public static void main(String[] args) {
        if (args.length < 1) {
            System.err.println("Usage: java Sum file...");
            return;
        }
        for (int i = 0; i < args.length; i++) {
            File f = new File(args[i]);
            try {
                sum(f);
            } catch (IOException x) {
                System.err.println(f + ": " + x);
            }
        }
    }