title: HashMap with date ranges
body: <p>I need to find some values <strong>very fast</strong> based on a multi-key.
  The key is composed by:
      <code>&lt; Int userId, String measureName, Date startDate, Date endDate &gt;</code>
  and the value is a double.</p>
  
  <p>The problem is that I have to ask for the value indicating a day, and not a date range.
  So if I'll ask for a <code>userId</code>, a <code>measureName</code> and a day, the data structure has to answer with the value where the day is between <code>startDate</code> and <code>endDate</code> (there are no overlap between date ranges).</p>
  
  <p>I cannot understand which is the better data structure to implement this. HashMap? TreeMap? MultiKeyMap? RangeMap? Help! :)</p>