Forward References in Java

[vedala V.S.Bharani kumar]

Dear Launchers,

Is threre any forward references in java? In khalid Mughal scjp1.4 book , in "initializers" chapter he gave some programs .The programs are not run in my system and I get Compilation Errors.

For Ex:

class Initialization
{
private static String msg(String msg)
{
System.out.println(msg); return msg;
}
--------------(1)
public Initialization() { m=msg("1") ; }
{ m=msg("2") ; }
String m=msg("3");
public static void main(String args[])
{
Object obj = new Initialization();
}
}

In this If I declare the String var. at (1) Compiled otherwise errors occured.


Thanks for all suggestions,

[email protected]

[Swati Thorve]

original code
class Initialization
{
private static String msg(String msg)
{
System.out.println(msg); return msg;
}
--------------(1)
public Initialization()
{//2
m=msg("1") ; //5
}
{
//3
m=msg("2") ;
}
String m=msg("3"); //4
public static void main(String args[])
{
Object obj = new Initialization(); //1
}
}

Here first look at the order in which code is executed.
In main method, at the time of creating object obj, its constructor is called.Look at the numbers provided by me, after that control goes to //2 i.e constructors first line. after that control moves to initializer block at //3. after that a string var at //4. and then return back to constructor.

We havent declared var m, before executing //3. m is declared at //4.
Here at //3, the var m is undeclared, hence it is giving compile time error of illegal forward reference.

if we change the code at //3 like
String m=msg("2") ;
then there wont be any error.
I hope u get it

[Balaji Sampath]

Swati,
Doesnt the the declaration need to be in line //2:
String m= msg("1") inside the constructor

Thanks

[Ram Roop]

The code provided here should not give any compilation errors. It would compile and run just fine. The output would be:

2
3
1

The order of execution would be:

1. initilization block execution.
2. member variable assignment.
3. constructor call.

Hope this is helpful.
[ January 12, 2006: Message edited by: Ram Roop ]

[Dmitryi Neverov]

Originally posted by Ram Roop:
The code provided here should not give any compilation errors. It would compile and run just fine. The output would be:

2
3
1

The order of execution would be:

1. initilization block execution.
2. member variable assignment.
3. constructor call.

Hope this is helpful.

[ January 12, 2006: Message edited by: Ram Roop ]

Agree, as it doesn't violate declare-before-read rule.

[sivaram omprakash]

so it seems forward referencing is allowed because during initializer block execution, variable m is not declared.Is there any scenario in java where forward referencing not allowed?.

[Gagan Deep]

I have confirmed, code is compiling fine(not giving any error).
Out put is
2
3
1

Thanks
Gagan.

[Thomas Drew]

If you try to use a varible in your code before you declare it you will get an compile error.

declaring the code

foo = "here is a string";
String foo= new String();

will give you a complie error you can not create forward reference in this manner.