如何在没有转换的情况下将String放入Java中的字节数组

I want to put a string value to an array byte without any conversion. the string length is always 14 and it's in fact an hexadecimal value presenting the date and time. so every 2 chars in this string is a byte so my byte array is a 7 bytes length. i didn't find a way to do so. the I want to put a string value to an array byte w