在 Java 中，为什么我在尝试显示 FileNotFoundException 时会收到 NullPointerException？

Question

I think it's easier to just show the code and the output I'm getting than trying to explain it :)

This is from my main method:

//prompt user for filename
    System.out.println("Please enter the text file name. (Example: file.txt):");
    String filename = ""; //will be used to hold filename

   //loop until user enters valid file name
    valid = false;
    while(!valid)
    {
        filename = in.next();
        try
        {
            reader.checkIfValid(filename);
            valid = true; //file exists and contains text
        }
        catch (Exception e)
        {
            System.out.println(e + "\nPlease try again.");
        }
    }

And this is the reader.checkIfValid method:

public void checkIfValid(String filename) throws InvalidFileException, FileNotFoundException
{
    try
    {
        in = new Scanner(new File(filename));

        if (!in.hasNextLine()) // can't read first line
            throw new InvalidFileException("File contains no readable text.");
    }
    finally
    {
        in.close();
    }
}

This is the output I get when a nonexistent file is entered:

Please enter the text file name. (Example: file.txt):

doesNotExist.txt

java.lang.NullPointerException

Please try again.

Why is the System.out.println(e) getting a NullPointerException? When I enter an empty file or a file with text, it works just fine. The empty file prints the InvalidFileException (a custom exception) message.

When I put a try-catch statement around the "in = new Scanner(new File(filename));", and have the catch block display the exception, I do get the FileNotFoundException printed out, followed by the NullPointerException (I'm not entirely sure why the catch block in the main method would be activated if the exception was already caught in the checkIfValid method...).

I've spent a while on this and I'm completely clueless as to what's wrong. Any help would be appreciated. Thanks!

Answer 1

edited: I think the null pointer comes from the call to reader, it is poor practise to catch all exceptions as you no longer know where they came from!

Maybe the checkIfValid method should just check if the filename is valid?

public boolean checkIfValid(String filename) {
    try {
        File file = new File(filename);
        return file.exists();   
    } catch (FileNotFoundException) {
        System.out.println("Invalid filename ["+filename+"] "+e);
    }
}

Then the code calling it could look like;

filename = in.next();
valid = reader.checkIfValid(filename);
if (valid)
    List<String> fileContents = readFromFile(filename);

Then contain all the file reading logic in it's own method like this;

public List<String> readFromFile(filename) {
    List<String> fileContents = new ArrayList<String>();
    try {
        in = new Scanner(new File(filename));
        while (in.hasNextLine()) {
            fileContents.add(in.nextLine);
        }
    } catch (IOException e){
        //do something with the exception
    } finally {
        in.close();
    }
    return fileContents;        
}

Answer 2

My mistake was something only I could've seen. I was catching all the exceptions so I wasn't able to see where it was coming from. Thank you for helping!