寫檔案儲存在Android SD 卡中。
public void writeData(String str) {
    String filePath = "/sdcard/Gyt/";
    String fileName = "data.txt";
    writeTxtToFile(str, filePath, fileName);
  }

  // 將字串寫入到文字檔案中
  public void writeTxtToFile(String strcontent, String filePath, String fileName) {
    //生成資料夾之後，再生成檔案，不然會出錯
    makeFilePath(filePath, fileName);

    String strFilePath = filePath + fileName;
    // 每次寫入時，都換行寫
    String strContent = strcontent + "\r\n";
    try {
      File file = new File(strFilePath);
      if (!file.exists()) {
        Log.d("TestFile", "Create the file:" + strFilePath);
        file.getParentFile().mkdirs();
        file.createNewFile();
      }
      RandomAccessFile raf = new RandomAccessFile(file, "rwd");
      raf.seek(file.length());
      raf.write(strContent.getBytes());
      raf.close();
    } catch (Exception e) {
      Log.e("TestFile", "Error on write File:" + e);
    }
  }

//生成檔案

  public File makeFilePath(String filePath, String fileName) {
    File file = null;
    makeRootDirectory(filePath);
    try {
      file = new File(filePath + fileName);
      if (!file.exists()) {
        file.createNewFile();
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
    return file;
  }

//生成資料夾

  public static void makeRootDirectory(String filePath) {
    File file = null;
    try {
      file = new File(filePath);
      if (!file.exists()) {
        file.mkdir();
      }
    } catch (Exception e) {
      Log.i("error:", e + "");
    }
  }