### Question:
DateValue extends Value { Value subtract (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 testSubtract () { assertEquals ("31-12-1999", mDate.subtract ("1", "days").toString ()); assertEquals ("01-12-1999", mDate.subtract ("1", "months").toString ()); assertEquals ("01-01-1999", mDate.subtract ("1", "years").toString ()); assertEquals ("27-12-1999", mDate.subtract ("5", "business").toString ()); }