Not Caffeine, that's the caching library. You probably meant one of FastUtil, Eclipse Collections, Koloboke, HPPC(-RT), they all provide primitive specializations of Maps and Collections.
For off-heap maps, now that's more interesting! I know about Chronicle Map, MapDB, https://github.com/snazy/ohc and https://github.com/RuedigerMoeller/fast-serialization/wiki/O....