Android, Java, Create directory on SD card programmatically using Storage Access Framework (SAF)

My problem is that I do not manage to create a directory on a SD card that is plugged in an Android portable device.
Below is the Java code I am trying to get to work: I am trying to create the directory sable under /storage/BF4F-1107/:

public class AnActivity extends AppCompatActivity
{
private static final int N_CREATE_DIRECTORY = 1;

    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        // SD card
        String s_sdCardStorage = "/storage/BF4F-1107/";

        Intent intent = new Intent(Intent.ACTION_CREATE_DOCUMENT);
        intent.addCategory(Intent.CATEGORY_OPENABLE);
        intent.setType("file/*");
        intent.putExtra(Intent.EXTRA_TITLE, s_destFilePath);
        startActivityForResult(intent, N_CREATE_DIRECTORY);
        // HERE
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data)
    {
        if(requestCode == N_CREATE_DIRECTORY)
        {
            if(data != null)
            {
                Uri uri = data.getData();
                DocumentFile docFileSDCardStorage = DocumentFile.fromSingleUri(this, uri);

                try
                {
                    DocumentFile docFileDir = docFileSDCardStorage.createDirectory("sable");
                }
                catch(UnsupportedOperationException exn)
                {
                    System.out.println(exn.getMessage());
                }
            }
        }
    }
}

What happens is:

• the code in onCreate() is executed.
• Then the program blocks at // HERE.
• On the portable device, appears a "dialog" which shows /storage/BF4F-1107/ and a button Save which I "press".
• Once "pressed", the code in onActivityResult() is executed.

But the result is that the directory sable under /storage/BF4F-1107/ is not created.
And the execution path goes through the catch clause, the exception UnsupportedOperationException is raised and null is printed at System.out.println(exn.getMessage());.

Edit 2: An empty file _storage_BF4F-1107_ is created under /storage/BF4F-1107/.

Can you help me make this code work?

Additionally, I would like the directory sable to be created silently.
I do not want to user to have to touch "Save".

I am trying to use the Storage Access Framework (https://developer.android.com/training/data-storage/shared/documents-files) because the mkdirs method of the java.io.File class doesn't work (I get permission denied exceptions) when I try to create a directory on the SD card.

Edit: my Android version is 6.0.1

Thank you.

Use ACTION_OPEN_DOCUMENT_TREE to let the user choose de SD card.

After that you can create as many files and directorys in the choosen directory.

If you only want to create one file with SAF use ACTION_CREATE_DOCUMENT where the user chooses the location and file name.