I keep getting NullPointerExceptions in the code below.  I have
also provided the execution output below.  It seems when I add a check
on an error (such as I did at Line 56), I just get the exception at a
different spot.

     I am new to Java.  I think this has something to do with
conversions from an Array (of files from listfiles method of File
class) to a List, but I cannot pinpoint the problem.

     Is there something obviously wrong?

     My intent was to create a general utility method that returns a
list of files in the designated directory and its subdirectories.

                                   Thanks, Alan

----jGRASP exec: java jat.util.file.AllFiles

... Entering method listAllFiles ...
dir = My Java Code
java.lang.NullPointerException
    at jat.util.file.AllFiles.listAllFiles(AllFiles.java:56)
    at jat.util.file.AllFiles.listAllFiles(AllFiles.java:73)
FilenameFilter selected . . .
filelist contents:
null

Directory found
Children: (size = 48)
URLget.java

... Entering method listAllFiles ...
dir = URLget.java
FilenameFilter selected . . .
    at jat.util.file.AllFiles.main(AllFiles.java:22)
java.lang.NullPointerException
    at jat.util.file.AllFiles.listAllFiles(AllFiles.java:73)
    at jat.util.file.AllFiles.main(AllFiles.java:22)

----jGRASP: operation complete.

package jat.util.file;

import java.io.*;
import java.io.FilenameFilter;
import java.util.*;

public class AllFiles
{
/**    The main method is used for testing other methods
    of the AllFiles class.
*/
  public static void main(String[] args)
  {
     File directory = new File(System.getProperty("user.dir"));
     if (args.length > 0)
     {
        File tempFile = new File(args[0]);
        if (tempFile.isDirectory()) { directory = tempFile; }
     }
     try
        {
           List<File> filelist = listAllFiles(directory, "java");
           for (Iterator it = filelist.iterator(); it.hasNext();)
           {
    File file = (File) it.next();
    System.out.println(file.getName());
           }
        }
     catch (Exception  e)
     { }
}

/**    The listAllFiles method provides a list of all files
    found in the user-specified directory and its
    subdirectories.

    @param    dir   the directory at which the file list
              should start
    @param    extension    a string indicating the file
                               extension that should be used to
filter the
                               results
    @return    File[]        a list of File objects
*/
public static List<File> listAllFiles(File dir, String extension)
throws Exception
{
  System.out.println("\n\n... Entering method listAllFiles ...");
  try
  {
     List<File> contents = null;
     List<File> filelist = null;
     System.out.println("dir = " + dir.getName());
     FilenameFilter select = new FileListFilter(dir.getName(),
                                                       extension);
     System.out.println("FilenameFilter selected . . .");
     File[] filearray = dir.listFiles(select);
     if (filearray.length > 0)
      { filelist = Arrays.asList(filearray); }
     System.out.println("filelist contents:\n"
                                      + filelist + "\n");
     if (filelist != null)
     {
        System.out.println("filelist is NOT empty");
        contents = filelist;
     }

     if (dir.isDirectory())
     {
       System.out.println("Directory found");
       String[] children = dir.list();
       System.out.println("Children: (size = "                     + children.length
+ ")");
       for (int i = 0; i < children.length; i++)
       {
            System.out.println(children[i]);
            contents.addAll(listAllFiles(new File(dir,
                        children[i]), extension));
       }
  }
  return contents;
}
catch ( SecurityException  e )
{
   e.printStackTrace();
   return null;
}
catch ( RuntimeException  e )
{
   e.printStackTrace();
   return null;
}
}
}

/**    The FileListFilter class implements the
                java.io.FilenameFilter interface.
*/
class FileListFilter implements FilenameFilter
{
    private String name, extension;

    public FileListFilter(String name, String extension)
    {
       this.name = name;
       this.extension = extension;
    }

/**   The accept method determines whether or not a File
      meets the filename filter criteria.

     @param  directory  a <code>File</code> object that indicates
                                  the directory
     @param   filename   a String that contains the filename
            of the file
     @return   boolean      indicates whether or not the file
          meets the filtering criteria
*/
  public boolean accept(File directory, String filename)
  {
     boolean fileOK = true;
     if (name != null)
     { fileOK &= filename.startsWith(name); }

     if (extension != null)
     { fileOK &= filename.endsWith('.' + extension); }

     return fileOK;
  }
}