Definite Assignment : in a for loop

[Binu K Idicula]

Hi,
Any local variable should be assigned before they are used, otherwise it is a compiler error. And there are some rules in JLS about the same. i could not fully digest the rules, but can somebody try this out in your JVM? In my JVM it DOES COMPILE fine.

 
public class CrazyAssignment{
       public static void main(String[] args){
            System.out.println("Hello");
        int k,x;
            for (;true||(k=readint())>0 ; )  {
    }
            for (boolean xF = true;xF || (x=readint())>0 ; )  {
System.out.println("x:"+x);
break;
    }
       }
       static int readint(){return 1;} 
}

However, notice the first for loop is just infinite loop. However if I change the code like as follows, the code complains of missing definite assignment.

    
        int k;
            for (;true||(k=readint())>0 ; )  {
    }
            int x;
            for (boolean xF = true;xF || (x=readint())>0 ; )  {
System.out.println("x:"+x);
break;
    }
            

    
    int k,x;
            for (;true||(k=readint())>0 ; )  {
                 break;
    }
            for (boolean xF = true;xF || (x=readint())>0 ; )  {
System.out.println("x:"+x);
break;
    }

If I change the code sample-3 break to continue, it again compiles fine. Is this related to reachability?

[ April 12, 2006: Message edited by: Binu K Idicula ]
[ April 12, 2006: Message edited by: Binu K Idicula ]

[Vishal Chawla]

Yes it is reachability.

The part x=readint() will never get called, so the code does not compile. When you dont have the break statement (code sample 2) , the code will never reach the block, where x is accessed, so you dont get any compilation error.

BTW, i am not getting any compilcation error for code sample 2

[Melle Lee]

I got compile error (x is possibly not initialized) for sample 2:

public class CrazyAssignment{
public static void main(String[] args){
System.out.println("Hello");
int k;
for (;true||(k=readint())>0 ; ) {}

int x;
for (boolean xF = true;xF || (x=readint())>0 ; ) {
System.out.println("x:"+x);
break;
}
}
static int readint(){return 1;}
}

[Vishal Chawla]

actually, it doesnt give me an error in eclipse. but if i use my command prompt and run the code. it does give me compile error.

[d jones]

Hi,

I am confused with this code. The difference between sample 1 and sample 2 is that in sample 1 k and x are declared on 1 line but in sample 2 they are declared on separate lines.

Sample 1 compiles fine for me but sample 2 I get a compilation error that x might not have been initialised.

Can anyone please explain why these 2 ways of declaring variables would make any difference.

I would have expected a compiliation error in both samples.

Thanks

[Binu K Idicula]

I think I should agree with Vishal in doubting unreachability which is not certain a compile time.
if I say,

1. int x;
2. if(true||(x=1)==1) return;
3. boolean y = false;

Compiler will not complain that statement 3 is unreachable. However I trust compiler should complain about chance of a local variable uninitialised and accessed. BUT IT DOESNOT COMPLAIN ABOUT LINE 3 in the following code sample, trying to access x, which is unitialized. I agree it is not reachable, but is it beyond rule?


1. int x;
2. if(true||(x=1)==1) return;
3. System.out.println(x);
4. boolean y = false;

Any Thoughts..

regards
Binu K Idicula
[ April 13, 2006: Message edited by: Binu K Idicula ]