Java Security

I am using the following code to calculate the hash values of files

public static String hash(File f, String algorithm)
            throws IOException, NoSuchAlgorithmException {
        if (!f.isFile()) {
            throw new IOException("Not a file");
        }
        RandomAccessFile raf = new RandomAccessFile(f, "r");
        byte b[] = new byte[(int) raf.length()];
        raf.readFully(b);
        raf.close();
        MessageDigest messageDigest = MessageDigest.getInstance(algorithm);
        messageDigest.update(b);
        return toHexString(messageDigest.digest());
    }

Now the problem is, for really big files, 100 MB or over, I get an OutOfMemoryError.

I have used the -Xms and -Xms options to increase the JVM heap size, and untimately made it to work. However, I think this is lame and there is also a limit to the -Xmx option.

Is there any other way I can calculate the hash values of these really big files?

Thanks a lot in advance.