//一个引用能够调用哪些成员（变量和函数），取决于这个引用的类型//一个引用调用的是哪一个方法，取决于这个引用所指向的对象class Test{    public static void main(String args []){        //Student s = new Student();        //Person p = s;        Person p = new Student();//向上转型        Student s = (Student)p;//向下转型，必须先向上转型才能向下转型，否则是错误的转型                        p.name = "zhangsan";        p.age = 20;        //错误的p.address = "Beijing";        p.introduce();            }}

class Person{    String name;    int age;        void introduce(){        System.out.println("my name is " + name +",my age is " + age);    }    }

class Student extends Person{    String address;    void study(){        System.out.println("我正在学习");    }    void introduce(){        super.introduce();        System.out.println("我的家在"+ address);            }}