How can I read a file as unsigned bytes in Java?

Question

How can I read a file to bytes in Java?

It is important to note that all the bytes need to be positive, i.e. the negative range cannot be used.

Can this be d

Answer 1

You wrote in a comment (please put such informations in the question - there is an edit link for this):

I need to be able to multiply the contents of a file by a constant. I was assuming that I can read the bytes into a BigInteger and then multiply, however since some of the bytes are negative I am ending up with 12 13 15 -12 etc and gets stuck.

If you want to use the whole file as a BigInteger, read it in a byte[], and give this array (as a whole) to the BigInteger-constructor.

/**
 * reads a file and converts the content to a BigInteger.
 * @param f the file name. The content is interpreted as
 *   big-endian base-256 number.
 * @param signed if true, interpret the file's content as two's complement
 *                  representation of a signed number.
 *               if false, interpret the file's content as a unsigned
 *                  (nonnegative) number.
 */
public static BigInteger fileToBigInteger(File f, boolean signed)
    throws IOException
{
    byte[] array = new byte[file.length()];
    InputStream in = new FileInputStream(file);
    int i = 0; int r;
    while((r = in.read(array, i, array.length - i) > 0) {
        i = i + r;
    }
    in.close();
    if(signed) {
        return new BigInteger(array);
    }
    else {
        return new BigInteger(1, array);
    }
}

Then you can multiply your BigInteger and save the result in a new file (using the toByteArray() method).

Of course, this very depends on the format of your file - my method assumes the file contains the result of the toByteArray() method, not some other format. If you have some other format, please add information about this to your question.

"I need to be able to multiply the contents of a file by a constant." seems quite a dubious goal - what do you really want to do?