You shouldnt. You should use this: https://github.com/sergeyksv/tingodb so that when you realize that you do need mongodb after all, you can just drop it in.
It sounds like the NoSQL equivalent of SQLite. Which is to say, if you just want a local cache in a standalone program, rather than a network service (with all the maintenance and deployment ceremony that implies).
If you just want a local cache in a standalone program, why not simply use a POJO? There's an awful lot of ceremony here and it doesn't seem to be adding any value.
