private void saveFileData() {
    BufferedWriter writer = null;
    try {
      FileOutputStream out = openFileOutput("data", MODE_PRIVATE);//保存的文件名为“data”
      writer = new BufferedWriter(new OutputStreamWriter(out));
      writer.write("this is a message");//文件中保存此字符串
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (writer != null) {
        try {
          writer.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }

  private void getFileData() {
    BufferedReader reader = null;
    try {
      FileInputStream fileInputStream = openFileInput("data");
      reader = new BufferedReader(new InputStreamReader(fileInputStream));

      String line = "";
      StringBuilder result = new StringBuilder();
      while ((line = reader.readLine()) != null) {
        result.append(line);
      }
      Log.d("Test", "result data is " + result);
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (null != reader) {
        try {
          reader.close();
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
    }
  }