this in java

[amal shah]

Hello,
I have a concern as follows:
Consider 2 classes:

public class Main {

public Main() {
new abc(this); // Line 1
new abc(new Main()); // Line 2
}

public static void main(String[] args) {

}
}

class abc
{
Main m;

abc(Main m)
{
this.m=m;
}
}


so passing the this reference as in line 1 is equivalent to constructing an object of the Main class.
In short this when passed,internally creates object of the class..is this conclusion correct...

help appreciated
cheers
amal shah

[Rob Spoor]

Originally posted by amal shah:
so passing the this reference as in line 1 is equivalent to constructing an object of the Main class.
In short this when passed,internally creates object of the class..is this conclusion correct...

Not quite correct.

When you use "this", you are NOT creating a new object, it uses the object that is already created.

Actually, you're code will never run properly, as you'll get a StackOverflowError. What happens is the following:
1) You create an instance of Main manually.
2) The Main() constructor runs.
3) The Main() constructor creates another instance of Main on // line 2
4) Go to step 2.

[amal shah]

When you use "this", you are NOT creating a new object, it uses the object that is already created.

which object does it use......there can n number of objects for a class

Actually, you're code will never run properly, as you'll get a StackOverflowError.

I don't think so....it ran properly.....it is just a parameter passed..

I am sort of confused with (this) concept....
pls help

cheers
amal shah

[Vaibhav Aggarwal]

Check out the below code

class Demo{
 
//Instance Variable
int a = 10;
 
void display(int a){
        // The a used here is local variable
System.out.println("The value of a in display method is "+a);
}
 
    void updateInstanceVariable(int a){
this.a = a;
System.out.println("The value of a in updateInstanceVariable method is "+a);
}
 
public static void main(String[] args){
Demo obj = new Demo();
obj.display(200);
System.out.println("The value of instance member a is "+obj.a);
obj.updateInstanceVariable(500);
System.out.println("The value of instance member a is "+obj.a);
}
}
 
    

The ouput for above code will be.

 The value of a in display method is 200
The value of instance member a is 10
The value of a in updateInstanceVariable method is 500
The value of instance member a is 500
Press any key to continue . . .   

You will see that when we use "this" then we are refering to the current instance of the class. So while the display method changed the value of the local variable, updateInstanceVariable method updated the instance member of the current object.
[ September 24, 2007: Message edited by: Vaibhav Aggarwal ]

[amal shah]

You will see that when we use "this" then we are refering to the current instance of the class. So while the display method changed the value of the local variable, updateInstanceVariable method updated the instance member of the current object.

Your code above displays use of (this) when the local variable hides the instance member of the class....

But i am confused with the code that i pasted in my first post with respect to usage of (this) passed in as parameter....

please help me understand this concept...

help appreciated
cheers
amal shah

[Henry Wong]

I don't think so....it ran properly.....it is just a parameter passed..

Amal,

When Rob is saying that your problem "will never run properly", he meant that you can't actually instantiate a Main object without an runtime exception. The reason your program doesn't generate a runtime exception, is because you never intantiate a Main object -- which means that you never tested any of the constructors that you are trying to figure out.

Henry

[amal shah]

Thanks Henry and Rob....I got the point...