Instanceof

class subclass{
public final void prt1(int a){
System.out.println("sub prt1");
}
public void prt2(){
System.out.println("sub prt2");
}
}
class sunclass extends subclass{
public void prt1(){
System.out.println("sun prt1");
}
public void prt2(){
System.out.println("sun prt2");
}
}
public class factory{
public static void main(String args[]){
subclass a=new subclass();
sunclass b=new sunclass();
boolean x=a instanceof b;
}
}

运行为什么会提示：factory.java:22: 找不到符号
符号： 类 b
位置： 类 factory
boolean x=a instanceof b;

关于java中的instanceof 请教java中instanceof的用法!!