code: public static String wantFilesPath(Context context, boolean externalStorageFirst) {
          String path = null;
          File f = null;
          if (externalStorageFirst && DiskFileUtils.hasSDCardMounted() && (f = context.getExternalFilesDir("xxx")) != null) {
              path = f.getAbsolutePath();
          } else {
              path = context.getFilesDir().getAbsolutePath();
          }
          return path;
      }
docs: external: "/storage/emulated/0/Android/data/in.srain.sample/files"
  internal: "/data/data/in.srain.sample/files"