class ByteUtil {
    public static byte[] longToBytes(long x) {
        ByteBuffer buffer = ByteBuffer.allocate(Long.BYTES);
        buffer.putLong(0, x);
        return buffer.array();
    }

    public static long bytesToLong(byte[] bytes) {
        ByteBuffer buffer = ByteBuffer.allocate(Long.BYTES);
        buffer.put(bytes, 0, bytes.length);
        buffer.flip();//need flip
        return buffer.getLong();
    }

    public static String randomAlphabetic(int length) {
        int leftLimit = 97; // letter 'a'
        int rightLimit = 122; // letter 'z'
        Random random = new Random();

        return random.ints(leftLimit, rightLimit + 1)
                .limit(length)
                .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
                .toString();
    }

    public static String randomNumeric(int length) {<FILL_FUNCTION_BODY>}

}

        int leftLimit = 48; // letter '0'
        int rightLimit = 57; // letter '9'
        Random random = new Random();

        return random.ints(leftLimit, rightLimit + 1)
                .limit(length)
                .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
                .toString();
    
<no_super_class>