Using Hacker Rank (Java version 8 top), read a file (no boilerplate code for the Java File reading/write nightmare + no way to google it) and store the content in N files according to the file content extension.
Anonimo
Java file management is not something you will learn, since it is something you don't use every day. I use List, which was a discussion on which implementation to use (LinkedList vs ArrayList). Time was consumed trying to make the file part work instead of focusing on the problem resolution.