@Test
    public void testObject() {
        ArrayList list = new ArrayList();
        System.out.println("a : " + list.hashCode());
        changeObject(list);
        System.out.println("d : " + list.hashCode());
    }
    
    public void changeObject(ArrayList list) {
        System.out.println("b : " + list.hashCode());
        list.add("1");
        System.out.println("c : " + list.hashCode());
    }