Why there is a compiler error?

[Supun Lakshan Dissanayake]

class Example{
	public static void main (String args[]){
		final int x;
		x=101;
		int y;
		if (x>100) {
		y=-1;
		}
		System.out.println(y);
	}
}

I know there is no compile error when i use following code

class Example{
	public static void main (String args[]){
		final int x=101;
		int y;
		if (x>100) {
		y=-1;
		}
		System.out.println(y);
	}
}

All I need to know what is the difference?

[fred rosenberger]

The compiler is dumb. In the first example, it is not able to determine that the if statement will always be true.

[Henry Wong]

Welcome to the ranch ... and next time, can you give full details? You stand a higher chance of getting responses if you describe the error and in as much details as possible. See ...

https://www.coderanch.com/how-to/java/TellTheDetails


Anyway ...

fred rosenberger wrote:The compiler is dumb. In the first example, it is not able to determine that the if statement will always be true.

And in the second case, the compiler is able to determine that the condition will always be true, as the variable x is a compile time constant variable.

Henry

[Campbell Ritchie]

Welcome again.
What it means is there is a possibility that there might be another line like x = 0; which would mean y was never assigned to. It is not possible for a compiler to follow the path of execution and test whether a value has changed. In the case of a final variable, however, there is no need to test it; the compiler knows it will not change.

[Buddhi Vikasitha]

Campbell Ritchie wrote:Welcome again.
What it means is there is a possibility that there might be another line like x = 0; which would mean y was never assigned to. It is not possible for a compiler to follow the path of execution and test whether a value has changed. In the case of a final variable, however, there is no need to test it; the compiler knows it will not change.

But sheriff, we have already told compiler that x is a final variable, then why thinking about a future x=0 line??

[Angus Comber]

final int x;  
x=101; 

Isn't this an error. You are declaring a final value (but not assigning a value). Then you set a value.

I didn't think this was possible if you declare a value final (unless you assign in constructor).

But interestingly in my Java it doesn't give an error.

[Jeff Verdegan]

Angus Comber wrote:

final int x;  
x=101; 

Isn't this an error. You are declaring a final value (but not assigning a value). Then you set a value.

I didn't think this was possible if you declare a value final (unless you assign in constructor).

But interestingly in my Java it doesn't give an error.

The rules are different for final static members vs. final non-static members vs. final locals.

static : Must be assigned by the time all static initializers have completed normally.

non-static : Must be assigned by the end of all possible paths through constructors have completed normally.

local : Must be assigned before it is read.

[Supun Lakshan Dissanayake]

Thanks for the posts guys!

[Campbell Ritchie]

You’re welcome

Are there several people in the same class who have been given the same question? We have had quite a few similar posts in the last two weeks.