@Test
public void testDifference() {

    IIsotope element1 = mock(IIsotope.class);
    IIsotope element2 = mock(IIsotope.class);
    when(element1.getSymbol()).thenReturn("H");
    when(element2.getSymbol()).thenReturn("C");
    IDifference difference = IsotopeDiff.difference(element1, element2);
    Assert.assertNotNull(difference);
}