using java.util.Scanner to read a file byte by byte

Question

I'm trying to read a one line file character by character using java.util.Scanner. However I'm getting this exception":

Exception in thread "main" java.util.InputMismatchException: For input string: "contents of my file"
    at java.util.Scanner.nextByte(Scanner.java:1861)
    at java.util.Scanner.nextByte(Scanner.java:1814)
    at p008.main(p008.java:18) <-- line where I do scanner.nextByte()

Here's my code:

public static void main(String[] args) throws FileNotFoundException {
    File source = new File("file.txt");
    Scanner scanner = new Scanner(source);
    while(scanner.hasNext()) {
        System.out.println((char)scanner.nextByte());
    }
    scanner.close()
}

Does anyone have any ideas as to what I might be doing wrong?

Edit: I realized I wrote hasNext() instead of hasNextByte(). However if I do that it doesn't print out anything.

Answer 1

Why on earth would you want to use a scanner to read a file byte by byte? That's like using a wheelbarrow to transport your pocket change. (If you really need a wheelbarrow for your pocket change, let me know so I can become your friend).

But seriously: Class InputStream reads bytes from a file, simply and reliably, and does nothing else.

Class scanner was recently introduced into the Java API so textbook examples could pull data out of a file with less pain than is usually involved with using the cascade of new BufferedReader(new InputStream). Its specialty is inputting numbers and strings from free-form input files. The nextByte() method actually reads one or a few decimal digits from the input stream (if they're there) and converts the number thus scanned into a single byte value.

And if you're reading bytes, why do you want to output them as chars? Bytes are not chars, and brute-force interconverting will fail in some places. If you want to see the values of those bytes, print them out as they are and you'll see small integers between 0 and 255.

If you want to read chars from a file, FileReader is the class for you.

Answer 2

Scanner is for parsing text data - its nextByte() method expects the input to consist of digits (possibly preceded by a sign).

You probably want to use a FileReader if you're actually reading text data, or a FileInputStream if it's binary data. Or a FileInputStream wrapped in an InputStreamReader if you're reading text with a specific character encoding (unfortunately, FileReader does not allow you to specify the encoding but uses the platform default encoding implicitly, which is often not good).

Answer 3

When troubleshooting Scanner, check for underlying I/O errors:

if(scanner.ioException() != null) {
  throw scanner.ioException();
}

Though I'm with the others - this probably isn't the right class for the job. If you want byte input, use an InputStream (in this case, FileInputStream). If you want char input, use a Reader (e.g. InputStreamReader).