@Test
    public void testRemoveMassSpringNetwork() {
        mass1 = new Mass(1, 2);
        mass2 = new Mass(3, 4);
        mass3 = new Mass(5, 6);
        mass4 = new Mass(5, 6, SHOULDER);
        Mass mass5 = new Mass(5, 6, ELBOW);
        Mass mass6 = new Mass(5, 6, HAND);
        Mass mass7 = new Mass(5, 6, ARM_SEGMENT);
        Mass mass8 = new Mass(5, 6, ARM_SEGMENT);
        Mass mass9 = new Mass(5, 6, INPUT);

        graph.addMasses(mass1, mass2, mass3, mass4, mass5, mass6, mass7, mass8, mass9);

        // removable 
        graph.addSpring(mass1, mass2);

        // not removable
        graph.addSpring(mass4, mass7);
        graph.addSpring(mass7, mass5);
        graph.addSpring(mass5, mass8);
        graph.addSpring(mass8, mass6);
        graph.addSpring(mass4, mass9);

        Spring spring1 = graph.getSprings().get(1);
        Spring spring2 = graph.getSprings().get(2);
        Spring spring3 = graph.getSprings().get(3);
        Spring spring4 = graph.getSprings().get(4);
        Spring spring5 = graph.getSprings().get(5);

        graph.removeMassSpringNetwork();
        assertEquals(6, graph.getMasses().size());
        assertSame(mass4, graph.getMasses().get(0));
        assertSame(mass5, graph.getMasses().get(1));
        assertSame(mass6, graph.getMasses().get(2));
        assertSame(mass7, graph.getMasses().get(3));
        assertSame(mass8, graph.getMasses().get(4));
        assertSame(mass9, graph.getMasses().get(5));

        assertEquals(5, graph.getSprings().size());
        assertSame(spring1, graph.getSprings().get(0));
        assertSame(spring2, graph.getSprings().get(1));
        assertSame(spring3, graph.getSprings().get(2));
        assertSame(spring4, graph.getSprings().get(3));
        assertSame(spring5, graph.getSprings().get(4));
    }