public static String toHex(long value) {
        return Strings.padStart(UnsignedLongs.toString(value, 16), 16, '0');
    }
@Test
    public void toHex() throws Exception {
        assertEquals("0f", Tools.toHex(15, 2));
        assertEquals("ffff", Tools.toHex(65535, 4));
        assertEquals("1000", Tools.toHex(4096, 4));
        assertEquals("000000000000000f", Tools.toHex(15));
        assertEquals("ffffffffffffffff", Tools.toHex(0xffffffffffffffffL));
        assertEquals("0xffffffffffffffff", Tools.toHexWithPrefix(0xffffffffffffffffL));

    }