instanceof编译时出错的时间和原因？

Below given code compiles:

下面给出的代码编译:

class Robot { }  
interface Animal { }  
class Feline implements Animal { }  
public class BarnCat extends Feline {  
    public static void main(String[] args) {  
        Animal af = new Feline();  
        Feline ff = new Feline();  
        BarnCat b = new BarnCat();  
        Robot r = new Robot();  
        if(af instanceof Animal) System.out.print("1 ");  
        if(af instanceof BarnCat) System.out.print("2 ");  
        if(b instanceof Animal) System.out.print("3 ");  
        if(ff instanceof BarnCat) System.out.print("4 ");  
        if(r instanceof Animal) System.out.print("5 ");  
    }  
}  
class