Here's a short article with code snippets by Damasia Maneiro who's working on an Android app that will use db4o. In her own words: "db4o is an object database, ie. forget about mapping of tables in a relational model. If you're a developer that translates into savings in time invested in your application and volume of code. db4o's great potential is that you can reuse your (plain, non-mapped) objects by saving and retrieving them as many times as you want. You can persist complex objects with nested collections or other complex objects, any level of complexity in your hierarchy. You're not limited to flat objects with primitive types to get a reasonable performance."

http://www.dzone.com/links/r/using_db4o_in_an_android_application.html