title: key-value store suggestion
body: <p>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). </p>
  
  <p>The map is <code>Map&lt;Integer,String&gt;</code>, and so I'm considering using GNU Trove <code>TIntObjectHashMap&lt;byte[]&gt;</code>, and storing the map value as an ascii byte array rather than String. </p>
  
  <p>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?</p>