Unfortunately this implementation is not very efficient for searches/updates of a certain value in a collection, as the whole collection needs to be instantiated to access any of its elements.
db4o brings some special collections with it. There are collections which support transparent persistence. See "Collections"
When you have a need for a huge collection, you might run into some performance bottleneck, since collections are always stored and retrieved as complete unit. You can use db4o special big-set to improve performance. See "Big Set"
You might wonder what is better to use, collection or arrays. Most of times it doesn't matter. See "Collections or Arrays?"