Previous state tax

Hi, 

 

For the year 2015, I was a visiting student researcher in the US and received a gross income of a bit less than 10k, I submitted my federal taxes and received a refund. Was I required to file state taxes too? I don't think I was required, but I just want to confirm. The state in question is California, and I was a nonresident alien for federal tax purposes. 

 

Thanks!