Android说无法解析方法'getExternalFilesDir(null)'

我正在使用Android Studio尝试将文件写入外部存储.

其他帖子建议我这样做getExternalFilesDir(null),但我收到了消息Cannot resolve method 'getExternalFilesDir(null)'.

 public void makeFile() {
    try {
        String storageState = Environment.getExternalStorageState();
        if (storageState.equals(Environment.MEDIA_MOUNTED)) {
            File file = new File(getExternalFilesDir(null), "outputFile.txt");
            FileOutputStream fos = new FileOutputStream(file);
            String text = "Hello, world!";
            fos.write(text.getBytes());
            fos.close();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}