扫描程序变量在try-catch块之外不起作用

Question

I tried to declare s.next() after the try-catch block but it wouldn't work! The s would only have the drop down if it is inside the try block.

I don't want to lump Parsing inputs, take appropriate actions all into the try block, because they wouldn't throw the FNFE and IOE. What can I do here?

public static void main(String[] args) 
      {
        // TODO Auto-generated method stub

        //Open file; file name specified in args (command line)
        try{
            FileReader freader = new FileReader(args[0]);
            Scanner s = new Scanner(freader);

        }catch(FileNotFoundException e){
            System.err.println("Error: File not found. Exiting program...");
            e.printStackTrace();
            System.exit(-1);
        }catch(IOException e){
            System.err.println ("Error: IO exception. Exiting...");
            e.printStackTrace();
            System.exit(-1);
        }
        // if i try to declare s.next() here it would not work

Answer 1

I think you mean you want to use s.next() and that it won't work.

to do that, declare s as a variable outside the try/catch block, set it to null there. Then assign it where you assign it now, but without the declaration. If my assumption is correct, your problem is that s is no longer an active variable outside the try/catch, because it is declared within that block.

FileReader freader = null;
Scanner    s       = null;
try { freader = new FileReader(args[0]); // risk null pointer exception here
      s = new Scanner(freader);
    }
catch { // etc.

Answer 2

because s variable which is an instance of Scanner class is limited to try block only. if you want s to be accessible outside the try-catch block, declare it outside the try catch.

 Scanner s = null;
 try{
        FileReader freader = new FileReader(args[0]);
         s = new Scanner(freader);

    }catch(FileNotFoundException e){
        System.err.println("Error: File not found. Exiting program...");
        e.printStackTrace();
        System.exit(-1);
    }catch(IOException e){
        System.err.println ("Error: IO exception. Exiting...");
        e.printStackTrace();
        System.exit(-1);
    }

Answer 3

In Java, variables are scoped by the block they are declared in. Since your Scanner is constructed inside the try block, it is not visible outside of it.

Is there any reason you want to do your actual scanning operations outside this block? In Java 7, a common idiom is the try-with-resources pattern:

try (Scanner s = new Scanner(new FileInputStream(file)) {
  //Do stuff...
}

Which will automatically close out the Scanner resource. As it is, you could potentially leak it, since there's no finally block in your code sample.