SOLR-2906 gave us an inefficient LFU cache modeled on FastLRUCache/ConcurrentLRUCache. It could use some serious improvement. The following project includes an Apache 2.0 licensed O(1) implementation. The second link is the paper (PDF warning) it was based on:
Using this project and paper, I will attempt to make a new O(1) cache called FastLFUCache that is modeled on LRUCache.java. This will (for now) leave the existing LFUCache/ConcurrentLFUCache implementation in place.