有一个反射的小程序引出的问题

public class second{
    public static void main(String[]args){
        try{
            Method sqrt=Math.class.getMethod("sqrt",double.class);
            rr(3,sqrt);
        }catch(Exception e){};
    }
    private static void rr(double c,Method f){
        try{
            double x=(double)f.invoke(null, c);
            System.out.printf("%.3f|%.3f \n",x,c);
            System.out.println(f);
        }catch(Exception e){};
    }
}