在我的以下程序中摆脱java.lang.NullPointerException

Question

This program works fine while I search for something inside my /home/meow directory and lists all the files, but when I try to list all the files on my system's "/" it crashes after it prints the contents of the /bin directory. I also tried to execute it as SUDO java pin

import java.io.*;

public class Pin
{
    public static void printFiles(String a)
    {
        File dir = new File(a);
        for(File file:dir.listFiles())
        {
            if(file.isFile())
            {
                System.out.println(file);
            }
            else
            {
                printFiles(file.toString());
            }
        }
    }
    public static void main(String[] args)
    {
        printFiles("/");
    }
}

This was my output ...

vikkyhacks java # sudo java Pin
/lib64/ld-linux-x86-64.so.2
/bin/ntfsmove
/bin/init-checkconf
/bin/chown
/bin/mt-gnu
/bin/ntfs-3g.usermap
/bin/mountpoint
/bin/plymouth
/bin/s
/bin/bunzip2
/bin/gzexe
/bin/fgconsole
/bin/ntfstruncate
/bin/i
/bin/plymouth-upstart-bridge
/bin/fgrep
/bin/ping
/bin/lesspipe
/bin/rbash
/bin/gzip
/bin/ntfsmftalloc
/bin/lowntfs-3g
/bin/tailf
/bin/bzcat
/bin/tempfile
/bin/domainname
/bin/touch
/bin/zcmp
/bin/mktemp
/bin/nano
/bin/unicode_start
/bin/ln
Exception in thread "main" java.lang.NullPointerException
    at Pin.printFiles(Pin.java:9)
    at Pin.printFiles(Pin.java:17)
    at Pin.printFiles(Pin.java:17)
    at Pin.main(Pin.java:23)

Answer 1

You need to check that a valid array of files are returned from File#listFiles . This can happen in the case of so-called logical files where the file is actually a view of physical files:

File[] files = dir.listFiles();
if (files != null) {
   for (File file : files) {
   ...

Alternatively you can just process anything this that is a directory

public static void printFiles(String a) {

   File[] files = new File(a).listFiles();
   if (files != null) {
      for (File file: files) {
         if (file.isFile()) {
            System.out.println(file);
         } else if (file.isDirectory()) {
            printFiles(file.toString());
         } 
      }
   }
}  

Answer 2

Your need to check whether the file or directory is exists, by using

File file = new File(a);
if (file.exists()){
 for(File file:dir.listFiles())
    {
        if(file.isFile())
        {
            System.out.println(file);
        }
        else
        {
            printFiles(file.toString());
        }
    } 
}