Why can File.listFiles return null when called on a directory?

问题

I'm creating an Android app, and I want to list the files in a directory. I do this by calling

File[] files = path.listFiles(new CustomFileFilter());

path is a File object, which is created by calling

File path = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);

When I then try to get the length of the files array by calling

int length = files.length;

This line gives me a NullPointerException, because files is null.

I have checked if the path which I try to list the files in exists by calling

System.out.println("Path exists: " + path.exists());

And when I run the app, it prints

Path exists: true

in the Android Studio console, so the directory exists.

I've also printed the path name, which is

/storage/emulated/0/Download

So the path is a directory, and not a file.

I have no idea about why I'm getting a NullPointerException, because the path is a directory.

EDIT: The CustomFileFilter class looks like this:

public class CustomFileFilter implements FileFilter {

    // Determine if the file should be accepted
    @Override
    public boolean accept(File file) {
        // If the file isn't a directory
        if(file.isDirectory()) {
            // Accept it
            return true;
        } else if(file.getName().endsWith("txt")) {
            // Accept it
            return true;
        }
        // Don't accept it
        return false;
    }
}

回答1:

Add this to AndroidManifest.xml:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

This permission allow you to read files; if you don't use this permission then listFiles() and list() will both throw NullPointerException.

And here is an example of showing all files in /storage/emulated/0/Download:

   String dir = "/storage/emulated/0/Download/";
        File f = new File(dir);
        String[] files = f.list();
        for(int i=0; i<files.length; i++){
            Log.d("tag", files[i]);
        }

回答2:

For those who are working on android SDK>23, you need to grant the permissions by code: use something like:

 boolean grantedAll = ContextCompat.checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED;
if (!grantedAll)
   {
     ActivityCompat.requestPermissions(thisActivity,
            new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
            REQUEST_PERMISSIONS);
   }

来源：https://stackoverflow.com/questions/27874706/why-can-file-listfiles-return-null-when-called-on-a-directory