NullPointerException when reading file from Android external storage

Question

I was having some trouble when trying to read zip file from Android external SD card.

Here is my code:

File[] fileArr = mVersionUpgradeViewModel.getZipFileFromSDCard();
for(int i = 0; i < fileArr.length; i++){
    System.out.println(fileArr[i]);
}

public File[] getZipFileFromSDCard() {
    File[] matchingFiles = new File[0];
    File f = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
    if(f.exists()) {
        matchingFiles = f.listFiles(new FilenameFilter() {
            public boolean accept(File dir, String name) {
                return name.endsWith("zip");
            }
        });
    }
    return matchingFiles;
}

And my emulator folder structure:

The zip folder is there. But with the code above, I am getting error message like:

Caused by: java.lang.NullPointerException: Attempt to get length of null array
    at com.mainapp.asynctask.VersionUpgradeTask.doInBackground(VersionUpgradeTask.java:86)
    at com.mainapp.asynctask.VersionUpgradeTask.doInBackground(VersionUpgradeTask.java:15)
    at android.os.AsyncTask$2.call(AsyncTask.java:333)

The error is pointing to the first for loop. It was working last time. But after I tried the newer version installation code, the method above breaks. Any ideas?

Thanks!

`NullPointerException` usually indicates problem with variables and/or prarameters that your code works with. Effective resolution calls for debugging the code. — soufrk, Jun 11 '18 at 07:29
But then the zip file is in the sd card. Do you have any ideas why is it so? — , Jun 11 '18 at 07:32
I don't think it's duplicate. Mine is trying to read from external sd card but it returns null although the file is there — , Jun 11 '18 at 07:35
@guest176969 Have you check that "mVersionUpgradeViewModel" is not null? and what is exectly "mVersionUpgradeViewModel" this class. — Mayur Patel, Jun 11 '18 at 07:50
`File.listFiles()` returns null under conditions which are clearly defined in the Javadoc. You aren't testing for it. Do so. — user207421, Jun 12 '18 at 01:09

Gautam Surani · Answer 1 · 2018-06-11T08:05:57.187

0

Are you sure you given permission of read external storage

List<File> fileArr = getZipFileFromSDCard();
    for (int i = 0; i < fileArr.size(); i++) {
        System.out.println(fileArr.get(i));
    }

 public List<File> getZipFileFromSDCard() {
    List<File> matchingFiles = new ArrayList<>();
    File wallpaperDirectory = new File(Environment.getExternalStorageDirectory().toString());
    File[] files = wallpaperDirectory.listFiles();
    for (File file : files) {
        if (file.getName().endsWith("pdf")) {
            matchingFiles.add(file);
        }
    }

    return matchingFiles;
}

edited Jun 11 '18 at 08:05

answered Jun 11 '18 at 07:33

Gautam Surani

1,136
10
21

1

Yeap in the manifest as well as runtime permission – Jun 11 '18 at 07:34
Do you have any ideas? – Jun 11 '18 at 07:42
yes you have set file array size is 0, please use list – Gautam Surani Jun 11 '18 at 07:44
Do you mind to provide me with some examples? Can you help me check the edited portion? It does not prompt the user for the permission on runtime though – Jun 11 '18 at 07:54
show my edited ans, set file directory as you requirement – Gautam Surani Jun 11 '18 at 08:06
@GautamSurani Thanks for the help. I solved it already – Jun 12 '18 at 01:40

score 0 · Answer 2 · answered Jun 11 '18 at 07:48

0

You initialize matchingFiles with an array of 0 elements:

File[] matchingFiles = new File[0];

and then you check whether Environment.DIRECTORY_DOWNLOADS exists and if so, you recreate matchingFiles to be an array of the files inside it ending with zip.

The stack trace clearly shows that the error is not thrown from this method, but it is safe to assume that the result of the method is problematic.

Now, matchingFiles is a null array later, so Environment.DIRECTORY_DOWNLOADS exists indeed, so you either do not have reading permission of the folder, or Environment.DIRECTORY_DOWNLOADS was not set to be the Download folder in your structure shown, or the zip file you are checking is restricted from the user running the application.

You may want to check whether matchingFiles is null and if so, set it to be new File[0] at the end of the method to improve your code, but the solution is clearly to check your permissions to the folder and the file.

answered Jun 11 '18 at 07:48

Lajos Arpad

64,414
37
100
175

Do you mind to provide me with some examples? – Jun 11 '18 at 07:56
@guest176969 I don't mind, but can you specify what do you need to be exemplified? – Lajos Arpad Jun 11 '18 at 08:05
Thanks for the help. I solved it already – Jun 12 '18 at 01:40
@guest176969 if this answer helped you solve your problem, then you may consider accepting it. If it was not helpful-enough, you might consider writing your own answer and accepting that. – Lajos Arpad Jun 12 '18 at 09:02

NullPointerException when reading file from Android external storage

2 Answers2