java 判断对象是否是某个类的类型方法

class Do1
{
public static void main(String[] args)
{
AA a=new CC();
if(a instanceof CC)
{
CC b=(CC)a;
b.run();
}

}
}
class AA
{
int y=3;
void brun()
{
System.out.println(y);
}
}

class CC extends AA
{
int x=1;
void run()
{
System.out.println(x);
}
}
class BB extends AA
{
int x=2;
void run()
{
System.out.println(x);
}
}