return to sender

[simcel jones]

hello happy new year and thankyou for reading this
i am experimenting with io at the moment and came across the following puzzle
why in the code below do I have to include the
return;
at the end of the try/catch statement for the file to compile without the
fin might not be initialized error

import java.io.*;

class CopyFile {
public static void main(String args[]) throws IOException {
int i;
int a=args.length;
int b;
int c=54;
FileInputStream fin;
FileOutputStream fout;
try {fin=new FileInputStream(args[0]); } catch (FileNotFoundException e) { System.out.println("filenferror"); return;}
try {
i=fin.read();
} catch (IOException e) { System.out.println("fileerror");
}
fin.close();
}
}

[Rob Acraman]

Happy New Year to you, too.
What happens if the "new FileInputStream" throws an exception? Answer: execution jumps immediately to the catch block - without even doing the assignment to 'fin'.
So, if you didn't have the 'return', the code would exit the catch block and move onto to "fin.read()" - But in this case, 'fin' is still unassigned !!
To avoid this situation, Java insists that all variables are "definitely assigned" before use - and will throw the compiler error you hit if there is even the possibility of unassigned variable usage.

[Peter Tran]

Simcel,
Rob is absolutely right in his reply. That's a nice feature about JAVA. The compiler checks all local variable must be initialized before you're allowed to use it. A better way to structure your program is:

class CopyFile {
  public static void main(String args[]) throws IOException {
    int i;
    int a=args.length;
    int b;
    int c=54;
    FileInputStream fin;
    FileOutputStream fout;
    try {
      fin=new FileInputStream(args[0]);
      try {
        i=fin.read();
      }
      catch (IOException e) {
        System.out.println("fileerror");
      }
      fin.close();
    }
    catch (FileNotFoundException e) {
      System.out.println("filenferror");
    }
  }
}

There's nothing to say that you can't imbed try...catch blocks.
-Peter