I am having a null pointer exception and i'm not sure where it is in my code

Question

I am having issues with various exceptions in my code. The class is designed to find all folders/subfolders on the computer and list them as an ArrayList. It does not care about the files in them. Here is the code. please help, I have been frustrated with this for a week now.

import java.io.File;
import java.util.ArrayList;


public class Detector
{
   private ArrayList<String> paths;

   public Detector()
   {
      File[] roots = File.listRoots();
      for(File drive : roots)
      {
         getPaths(drive.listFiles());
      }
   }
   public Detector(String str)
   {

      File[] bob = new File(str).listFiles();
      getPaths(bob);
   }

   public void getPaths(File[] files)
   {
      for (File file : files)
      {
          if(file.isDirectory())
          {
             paths.add(file.getName());
             getPaths(file.listFiles());
          }
      }
   }

   public ArrayList<String> getPathList()
   {
      return paths;
   }

   public void printPaths()
   {
      for(String str: this.getPathList())
      {
         System.out.println(str);
      }
   }
   public static void main(String[] args)
   {
         Detector tom = new Detector("/F:");
         tom.printPaths();
   }
}

not sure `"/F:"` is really supposed to work – njzk2 Sep 16 '13 at 15:18 — njzk2, Sep 16 '13 at 15:18
you never initialize your arraylist ... – njzk2 Sep 16 '13 at 15:19 — njzk2, Sep 16 '13 at 15:19

score 2 · Answer 1 · answered Sep 16 '13 at 15:19

2

You never initialized paths:

private ArrayList<String> paths = new ArrayList<String>();

Note that I've also used a type parameter of String here, as you should. In general try to stay away from using raw types.

answered Sep 16 '13 at 15:19

arshajii

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Want to mention Liskov? – Bohemian Sep 16 '13 at 15:27
@Bohemian What do you mean? – arshajii Sep 16 '13 at 15:35
[Liskov substitution principle](http://en.m.wikipedia.org/wiki/Liskov_substitution_principle): the type of `paths` should be declared as the abstract type `List` if best practices are followed (not as fthe concrete implementation type `ArrayList`) – Bohemian Sep 16 '13 at 21:11

score 1 · Answer 2 · answered Sep 16 '13 at 15:30

You have incorrect syntax at this line

private ArrayList<String> paths;

You are not initializing arraylist properly. in order to initialize it properly you would have to do

private ArrayList<String> paths = new ArrayList<>();

Note: I did not put String inside <> as its redundant in java 1.7.

score 0 · Answer 3 · answered Sep 16 '13 at 16:21

Additional remark:

File.listFiles() might return null which might be your NullPointerException. So do:

public void getPaths(File[] files)
{
    if (files == null) {
        return;
    }

And Java 7 has a Files.walkFileTree which comes with a couple of new classes, but is worth getting accustomed to.

I am having a null pointer exception and i'm not sure where it is in my code

3 Answers3