I will add another answer which is the fastest one possible ׂ(yes, even more than the accepted answer), BUT it will not work for every single case. HOWEVER, it WILL work for every conceivable scenario:
You can simply use String as intermediate. Note, this WILL give you the correct result even though it seems like using String might yield the wrong results AS LONG AS YOU KNOW YOU'RE WORKING WITH "NORMAL" STRINGS. This is a method to increase effectiveness and make the code simpler which in return must use some assumptions on the data strings it operates on.
Con of using this method: If you're working with some ASCII characters like these symbols in the beginning of the ASCII table, the following lines might fail, but let's face it - you probably will never use them anyway.
Pro of using this method: Remember that most people usually work with some normal strings without any unusual characters and then the method is the simplest and fastest way to go.
from Long to byte[]:
byte[] arr = String.valueOf(longVar).getBytes();
from byte[] to Long:
long longVar = Long.valueOf(new String(byteArr)).longValue();