java - NullPointerException with Scanner

Question

I am getting a NullPointerException on the 5th line. I am not really sure why or how to fix it...

    public static Scanner getInputScanner(Scanner console){
    Scanner inputFile = new Scanner(System.in);
    Scanner file = null;
    String userInputFile = null;
    while (file.equals(null)) {
        try {
            System.out.print("Enter input file: ");
            userInputFile = inputFile.nextLine();
            file = new Scanner(new File(userInputFile));
        } catch (FileNotFoundException e) {
            System.out.print(userInputFile + " (No such file or directory)");
            file = null;
            return file;
        }
    }
    return file;
}

Any pointers?

Answer 1

Although you generally compare Java objects using the equals method, null comparison is a notable exception: no Java Object compares equal to null - Java classes need to satisfy this requirement:

For any non-null reference value x, x.equals(null) should return false.

Moreover, file is null to start with, so calling any methods on it, including equals, will result in NPE.

Therefore, you need to use reference equality instead:

while (file == null) {
    ...
}