Problems initializing a final variable in Java

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 = buildFi开发者_如何学JAVAleName();

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...?

How about

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

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.

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...

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...?

Along the same lines as the OP's issue, I had to be able to find a way to assign values to final fields to be read in from a .properties file on the filesystem, so the values couldn't be known by my app until that happened. Using a generalized method call to assign the value after reading the content of the .properties file into a Properties object on app startup was a Hail Mary pass that thankfully worked out. It also limits the no. of times the file has to be read to once per the app's getting loaded into the memory simply by the code checking to see if the Properties object is or is not currently null. But of course, once assigned, the final field's value cannot be altered except by altering its "final" status via manuipulating the field's modifying definition at runtime (as discussed in some other places here on SO, such as https://stackoverflow.com/a/3301720/1216686 - sneaky, but I love it!). Code example, with typical runtime error checking such as for NPEs omitted for brevity:

import java.util.Properties;

public class MyConstants {

  private static Properties props; // declared, not initialized,
                                   // so it can still be set to
                                   // an object reference.

  public static String MY_STRING = getProperty("prop1name", "defaultval1");
  public static int MY_INT = Integer.parseInt(getProperty("prop2name", "1"));
  // more fields...

  private static String getProperty(String name, String dflt) {
   if ( props == null ) {
     readProperties();
   }
   return props.getProperty(name, dflt);
  } 

  private static void readProperties() {
     props = new Properties(); // Use your fave way to read
                      // props from the file system; a permutation
                      // of Properties.load(...) worked for me.
  } 

  // Testing...
  public static void main(String[] args) {
      System.out.println(MY_STRING);
      System.out.println(MY_INT);
  }

}

This lets you externalize properties to be read into the app and still mark the fields used to hold their values as "final". It also allows you to guarantee a returned value for the final field value since getProperty() in the Properties class allows the method's calling code to pass in a default value to use in case the property's key-value pair wasn't found in the external .properties file.