关于引用地址和物理地址的一个困惑,该如何处理

@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());    }