
 
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| 11:29
 
| 11:29
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|  11:46
 
|  11:46
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|  This is because we had explicitly initialized the fields of the '''Student''' class to''' 50''' and ''' Raju'''.
+
|  This is because we had explicitly initialized the fields of the '''Student class''' to''' 50''' and ''' Raju'''.
  
 
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| 12:13
 
| 12:13
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| Create an  object '''emp2''' in the '''Test Employee''' '''class''' already created.
+
| Create an  object '''emp2''' in the '''TestEmployee''' '''class''' already created.
  
 
|-
 
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| 12:18
 
| 12:18
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|  Then initialize the values of the two objects using dot operator.
+
|  Then initialize the values of the two objects using '''dot operator'''.
  
 
|-
 
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| 12:23
 
| 12:23
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|  Use 55 and Priya as values for first object.
+
|  Use '''55''' and '''Priya''' as values for first object.
  
 
|-
 
|-
 
| 12:27
 
| 12:27
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|  Use 45 and Sandeep as values for second object.
+
|  Use '''45''' and '''Sandeep''' as values for second object.
  
 
|-
 
|-

Revision as of 22:29, 30 March 2015

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 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 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 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 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 [1]
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 
[2]
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

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