@Test
    public void testGetSpringsByMass() {
        assertTrue(graph.getSprings(mass1).isEmpty());

        graph.addSpring(mass1, mass2);
        List<Spring> springs = graph.getSprings(mass1);

        assertEquals(1, springs.size());
        assertSame(mass1, springs.get(0).getSource());
        assertSame(mass2, springs.get(0).getDestination());

        graph.addSpring(mass1, mass3);
        springs = graph.getSprings(mass1);

        assertEquals(2, springs.size());
        assertSame(mass1, springs.get(0).getSource());
        assertSame(mass2, springs.get(0).getDestination());
        assertSame(mass1, springs.get(1).getSource());
        assertSame(mass3, springs.get(1).getDestination());
    }