Why I am getting null pointer exception

Question

//Author: Darin Park
//Date: 24 October, 2014
//Version: 2
package readandcopyupdate1;

import java.util.ArrayList;
import java.io.*;

public class ReadAndCopyUpdate1{

public static void main(String[] args){
    ReadAndCopyUpdate1 rc = new ReadAndCopyUpdate1();
    final File folder1 = new File("/root/avatar/default/upload/member");
    final File folder2 = new File("/root/avatar/default/upload/Transfer");
    rc.listFilesForOldFolder(folder1,rc.oldFiles);
    rc.listFilesForNewFolder(folder2,rc.newFiles);
            rc.oldFiles.stream().forEach((oldFile) -> {
                System.out.println(oldFile);
        });

    System.out.println("\n\n");
            rc.newFiles.stream().forEach((newFile) -> {
                System.out.println(newFile);
        });


}

private void listFilesForOldFolder(final File folder, ArrayList arrayList) {
   /* 
    * This method takes two arguments. 
    * The first argument is the Original Avatar Upload Folder which we want to scan.
    * The second argument is the ArrayList where we want to store all the avatar file names.
    * The first argument is not a string, it's a File.
    * So we need to first convert string to a File by using File Method. Look below for example.
    * final File folder1 = new File("/root/avatar/default/upload/member");
   */
    for (final File fileEntry : folder.listFiles()) {
        if (fileEntry.isDirectory()) {
            listFilesForOldFolder(fileEntry, arrayList);
        } else {
            String str = fileEntry.getName(); 
            if(str.equals("index.html")){
              continue;
            }          
            if(str.charAt(32) == '9'){
                arrayList.add(str);
            }
        }
    }
}

private void listFilesForNewFolder(final File folder, ArrayList arrayList) {
    /*
     * This method takes two arguments.
     * The first argument is the New Transfer folder where we want to store avatar's copy.
     * The second argument is the ArrayList where we want to store all the avatar names in the new Transfer folder.
     * The first argument is not a string, it's a File.
     * So we need to first convert string to a file by using File method. Look below for example.
     * final File folder2 = new File("/root/avatar/default/upload/Transfer");
    */  
    for (final File fileEntry : folder.listFiles()) {
        if (fileEntry.isDirectory()) {
            listFilesForNewFolder(fileEntry, arrayList);
        } else {
            String str = fileEntry.getName();          
            arrayList.add(str);
        }
    }
}



private final ArrayList<String> oldFiles = new ArrayList<>(5);
private final ArrayList<String> newFiles = new ArrayList<>(5);

}

These method used to work in my first version without any problem and now they are suddenly throwing null pointer exception when I am rewriting the code and was doing unit testing for these methods.

Exception in thread "main" java.lang.NullPointerException
    at readandcopyupdate1.ReadAndCopyUpdate1.listFilesForOldFolder(ReadAndCopyUpdate1.java:33)
at readandcopyupdate1.ReadAndCopyUpdate1.main(ReadAndCopyUpdate1.java:15)

What I am trying to accomplish is the following: 1. folder1 consist of files which have 33 characters in their file name. I want to pick all those files which have number 9 in their 33th position.

2. I'll store all such file names in an arrayList 'oldFiles'
3. I'll scan folder2 and store all files there in arrayList 'newFiles'
   4. I am testing them by printing arrayLists values on sceen.

Answer 1

Here, the caveats are:

Line: "for (final File fileEntry : folder.listFiles())"

This line may return null, if the folder path does not exist. So check for null for fileEntry variable before processing it.

Line: "String str = fileEntry.getName();"

Check for null/empty for this str variable, before checking it with other values.

Answer 2

Do a test that str is at least 33 characters long:

...
else {
    String str = fileEntry.getName();
    if(str == null || "index.html".equals(str)){
        continue;
    }          
    if(str.length() >= 33 && str.charAt(32) == '9'){
        arrayList.add(str);
    }
}
...

If, however, it is certain that the last character is 9, then do this instead:

...
else {
    String str = fileEntry.getName(); 
    if(str == null || "index.html".equals(str)){
        continue;
    }          
    if(str.charAt(str.length() - 1) == '9'){
        arrayList.add(str);
    }
}
...

Answer 3

This will fix it:

String str = fileEntry.getName();
if (str == null || "index.html".equals(str)) {
    continue;
}
if (str.length() >= 33 && str.charAt(32) == '9') {
    arrayList.add(str);
}

We missed one big thing:

final File folder1 = new File("/home3/not_existing_follder");
if (!folder1.exists() || !folder1.isDirectory()){
    System.out.print("Folder! does not exist");
    return;
}

When File is given with an not existing dir, it will return a null. Apply this to both directories and you will be fine.