この例では、ファイル・リストの16ビットのチェックサムを計算します。高速化のため、NIOマップ・バイト・バッファを使用します。

public class Sum {

    // Compute a 16-bit checksum for all the remaining bytes
    // in the given byte buffer

    private static int sum(ByteBuffer bb) {
        int sum = 0;
        while (bb.hasRemaining()) {
            if ((sum & 1) != 0)
                sum = (sum >> 1) + 0x8000;
            else
                sum >>= 1;
            sum += bb.get() & 0xff;
            sum &= 0xffff;
        }
        return sum;
    }

    // Compute and print a checksum for the given file

    private static void sum(File f) throws IOException {

        // Open the file and then get a channel from the stream
        try (
            FileInputStream fis = new FileInputStream(f);
            FileChannel fc = fis.getChannel()) {

            // Get the file's size and then map it into memory
            int sz = (int) fc.size();
            MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, sz);

            // Compute and print the checksum
            int sum = sum(bb);
            int kb = (sz + 1023) / 1024;
            String s = Integer.toString(sum);
            System.out.println(s + "\t" + kb + "\t" + f);
        }
    }

    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 e) {
                System.err.println(f + ": " + e);
            }
        }
    }
}