code: public final StringBuffer 
          format(Object obj, StringBuffer appendTo, FieldPosition fieldPosition)
      {
          if ( obj instanceof DateInterval ) {
              return format( (DateInterval)obj, appendTo, fieldPosition);
          }
          else {
              throw new IllegalArgumentException("Cannot format given Object (" + obj.getClass().getName() + ") as a DateInterval");
          }
      }
docs: Format an object to produce a string. This method handles Formattable
  objects with a DateInterval type.
  If a the Formattable object type is not a DateInterval,
  IllegalArgumentException is thrown.
  
  @param obj               The object to format.
  Must be a DateInterval.
  @param appendTo          Output parameter to receive result.
  Result is appended to existing contents.
  @param fieldPosition     On input: an alignment field, if desired.
  On output: the offsets of the alignment field.
  There may be multiple instances of a given field type
  in an interval format; in this case the fieldPosition
  offsets refer to the first instance.
  @return                  Reference to 'appendTo' parameter.
  @throws    IllegalArgumentException  if the formatted object is not
  DateInterval object