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

如果aioobe @的建议对您不起作用，并且您需要找出应用程序运行的目录，请尝试记录以下内容：

new File(".").getAbsolutePath()

Answer 2

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 3

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 4

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 5

Since your working directory is “D:/java-projects/HelloWorld”

@pdbartlett's id is great, But String filePath = new File(".").getAbsolutePath() will output "D:/java-projects/HelloWorld/." which is not easy to add your extra relative path like "filePath" + "/src/main/resources/" + FILENAME which located in resources folder.

I suggest with String filePath = new File("").getAbsolutePath() which return the project root folder

Answer 6

In Eclipse, you can also look under "Run Configurations->Than TAB "Classpath".

By default the absolut path is listed under "User Entries" in [ icon ] ' your.path ' (default classpath)

Answer 7

Put the file in resources folder.

ClassLoader classLoader = getClass().getClassLoader();
String file1 = classLoader.getResource("myfile.csv").getPath();