It should be specified that the random tests may include non-letter, non-space characters.

Good Kata. Feels like 7 kyu, though, not 6.

Minor issue, but it appears that you need to add

import java.math.BigInteger;

at the top of the solution to make it work, unless of course you intended for us to figure this out ourselves for some reason.

Ah, I did this too, but mine was so much more messy. I didn't shorten the given variable name "numbers" to just "n"! Nice catch there.

I as well. I was using Get methods, but wasn't sure if it was what the Kata was asking.