对子类赋给父类的思考

2014-12-03

13:57:48

 1 /**  2  * @author nlee 3  * @version 1:09:38 PM Dec 3, 2014 4  */ 5 package test; 6  7 public class T extends T1 8 { 9     int b;10 11     public T(int s1)12     {13         super(s1);14         b = 111;15     }16 17     public void fun2()18     {19         System.out.PRintln("T===fun2");20     }21 22     public static void main(String[] args)23     {24         T t = new T(5);25         t.fun1();26         t.fun2();27         System.out.println(t.b);28         System.out.println(t.a);29         T1 t1 = t;30         t1.fun1();31         t1.fun2();32         System.out.println(t1.a);33         // System.out.println(t1.b);34     }35 }36 37 class T138 {39     int a;40 41     public T1(int s)42     {43         a = s;44     }45 46     public int get()47     {48         return a;49     }50 51     public void set(int d)52     {53         a = d;54     }55 56     public void fun1()57     {58         System.out.println("T1---fun1");59     }60 61     public void fun2()62     {63         System.out.println("T1---fun2");64     }65 }

33行被注释的代码

33         // System.out.println(t1.b);

若执行则会报错。

因为29行代码

29         T1 t1 = t;

将子类对象实例赋给父类对象，t1和t指向是同一内存块，但是t1不能访问子类的字段，因此t1不能访问变量b。