Doubt in this()

[Niyas Ahmed Sheikh]

Hi,

What is the main use of this(); and this.a = a.
Why we are using this. I referred in some books, it's simply saying it's points to the current object. Could u pls explain with one simple example.

[fred rosenberger]

it's used a lot in constructors, to differentiate between the parameter passed in and the class variable with the same name... i.e.

public class Foo{
int param1;
int param2;
 
public Foo(int param1, int param2)
   {
   this.param1 = param1;
   this.param2 = param2;
   }
}

note that without the keyword "this", the compiler can't tell which param1 you are talking about.

[Ryan McGuire]

Originally posted by Niyas Ahmed Sheikh:
Hi,

What is the main use of this(); and this.a = a.
Why we are using this. I referred in some books, it's simply saying it's points to the current object. Could u pls explain with one simple example.

1  public class Ryan {
2      int age;
3      String address;
4      String phone;
5  
6      // Make a new Ryan object with default values.
7      public Ryan() {
8          age = 0;
9          address = "";
10         phone = "";
11     }
12     public Ryan(int age) {
13         this();  // Do the default initialization.
14         this.age = age;
15     }
16 }

The this() in line 13 calls the no-argument constructor.
Line 14 sets the instance field called age to the value of the passed in argument called age. The this is required to distinguish the instance field from the method argument of the same name.

Make sense?

Ryan

[ganesh pol]

hi my dear friend
you can refer this particualr site
http://www.developer.com/java/article.php/1440571

or

http://java.sun.com/docs/books/tutorial/java/javaOO/thiskey.html

[Kumar Sub]

I use it for member help in JBuilder. When editing code, I forget the member variable's name, it is easier to type in 'this.', select a member and continue. Helps you to continue coding and also avoid typing errors.

Then to standardize you might want to leave the 'this.' in all member references or get rid of it completely. I try to stay away from using member names for parameters, but that also happens sometimes and the 'this.' remains until I go back and clean.

[M Beck]

i try to always preface instance variables and instance methods (members) with "this", to make it explicit that i'm calling something specific to the current object. explicit is better than implicit.

[Niyas Ahmed Sheikh]

Originally posted by Ryan McGuire:

1  public class Ryan {
2      int age;
3      String address;
4      String phone;
5  
6      // Make a new Ryan object with default values.
7      public Ryan() {
8          age = 0;
9          address = "";
10         phone = "";
11     }
12     public Ryan(int age) {
13         this();  // Do the default initialization.
14         this.age = age;
15     }
16 }

The this() in line 13 calls the no-argument constructor.
Line 14 sets the instance field called age to the value of the passed in argument called age. The this is required to distinguish the instance field from the method argument of the same name.

Make sense?

Ryan

So your are telling the main use of "this" is distinguish the instance variable from the method argument. Correct me If i'm wrong, instead of assigning "age" to one new variable(x), we can use this operator.

12 public Ryan(int age) {
13 this(); // Do the default initialization.
14 x = age;
15 }

Another doubt in this(); Whether we can have any argument in the this();
this() is mainly to call constructor alone?

[John Dell'Oso]

Yes, you can pass an argument using this() - as in this(x, y, z). For example:

public class Ryan {
 
    int age;
    String address;
    String phone;
 
    public Ryan() {
 
        this(0);  // call the 1-arg constructor
        address = "";
        phone = "";
    }
 
    public Ryan(int age) {
 
        this.age = age;
    }
}  

Although not really a useful example - it does demonstrate the use of this(). And now I have had enough of that() ;-)

Regards,
JD

[fred rosenberger]

note that there is a difference between

this.myVar

and

this(myVar)

the first is saying "use the 'myVar' that is a member variable of the class i'm currently in".

the latter is saying "call the constructor for the class i'm currently in that takes a single parameter of whatever type myVar is".

[John Dell'Oso]

Niyas,

Just a couple of points I forgot to make in my previous post:

1) this() can only be used from another constructor.
2) this() must be the first line in the constructor.

Regards,
JD