java遍历文件时提示空指针错误

import java.io.*; 
public class FileList 
{ 
public static int fileTotalNum = 0;    //文件总数 
public static int directoryNum = 0;    //文件夹总数 

public void FList(File f) 
{ 
String[] s = f.list(); 
fileTotalNum+=s.length; 

for(int i = 0;i < s.length;i++) 
{ 
File filelist = new File(f.getPath()+s[i]); 
if(filelist.isDirectory()) 
{ 

directoryNum++; 
FList(filelist); 
} 
} 
} 

public static void main(String[] args) 
{ 
File file = new File("C:\\"); 
FileList fl = new FileList(); 
fl.FList(file); 
System.out.println("一共有文件："+fileTotalNum); 
} 
} 



执行以上代码后提示空指针错误，具体如下： 
Exception in thread "main" java.lang.NullPointerException 
        at FileList.FList(FileList.java:10) 
        at FileList.FList(FileList.java:19) 
        at FileList.main(FileList.java:28)

大家，知道，这是为什么吗？谢谢了