Consider ehcache. I show here a class for wrapping it as a java.util.Map. You can easily store Lists or other data structures as your values, avoiding the O(m*n) issue you are concerned with. ehcache is Apache 2.0 license, with an commercial enterprise version available by Terracotta. The open source version will allow you to spill your cache to disk, and if you choose not to evict cache entries it is effectively a persistent key-value store.
