@Test
public void testDifference() {

    IAtom atom1 = mock(IAtom.class);
    IAtom atom2 = mock(IAtom.class);
    when(atom1.getSymbol()).thenReturn("H");
    when(atom2.getSymbol()).thenReturn("C");
    IDifference difference = AtomDiff.difference(atom1, atom2);
    Assert.assertNotNull(difference);
}