Java/C2/Using-this-keyword/English-timed

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.
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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