Java文件列表：使用for循环时的NPE

Question

I've written simple java code to display file list in Linux to use it for FTP program using TCP(I'm going step by step, using all features). I'm posting this question after googling for about half an hour.

import java.io.*;
class FileList
{
public static void main(String args[])
{
String dirname="/home/vaibhav/Test";
File f1=new File(dirname);
String sl[]=f1.list();
System.out.print(f1.list()+"......."+sl.length+" files found.");
for(int i=0;i<sl.length;i++)
{ 
System.out.print(sl[i]+" ");
}      
}
}

Output:

vaibhav@vaibhav-VirtualBox:~/Java$ javac FileList.java
vaibhav@vaibhav-VirtualBox:~/Java$ java FileList
Exception in thread "main" java.lang.NullPointerException
    at FileList.main(FileList.java:10)

My Directories Test and Java contents:

vaibhav@vaibhav-VirtualBox:~$ cd /home/vaibhav/Test
vaibhav@vaibhav-VirtualBox:~/Test$ dir
1  2
vaibhav@vaibhav-VirtualBox:~/Test$ ls -al
total 8
drwxrwxrwx  2 vaibhav vaibhav 4096 Jan 18 15:04 .
drwxr-xr-x 25 vaibhav vaibhav 4096 Jan 18 16:50 ..
-rw-rw-rw-  1 vaibhav vaibhav    0 Jan 18 15:04 1
-rw-rw-rw-  1 vaibhav vaibhav    0 Jan 18 15:04 2
vaibhav@vaibhav-VirtualBox:~/Test$ cd
vaibhav@vaibhav-VirtualBox:~$ cd /home/vaibhav/Java
vaibhav@vaibhav-VirtualBox:~/Java$ ls -al
total 20
drwxr-xr-x  2 vaibhav vaibhav 4096 Jan 18 16:52 .
drwxr-xr-x 25 vaibhav vaibhav 4096 Jan 18 16:50 ..
-rw-r--r--  1 vaibhav vaibhav  990 Jan 18 16:52 FileList.class
-rw-r--r--  1 vaibhav vaibhav  477 Jan 18 16:52 FileList.java
-rw-r--r--  1 vaibhav vaibhav  487 Jan 18 16:52 FileList.java~

When I changed String dirname to /home/vaibhav/Java which is pwd too, program executed well.

What was the issue? Thank you.

Answer 1

Better to try this:

File folder = new File("your/path");
File[] listOfFiles = folder.listFiles();

    for (int i = 0; i < listOfFiles.length; i++) {
      if (listOfFiles[i].isFile()) {
        System.out.println("File " + listOfFiles[i].getName());
      } else if (listOfFiles[i].isDirectory()) {
        System.out.println("Directory " + listOfFiles[i].getName());
      }
    }

Hope this helps you