Problem with relative file path

Question

So here is my program, which works ok:

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;
import java.util.Scanner;
import java.util.Locale;

public class ScanSum {
    public static void main(String[] args) throws IOException {
        Scanner s = null;
        double sum = 0;
        try {
            s = new Scanner(new BufferedReader(new FileReader("D:/java-projects/HelloWorld/bin/usnumbers.txt")));
            s.useLocale(Locale.US);

            while (s.hasNext()) {
                if (s.hasNextDouble()) {
                    sum += s.nextDouble();
                } else {
                    s.next();
                }
            }
        } finally {
            s.close();
        }

        System.out.println(sum);
    }
}

As you can see, I am using absolute path to the file I am reading from:

s = new Scanner(new BufferedReader(new FileReader("D:/java-projects/HelloWorld/bin/usnumbers.txt")));

The problem arises when I try to use the relative path:

s = new Scanner(new BufferedReader(new FileReader("usnumbers.txt")));

I get an error:

Exception in thread "main" java.lang.NullPointerException
    at ScanSum.main(ScanSum.java:24)

The file usnumbers.txt is in the same directory as the ScanSum.class file:

D:/java-projects/HelloWorld/bin/ScanSum.class
D:/java-projects/HelloWorld/bin/usnumbers.txt

How could I solve this?

Answer 1

From which directory is the class file executed? (That would be the current working directory and base directory for relative paths.)

If you simply launch the application from eclipse, the project directory will be the working directory, and you should in that case use "bin/usnumbers.txt".

Answer 2

The NullPointerException is due to the fact that new FileReader() expression is throwing a FileNotFoundException, and the variable s is never re-assigned a non-null value.

The file "usnumbers.txt" is not found because relative paths are resolved (as with all programs) relative to the current working directory, not one of the many entries on the classpath.

To fix the first problem, never assign a meaningless null value just to hush the compiler warnings about unassigned variables. Use a pattern like this:

FileReader r = new FileReader(path);
try {
  Scanner s = new Scanner(new BufferedReader(r));
  ...
} finally {
  r.close();
}

For the second problem, change directories to the directory that contains "usnumbers.txt" before launching java. Or, move that file to the directory from which java is run.

Answer 3

It must be a FileNotFoundException causing NPE in the finally block. Eclipse, by default, executes the class with the project folder (D:/java-projects/HelloWorld in your case ) as the working directory. Put the usnumbers.txt file in that folder and try. Or change the working directory in Run Configuration -> Argument tab

Answer 4

If aioobe@'s suggestion doesn't work for you, and you need to find out which directory the app is running from, try logging the following:

new File(".").getAbsolutePath()