Difference between revisions of "Java/C2/Instance-fields/English-timed"
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Revision as of 17:05, 7 April 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. |
| 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. |
| 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 to 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. |
