Почему нет доступа к файлу?

public class ExternalStorageTest extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView textView = new TextView(this);
        setContentView(textView);
        String state = Environment.getExternalStorageState();
        if (!state.equals(Environment.MEDIA_MOUNTED)) {
            textView.setText("No external storage mounted");
        } else {
            File externalDir = Environment.getExternalStorageDirectory();
            File textFile = new File(externalDir.getAbsolutePath()+ File.separator + "text.txt");
            try {
                writeTextFile(textFile, "This is a test. Roger");
                String text = readTextFile(textFile);
                textView.setText(text);
                if (!textFile.delete()) {
                    textView.setText("Couldn't remove temporary file");
                }
            } catch (IOException e) {
                textView.setText("something went wrong! " + e.getMessage());
            }
        }
    }
    private void writeTextFile(File file, String text) throws IOException {
        BufferedWriter writer = new BufferedWriter(new FileWriter(file));
        writer.write(text);
        writer.close();
    }
    private String readTextFile(File file) throws IOException {
        BufferedReader reader = new BufferedReader(new FileReader(file));
        StringBuilder text = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            text.append(line);
            text.append("\n");
        }
        reader.close();
        return text.toString();
    }
}

if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) !=
                PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this,
                    new String[] {
                            Manifest.permission.WRITE_EXTERNAL_STORAGE
                    }, 0);
        }