Despite this List<T> requirement this is a pretty nice task. Really strange they ask Java questions on a Ruby interview.

However, I think it’s pretty manageable to finish this by 30 mins. Especially if you extend AbstractSequentialList, an abstract class implementing a List interface (so I believe it meets the requirements), helping you avoid implementing dozens of methods by hand. If that would be a Java interview, I would expect a candidate to finish that in 30 mins (not sure if “junior” though).