@Test
public void testDifference() {

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