Problems initializing a final variable in Java

Question

I keep running into slight variations of a problem in Java and it's starting to get to me, and I can't really think of a proper way to get around it.

I have an object property that is final, but dynamic. That is, I want the value to be constant once assigned, but the value can be different each runtime. So I declare the class level variable at the beginning of the class - say private final FILE_NAME;. Then, in the constructor, I assign it a value - say FILE_NAME = buildFileName();

The problem begins when I have code in the buildFileName() method that throws an exception. So I try something like this in the constructor:

try{
   FILE_NAME = buildFileName();
}
catch(Exception e){
   ...
   System.exit(1);
}

Now I have an error - "The blank final field FILE_NAME may not have been initialized." This is where I start to get slightly annoyed at Java's strict compiler. I know that this won't be a problem because if it gets to the catch the program will exit... But the compiler doesn't know that and so doesn't allow this code. If I try to add a dummy assignment to the catch, I get - "The final field FILE_NAME may already have been assigned." I clearly can't assign a default value before the try-catch because I can only assign to it once.

Any ideas...?

Answer 1

On second thought, I think I just came up with a solution! - use an intermediate variable.

String fileName = null;
try{
   fileName = buildFileName();
}
catch(Exception e){
   ...
   System.exit(1);
}
FILE_NAME = fileName;

Don't know why it took me so long to think of this...

Answer 2

How about

String tempName = null;
try{
   tempName = buildFileName();
}
catch(Exception e){
   ...
   System.exit(1);
}
FILE_NAME = tempName;

Answer 3

Either

try {
   FILE_NAME = buildFileName();
} catch (Exception e){
   ...
   System.exit(1);
   throw new Error();
}

Or some prefer:

private static final String FILE_NAME = fileName();

private static String fileName() {
    try {
        return buildFileName();
    } catch (Exception e){
        ...
        System.exit(1);
        throw new Error();
    }
}

But calling System.exit in a static initialiser is probably a bad idea. It's going to mess your unit tests up.

Answer 4

I would personally just throw an Error -- if your error flow is properly designed, the System.exit() should be redundant. Your program presumably doesn't plough on into the wilderness if an Error is thrown...?