The .db extension on the file tells us nothing, because it depends on the program that is generating it as to what format it takes. If you are interested in using databases from Java, and Java itself, then your time would be better spent studying the Java Tutorials[^].
Ah, yes. I just got familiar with that file extension meaning nothing. I actually tried The Java Tutorials but I had a hard time understanding everything there. Perhaps I didn't had enough programming experience yet because I think they assume you've had C++ knowledge already. So I'm currently learning Java with other tutorials.
Anyway I'm familiar with MySql.
I think what you mean is to have an embedded database in your application. Well in that case, you have a number of choices. I would suggest you look at [Hypersonic SQL^]. It is lightweight and quite mature.
Another option is to look at [Sqlite^]. I have not used it in a Java project before, but it is quite a nice and fast little database.
A great thing about SQLite is the licence agreement which includes the text
"Anyone is free to copy, modify, publish, use, compile, sell, or distribute the original SQLite code, either in source code form or as a compiled binary, for any purpose, commercial or non-commercial, and by any means."
Well I think I'll settle with HyperSQL. I downloaded it. Comes with a nice documentation and that's what I'm reading now to learn it. I discovered the embedded database I saw on the example on my original post was something called H2. I think it's similar.
The first thing you need to do is to learn how to use Google to help you in your research. A quick search for "Java tutorials" would have found you The Java Tutorials[^]; clear, concise and useful for anyone new to programming.