@Test
public void testDiff() {

    IIsotope element1 = mock(IIsotope.class);
    IIsotope element2 = mock(IIsotope.class);
    when(element1.getSymbol()).thenReturn("H");
    when(element2.getSymbol()).thenReturn("C");
    String result = IsotopeDiff.diff(element1, element2);
    Assert.assertNotNull(result);
    Assert.assertNotSame(0, result.length());
    assertContains(result, "IsotopeDiff");
    assertContains(result, "H/C");
}