@Override
 public int compare(CacheSpan lhs, CacheSpan rhs) {
   long lastAccessTimestampDelta = lhs.lastAccessTimestamp - rhs.lastAccessTimestamp;
   if (lastAccessTimestampDelta == 0) {
     // Use the standard compareTo method as a tie-break.
     return lhs.compareTo(rhs);
   }
   return lhs.lastAccessTimestamp < rhs.lastAccessTimestamp ? -1 : 1;
 }