public static String toHexString(final byte[] bytes) {
        return toHexString(bytes, ":");
    }
@Test
    public void testToStringBytes() {
        byte[] dpid = {0, 0, 0, 0, 0, 0, 0, -1 };
        String valid = "00:00:00:00:00:00:00:ff";
        String testString = HexString.toHexString(dpid);
        assertEquals(valid, testString);

        String validNoSep = "00000000000000ff";
        assertEquals(validNoSep, HexString.toHexString(dpid, null));
    }