public static String u8(long v) {
        char[] result = new char[16];
        for (int i = 0; i < 16; i++) {
            result[15 - i] = Character.forDigit((int) v & 0x0f, 16);
            v >>= 4;
        }

        return new String(result);
    }
@Test
    public void testU8() {
        Assert.assertEquals("0000000000000000", Hex.u8(0L));
        Assert.assertEquals("0000016b5086c128", Hex.u8(1560424137000L));
        Assert.assertEquals("000462d53c8abac0", Hex.u8(1234567890123456L));
    }