Difference between revisions of "Java/C2/Using-this-keyword/English-timed"

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| ''' I am a parameterized constructor'''
 
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'''I am parameterized constructor'''
 
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'''Raju'''
 
'''Raju'''
  

Revision as of 11:19, 8 April 2015

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:
  • Ubuntu version 11.10
  • jdk 1.6
  • Eclipse 3.7.0
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.

http://www.spoken-tutorial.org

00:40 Now we will see the use of this keyword.
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 press Ctrl, S and 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 "Raju" into the 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 refer 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 conflict between local and instance variables we use "this" keyword.
05:29 Now, we will see the use of "this" keyword 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 a 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 a 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 * The use of 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 [1].
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 to 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

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