abstract protected Comparable getComparable(String termtext)
Returns an object which, when sorted according to natural order,
will order the Term values in the correct order.
For example, if the Terms contained integer values, this method
would return new Integer(termtext). Note that this
might not always be the most efficient implementation - for this
particular example, a better implementation might be to make a
ScoreDocLookupComparator that uses an internal lookup table of int.

 public ScoreDocComparator newComparator(IndexReader reader,
    String fieldname) throws IOException 
{
    final String field = fieldname.intern();
    final Comparable[] cachedValues = FieldCache.DEFAULT.getCustom (reader, field, SortComparator.this);
    return new ScoreDocComparator() {
      public int compare (ScoreDoc i, ScoreDoc j) {
        return cachedValues[i.doc].compareTo (cachedValues[j.doc]);
      }
      public Comparable sortValue (ScoreDoc i) {
        return cachedValues[i.doc];
      }
      public int sortType(){
        return SortField.CUSTOM;
      }
    };
}