Null pointer exceptions

A

Alan

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);
contents.addAll(listAllFiles(new File(dir,
children), 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;
}
}
 
D

d0ngd0ng

public static List<File> listAllFiles(File dir, String extension)

You have to check if the parameter dir is a directior with
dir.isDirectory.

Because the invoking of the method listFiles on a no directory File
object will return null.


   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);
             contents.addAll(listAllFiles(new File(dir,
                             children), 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;
   }

}
 
L

Lew

Kira said:
NullPointerException is certainly java's version of C's dangling pointer
problem.

Crude but effective analogy. Note that the scope, causes and impact of the
two are not equivalent. Analysis and remediation also differ markedly for the
two types of error.
 

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