I've done a lot of stuff, mostly Minecraft modding, with Java. So I don't really expect any surprises.
After Groovy, Java feels good again. It is indeed pretty verbose but feels a lot more concise than Groovy.
I just want to say I really liked today's task. It really tricked you into a naive approach first and then punched you on the nose for the second part. I love the fact that it let you run into a memory space problem ^^