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

|| '''Time'''

|  Welcome to the Spoken Tutorial on '''Using "this" keyword''' in '''java'''.  

|  In this tutorial, we will learn:

| About use of "this" keyword  

|To use "this" keyword with '''fields'''

|To use '''this''' keyword for chaining of '''constructors'''.  

| Here we are using:

'''Ubuntu version 11.10'''

'''jdk 1.6'''

'''Eclipse 3.7.0'''

| To follow this tutorial, you must know  

|  how to create a '''constructor''' in java, using '''eclipse'''.  

|  If not, for relevant tutorials, please visit our website which is as shown. http://www.spoken-tutorial.org  

|Now we will see the use of '''this''' keyword.

| Within a constructor, "this" is a reference to the current '''object'''.  

|  We can refer any member of the current '''object''', within a '''constructor''', using "this".  

|Now we will see the use of '''this''' keyword with fields.   

| '''this''' keyword helps us to avoid name conflicts.  

|  We can see such an example here.  

|  For that, let us open '''Eclipse'''.  

|  Open the '''Student class''' we had created in the earlier tutorial.  

|  Comment the '''default constructor''', comment the '''constructor''' with 1 parameter.  

|  Also comment the code for creating the first two '''objects'''.  

|  Now, notice the '''parameterized constructor'''.

| '''the_roll_number''' and '''the_name''' are the arguments passed to the constructor.  

|  '''roll_number''' and '''name''' are the '''instance''' variables.  

|  Now, let me change the arguments to '''roll_number''' and '''name''' itself.  

|  So inside the constructor we have:  

|  '''roll_number''' equal to '''roll_number''' and '''name''' equal to '''name'''.

|Now '''Save''' and '''run''' the file. So press '''Ctrl, S''' and '''Ctrl, F11'''.

|  We get the output as follows:  

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

'''0'''

''' null'''

| Now come back to the code.  

| We see 2 warnings in the code.  

|  Hover your mouse over the warning symbol.  

|We can see '''The assignment to the variable roll_number has no effect.'''

| And '''The assignment to the variable name has no effect'''.

|  This is because, in the '''constructor''', 'roll_number' and 'name' are ''local variables''.  

|  ''Local variables'' are variables that are accessible within the method or block.  

|  Here, 'roll_number' and 'name' will be initialized to 11 and "Raju"

| because we have passed the values 11 and "Raju" into the constructor.  

| But once they come out of the constructor, it is not accessible.  

| Then the only '''roll_number''' and '''name''' we know, are the instance variables.  

|  They have been initialized to '0' and ''null' already once the object is created.  

|  So we got the output as '''0''' and '''null'''.  

|  Now, let us make a small change inside the '''constructor'''.  

| So, type: '''this''' dot '''roll_number''' equal to '''roll_number'''.  

|  And, '''this''' dot '''name''' equal to '''name'''.  

|  Now '''save''' and '''run''' the file. So press '''ctrl, S''' And '''Ctrl, F11''' keys.

| We get the output as:

|   

'''I am parameterized constructor'''

|  This is because '''this''' dot '''roll_number''' and '''this''' dot '''name''' refer to the instance variables 'roll_number' and 'name'.

|And here '''roll_number''' and '''name''' are the arguments, passed in the '''method'''.  

|  To avoid conflict between '''local''' and '''instance''' variables we use "this" keyword.  

|  Now, we will see the use of "this" keyword for chaining of '''constructor'''.  

|  We can use this keyword inside a '''constructor''' to call another one.  

|  The '''constructors''' must be in the same class.

| This is called '''explicit constructor invocation'''.

| So, let us come back to the '''Student''' class which we created.  

|Now remove the comments.  

| Now, comment the part to assign the instance variables to their values in the first two '''constructors'''.

|Then '''comment''' the part which creates the second and third objects.

| Now, let us first come to the '''constructor''' with no parameters.  

|  After curly brackets type: '''this''' within brackets '''11''' and semicolon.  

| Inside the second constructor, type: '''this''' within brackets '''11''' comma within double quotes "Raju" semicolon.  

| Now, '''Save''' and '''Run''' the file. So, press '''Ctrl, S''' and '''Ctrl , F11'''.  

|  We get the output as: 

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

| '''I am a constructor with a single parameter'''

| '''I am Default Constructor'''

'''11''' and  

| Now, I will explain the output.  

|  When the object is created, the respective '''constructor''' gets called.  

|  The constructor here is the ''no argument'' '''constructor'''.

| It encounters the '''this''' within brackets '''11''' statement.  

|  Hence it calls the '''constructor''' that accepts single integer argument.  

|  Then the control comes to '''this''' within brackets '''11''' comma '''Raju'''.

|  Hence it calls the '''constructor''' that accepts '''1 integer''' and '''1 String''' argument.  

|  So, this '''constructor''' is executed and we get the output as '''I am a parameterized constructor'''.  

|Now the instance variables will be initialized to '''11''' and '''Raju''' as we have passed.  

|  Now, the control goes back to the calling '''constructor'''.  

|  So, the second constructor gets executed.  

|  We get the output as '''I am constructor with a single parameter'''.

|  Then, the control goes to the first '''constructor''' and executes it.  

|  So, we get output as '''I am a default constructor'''.

|  Then '''studentDetail''' method is executed.  

|  So, we get '''11''' and '''Raju'''.  

|  Now, let us make a small change.  

| Make the '''this''' statement the last one in the '''constructor'''.  

|  We get a compiler error.  

|  Hover the mouse over the error symbol.  

|  We get the error as:  

|  '''Constructor call must be the first statement in a constructor'''.

| So, we must make it the first line of the '''constructor'''.

| So, make it the first line of the '''constructor'''.

|  Now we can see that the error has gone.  

|  So, in this tutorial, we learnt:

|The use of '''this''' keyword with '''fields'''.

|To use '''this''' keyword for chaining '''constructors'''

|  How '''this''' keyword should be used within a '''constructor'''.

|  For self assessment, in the '''Employee''' '''class''' created earlier:  

|  Create a '''constructor''' with two parameters.

|  Use '''this''' keyword to initialize the '''instance variables''' .

| 10:57  

| Also, create a '''constructor''' with 1 and no parameters.  

|  Try chaining the '''constructors''' using '''this''' as explained in the tutorial.  

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