NullPointerException: Attempt to get length of null array

Question

I am developing an Android App. I have the below code to write a list of all files starting with XLR in a particular folder:

private List<File> getListFiles(File parentDir) {
    ArrayList<File> inFiles = new ArrayList<File>();
    File[] files = parentDir.listFiles();
    for (File file : files) {
        try {
            if ((file.exists()) && (file != null)) {
                if (file.isDirectory()) {
                    inFiles.addAll(getListFiles(file));
                } else {
                    if (file.getName().startsWith("XLR")) {
                        inFiles.add(file);
                    }
                }
            }


        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    return inFiles;
}

On Android 10 and 11 it seems to be a problem. For Android 10 I have legacy storage enabled and Android 11 I have got access to all files. So the issue is not file access. The folder itself might not be in my app's folder, hence the need to be able to list out files

I know that there is an extensive page here on NPE's and I have tried my best to follow the advice there (I am checking that files exist and are not null), but still to no avail.

I feel that there's probably something so stupid, that a more experienced programmer would probably pick out in 2 seconds...

@blackapps would that even make a difference ? the for loop will only iterate over actual instances, meaning that they can't be null, right ? or am i wrong, i haven't done java in a long time — a_local_nobody
– a_local_nobody, Commented Oct 19, 2021 at 11:27
open listFiles method and read if there can be null in result array. Also on which line it fires NPE? — Autocrab
– Autocrab, Commented Oct 19, 2021 at 11:28

Daniel Campos Olivares · Accepted Answer · 2021-10-19 11:49:08Z

2

This problem would be easily solved just reading the documentation of the actual File package.

Documentation on the method File::listFiles:

An array of abstract pathnames denoting the files and directories in the directory denoted by this abstract pathname. The array will be empty if the directory is empty. Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

As you can see, that method can actually return null and not only an empty array, so you should check that the value isn't null before even trying to iterate over it.

answered Oct 19, 2021 at 11:49

Daniel Campos Olivares

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1 Comment

Craig Archer Over a year ago

Thanks for this - I changed to top few lines to this:private List<File> getListFiles(File parentDir) { ArrayList<File> inFiles = new ArrayList<File>(); File[] files = parentDir.listFiles(); if (files != null) {

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