Hello everyone
In this tutorial
I am going to teach something interesting about the "this" keyword in java
Before this
Keep into mind that
Whenever we are creating an object, the copies of the instance variables are created for each object
for example in the following example I am creating two objects called td and td1
and in the class there are two instance variables called int a and int b.
Now what I am saying is that there will be two separate objects and for those separate objects
there will be separate variables int a and int b
This diagram elaborates more
Now that you got this we will move to our point
now when we use this keyword we are referring to the current object
now look at the program
Using this keyword with constructors
/* A program to demonstrate the use of this keyword
Developer: Prasad Kharkar
Date : 25th February 2010
*/
class ThisDemo1
{
int a = 0;
int b = 0;
ThisDemo1(int x, int y)
{
this.a = x;
this.b = y;
}
public static void main(String [] args)
{
ThisDemo1 td = new ThisDemo1(10,12);
ThisDemo1 td1 = new ThisDemo1(100,23);
System.out.println(td.a); // prints 10
System.out.println(td.B); // prints 12
System.out.println(td1.a);// prints 100
System.out.println(td1.B);// prints 23
}
}
when we compile and run the program we get the output as
Quote
F:\Java\Concepts\DeclarationAndAccessControl>java ThisDemo1
10
12
100
23
10
12
100
23
Now we will look step by step
- When we created the object td then the constructor was called for td
- The respective values are passed to the copies of the instance variables of that object (as shown in the figure)
- In the constructor using this.a = x and this.b = y, we copied the values into the instance variables
- using System.out.println() statements we confirmed that those values referred to respective objects
Chaining of constructors using this keyword
Now what is chaining?
this is nothing but calling of one constructor from other constructor.
This is possible by using this keyword
Now look at the following program
/* A Program to demonstrate the use of This keyword
Developed By : Prasad kharkar
*/
class ThisDemo
{
public ThisDemo()
{
this(10);
System.out.println("First Constructor");
}
public ThisDemo(int a) // overloaded constructor
{
this(10,20);
System.out.println("Second Constructor");
}
public ThisDemo( int a, int B) // another overloaded constructor
{
this("Prasad");
System.out.println("Third Constructor");
}
public ThisDemo(String s) // and still another
{
System.out.println("Fourth Constructor");
}
public static void main(String args[])
{
ThisDemo first = new ThisDemo(); // one object should be created so that we can access other constructos through the constructor which is called first
}
}
This is interesting
Now this is the output of the program
Quote
F:\Java\Concepts\DeclarationAndAccessControl>java ThisDemo
Fourth Constructor
Third Constructor
Second Constructor
First Constructor
Fourth Constructor
Third Constructor
Second Constructor
First Constructor
now what happened here?
let us go step by step
- When the object is created then the respective constructor(No argument constructor) gets called
- The control comes to the first line in the constructor where it encounter with "this(10)" statement
- Hence it calls the constructor that accepts one integer argument
- Again the control comes to the this(10,20) statement and again it jumps to the constructor which takes two integer arguments
- Again the same thing, I need not explain it anymore, you will get bored
- Now the control is in the constructor which takes string argument
- here there is no this keyword
- hence the constructor gets executed and control goes back to the calling constructor
- This process continues till the first this keyword was encountered
Well this is the total procedure
I hope you got this thoroughly
similarly we can call methods using this keywordthis.someMethod();
Note that here we have called all the constructors using one object only
Using this with instance variables
well we already have used it
hehehe
just look at the part of the code
ThisDemo1(int x, int y)
{
this.a = x;
this.b = y;
}
I need not elaborate this more
Now there are two most important points regarding this
- Whenever we are using this in a constructor or a method for calling , then we must call it in the first line of method, otherwise it gives compile time error, but as KYA sir has explained we can write this for assignments anywhere
this is code given by him
public class Set { protected int a; public Set(){ System.out.println(); this.a = 10; } public int getA() {return a;} public static void main(String [] args){ Set set = new Set(); Setter s = new Setter(); System.out.println(s.getA()); System.out.println(set.getA()); } } class Setter extends Set{ public Setter(){ super(); this.a = 15; } }
- We cannot write super and this in the same method as both need to be written in the first line
to know more about the super keyword, check out this tutorial by japanir sir.
Hope this tutorial was useful to you
Have a great time ahead!!
This post has been edited by pdkharkar: 25 February 2010 - 04:37 PM
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