Google has an open-source, in-memory implementation of Java 7’s FileSystemProvider. The project is called jimfs.
If you use Java 6 or earlier, there is an alternative: I’ve used Apache Commons VFS before to great success. It seems to be much like the custom FileSystemProvider another answerer mentioned is in Java 7.
It comes pre-loaded with several file-system implementations: File, RAM, S/FTP, and Jar to name a few. I’ve also seen a plugin for S3.