I'm interested how this came about. They usually either try to match people who know common languages or they do more generic problems which don't depend on language. One important thing is if you said you knew C well (or even only "working knowledge of") then it's perfectly reasonable for them to ask you to write it in C instead of Java/Python/etc. Did the interviewers state they didn't Java beforehand? Did they say they knew another language you'd mentioned you knew on your resume?

My interview experience actually involved them hammering me on the languages I said I knew but wasn't an expert in. This is likely as they were trying to find out whether I was lying (or, put kindly, overstating) on my resume. From what I've seen there are a few people who put every language down that they've ever written hello world in ;)

(disclaimer: recent intern from Google Sydney office)