Sometimes its handy to have a database embedded within an application. It means that your users don't need to install any database software themselves. Saves a lot of hassle all round.
There've been some questions around that scenario recently, so I uploaded the sources of the embedded database scenario here:
Use the above, i.e., use Subversion with the above link to check out a growing set of NetBeans API samples. One of them is this one:
Once you've opened the above in the IDE, you should see this structure, i.e., notice the Derby database CustDB below, in the Files window:
When you run the app, you should see this, i.e., data from an embedded database displayed in a NetBeans Platform application:
I.e., a very simple, yet functioning, sample of an embedded database within a Swing application.