### Question:
DateValue extends Value { Value add (String number, String period) { int nr = parseInteger (number); return addPeriod (nr, period); } DateValue(Calendar date);   DateValue(String stringValue); @Override String toString();  }### Answer:
@Test public void testAdd () { assertEquals ("02-01-2000", mDate.add ("1", "days").toString ()); assertEquals ("01-02-2000", mDate.add ("1", "months").toString ()); assertEquals ("01-01-2001", mDate.add ("1", "years").toString ()); assertEquals ("07-01-2000", mDate.add ("5", "business").toString ()); assertEquals ("01-01-2000", mDate.add ("0", "business").toString ()); try { mDate.add ("a", "days"); fail ("no exception"); } catch (ExecutionException ee) { } }