java的向下转型

class A{
public void fun1(){
System.out.println("A-->public void fun1()");
}
public void fun2(){
this.fun1();
}
}
class B extends A{
public void fun1(){
System.out.println("B-->public void fun1()");
}
public void fun3(){
System.out.println("B-->public void fun3()");
}
}
class C extends A{
public void fun1(){
System.out.println("C-->public void fun1()");
}
public void fun4(){
System.out.println("C-->public void fun4()");
}
}
public class Demo03{
public static void main(String args[]){
fun(new B());
fun(new C());
}
public static void fun(A a){
//a.fun1();
if(a instanceof B){
B b = (B)a;
b.fun3();
}
if(a instanceof C){
C c = (C)a;
c.fun4();
}
}
}