Difference between revisions of "Java/C2/Using-this-keyword/English-timed"
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Revision as of 17:26, 16 January 2014
| Time' | Narration |
| 00:02 | Welcome to the Spoken Tutorial on using this keyword in java. |
| 00:07 | In this tutorial we will learn |
| 00:09 | About use of this keyword |
| 00:11 | To use this keyword with fields |
| 00:14 | To use this keyword for chaining of constructors. |
| 00:17 | Here we are using
|
| 00:28 | To follow this tutorial you must know |
| 00:30 | how to create a constructor in java using eclipse. |
| 00:34 | If not, for relevant tutorials please visit our website which is as shown, |
| 00:40 | Now we will see the use of this key word |
| 00:44 | Within a constructor, this is a reference to the current object. |
| 00:48 | We can refer any member of the current object within a constructor using this. |
| 00:55 | Now we will see the use of this keyword with fields. |
| 01:00 | this keyword helps us to avoid name conflicts. |
| 01:07 | We can see such an example here. |
| 01:10 | For that let us open Eclipse. |
| 01:17 | Open the Student class we had created in the earlier tutorial. |
| 01:23 | Comment the default constructor comment the constructor with 1 parameter. |
| 01:40 | Also comment the code for creating the first two objects. |
| 02:03 | Now, notice the parameterized constructor. |
| 02:11 | the_roll_number and the_name are the arguments passed to the constructor. |
| 02:20 | roll_number and name are the instance variables. |
| 02:26 | Now, let me change the arguments to roll_number and name itself. |
| 02:39 | So inside the constructor we have: |
| 02:42 | roll_number equal to roll_number and name equal to name. |
| 02:55 | Now Save and run the file.So pressCtrl ,Sand Ctrl, F11 |
| 03:04 | We get the output as follows: |
| 03:07 | I am a Parameterized Constructor
0 null |
| 03:12 | Now come back to the code. |
| 03:17 | We see 2 warnings in the code. |
| 03:20 | Hover your mouse over the warning symbol. |
| 03:23 | We can see The assignment to the variable roll_number has no effect.
|
| 03:29 | And The assignment to the variable name has no effect. |
| 03:33 | This is because in the constructor roll_number and name are local variables. |
| 03:40 | Local variables are variables that are accessible within the method or block. |
| 03:47 | Here, roll_number and name will be initialized to 11 and Raju. |
| 03:54 | Because we have passed the values 11 and Rajuthe constructor. |
| 04:01 | But once they come out of the constructor, it is not accessible. |
| 04:06 | Then the only roll_number and name we know are the instance variables. |
| 04:13 | They have been initialized to 0 and null already once the object is created. |
| 04:18 | So we got the output as 0 and null. |
| 04:21 | Now, let us make a small change inside the constructor. |
| 04:29 | So type this dot roll_number equal to roll_number. |
| 04:37 | And this dot name equal to name. |
| 04:44 | Now save and run the file. So press ctrl, S And Ctrl, F11 keys |
| 04:51 | We get the output as |
| 04:53 |
I am Parameterized Constructor 11 and Raju |
| 04:58 | This is because this dot roll_number and this dot name refers to the instance variables roll_number and name. |
| 05:12 | And here roll_number and name are the arguments passed in the method. |
| 05:19 | To avoid confliction between local and instance variables we use this keyword. |
| 05:29 | Now we will see the use of this keywords for chaining of constructor. |
| 05:34 | We can use this keyword inside a constructor to call another one. |
| 05:39 | The constructors must be in the same class. |
| 05:43 | This is called explicit constructor invocation. |
| 05:46 | So let us come back to the Student class which we created. |
| 05:53 | Now Remove the comments. |
| 06:28 | Now comment the part to assign the instance variables to their values in the first two constructors. |
| 06:52 | Then Comment the part which creates the second and third objects. |
| 07:08 | Now let us first come to the constructor with no parameters. |
| 07:16 | After curly brackets type this within brackets 11 and semicolon. |
| 07:28 | Inside the second constructor type this within brackets 11 comma within double quotes Raju semicolon. |
| 07:42 | Now Save and Run the file. So press Ctrl,S and Ctrl , F11. |
| 07:49 | We get the output as |
| 07:51 | I am a Parameterized Constructor |
| 07:54 | I am a constructor with a single parameter |
| 07:57 | I am Default Constructor
11 and Raju |
| 08:02 | Now, I will explain the output. |
| 08:08 | When the object is created, the respective constructor gets called. |
| 08:13 | The constructor here is the no argument constructor. |
| 08:20 | The control comes to the first line in the constructor. |
| 08:24 | It encounters the this within brackets 11 statement. |
| 08:26 | Hence it calls the constructor that accepts single integer argument. |
| 08:36 | Then the control comes to this within brackets 11 comma Raju. |
| 08:44 | Hence it calls the constructor that accepts 1 integer and 1 String argument. |
| 08:53 | So this constructor is executed and we get the output as I am Parameterized Constructor. |
| 09:02 | Now the instance variables will be initialized to 11 and Raju.As we have passed. |
| 09:11 | Now, the control goes back to the calling constructor. |
| 09:16 | So the second constructor gets executed. |
| 09:19 | We get the output as I am constructor with a single parameter. |
| 09:25 | Then, the control goes to the first constructor and executes it. |
| 09:30 | So we get output as I am a default constructor. |
| 09:36 | Then studentDetail method is executed. |
| 09:42 | So, we get 11 and Raju. |
| 09:45 | Now, let us make a small change. |
| 09:47 | Make the this statement the last one in the constructor. |
| 10:00 | We get a compiler error. |
| 10:03 | Hover the mouse over the error symbol. |
| 10:06 | We get the error as: |
| 10:07 | Constructor call must be the first statement in the constructor. |
| 10:12 | So we must make it the first line of the constructor. |
| 10:16 | So make it the first line of the constructor. |
| 10:27 | Now we can see that the error has gone. |
| 10:31 | So in this tutorial, we learnt |
| 10:35 | To use this keyword with fields. |
| 10:38 | To use this keyword for chaining constructors |
| 10:41 | How this keyword should be used within a constructor. |
| 10:45 | For self assessment, in the Employee class created earlier: |
| 10:49 | Create a constructor with two parameters |
| 10:52 | Use this keyword to initialize the instance variables . |
| 10:57 | Also create a constructor with 1 and no parameters. |
| 11:01 | Try chaining the constructors using this as explained in the tutorial. |
| 11:07 | To know more about the Spoken Tutorial Project |
| 11:09 | Watch the video available at the following link http://spoken-tutorial.org/What_is_a_Spoken_Tutorial |
| 11:12 | It summarizes the Spoken Tutorial project |
| 11:16 | If you do not have good bandwidth, you can download and watch it |
| 11:19 | The Spoken Tutorial Project Team |
| 11:23 | Conducts workshops using spoken tutorials |
| 11:26 | Gives certificates for those who pass an online test |
| 11:30 | For more details, please write to contact@spoken-tutorial.org |
| 11:36 | Spoken Tutorial Project is a part of the Talk to a Teacher project |
| 11:40 | It is supported by the National Mission on Education through ICT, MHRD, Government of India |
| 11:46 | More information on this Mission is available at http://spoken-tutorial.org/NMEICT-Intro |
| 11:55 | Thus We have come to the end of this tutorial. |
| 11:58 | This is Arya Ratish signing off. Thanks for joining. |
Contributors and Content Editors
Arya Ratish, Gaurav, Kaushik Datta, PoojaMoolya, Sandhya.np14, Sneha
