If you are already familiar with using the Apache DB I believe you will find it very simple to start using it as an in-memory database. You just need to change connection string you are using.
Using Apache DB as an internal database running together with our application in the same process you just need to add the word memory to the connection string (e.g. jdbc:derby:memory:nameOfOurDB;create=true).
Using Apache DB as an application running separated from ours just add the word memory together with the name of the database to the connection string your are using (e.g. jdbc:derby://127.0.0.1:1527/memory:nameOfOurDB;create=true).