I need a very basic key-value store for java. I started with a HashMap but it seems that HashMap is somewhat space inefficient (I'm storing ~20 million records, and seems to require ~6GB RAM).
The map is Map<Integer,String>, and so I'm considering using GNU Trove TIntObjectHashMap<byte[]>, and storing the map value as an ascii byte array rather than String.
As an alternative to that, is there a key-value store that only requires adding jar files, does not hold the entire map in RAM at once, and is still reasonably fast?