public void testThreeArgsConstructorException()
  {
    
    // first case : the nominator is null. constructor should fail.
    try 
    {
      Quantity testo = new Quantity(100 , null, Arrays.asList("this homework is too hard"));
      
      fail();
    }
    
    // test is passed. no need to catch anything.
    catch(IllegalArgumentException e)
    {}
    
    // second case : denumerator is null. constructor should fail.
    try 
    {
      Quantity testo = new Quantity(100 , Arrays.asList("Idon'twanttogotoKoreanarmy"), null);
      
      fail();
    }
    
    // test is passed. no need to catch anything.
    catch(IllegalArgumentException e)
    {}
    
  }