Error java IO total size of folder

my program counts and prints out the number of files of folder and the total size in bytes of all files but i have error ("Exception in thread "main" java.lang.NullPointerException")

import java.io.*;

public class Ex5 { public static void a(String s) throws IOException{

    long size=0;
    File f=new File(s);
    File [] a=f.listFiles();
    System.out.println("the number of files in this folder :"+a.length);
    for(int i=0;i<a.length;i++){
    if(a[i].isFile()){
        size=size+a[i].length();

    }else
        a(a[i].getName());

    }
System.out.println("the folder size is :"+size);
}
public static void main(String[] args) throws IOException {
    // TODO Auto-generated method stub
  a("C:\Users\hackour\Documents\javablue\applet");
}

}