Collection a = new ArrayList();
        a.add(new Student("tom",3));
        a.add(new Student("jerray",2));
        a.add(new Student("jack",43));
        System.out.println(a.size());
        Object[] o = a.toArray();
        Object ob=a.toArray();//对象接收数组?

Object是最顶层的类，Object[] o = a.toArray()；父类引用指向子类对象