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.
Or NeDB which solve the same constraints :)
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.