@Test
    public void testGetMassWithHigherIndex() {
        mass1 = new Mass(1, 1);
        mass2 = new Mass(2, 2);
        graph.addMasses(mass1, mass2);

        graph.addSpring(mass1, mass2);
        assertSame(mass1, graph.getSprings().get(0).getMassWithLowerIndex());
        assertSame(mass2, graph.getSprings().get(0).getMassWithHigherIndex());
    }