Difference between revisions of "Java/C2/Instance-fields/English-timed"
From Script | Spoken-Tutorial
Arya Ratish (Talk | contribs) |
|||
| Line 1: | Line 1: | ||
{| border=1 | {| border=1 | ||
| − | || ''Time''' | + | || '''Time''' |
|| '''Narration''' | || '''Narration''' | ||
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. |
