@Test
  public void should_return_preferred_instance() {
    final DividendYieldCalculator common =
        dividendYieldCalculatorFactory.newInstance(StockType.PREFERRED);

    assertNotNull(common);
    assertEquals(DividendYieldPreferredCalculatorImpl.class, common.getClass());
    assertNotEquals(DividendYieldCommonCalculatorImpl.class, common.getClass());
  }
 @Test(expected = NullPointerException.class)
 public void should_throw_null_pointer_exception() {
   dividendYieldCalculatorFactory.newInstance(null);
 }