/**用這個方法需要加許可權
 * 往SDCard寫入資料許可權
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
 */

if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
    //存放的路徑
   String path =  Environment.getExternalStorageDirectory().getPath()+ File.separator+"json.text"
 
;
   File file = new File(path);
   //檔案存在就刪除
    if(file.exists()){
        file.delete();
    }

    try {
        file.createNewFile();//建立檔案
        PrintStream ps = new PrintStream(file);//向檔案中寫資訊
        ps.print(str);//str是解析出來的物件
        ps.flush();
        ps.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

特別簡單的

FileOutputStream open = new FileOutputStream("wenjianming.txt",MODE_PRIVATE);
open.write(list.toString().getBytes());
open.close();
//路徑
data/data/包名/資料夾：files/wenjianming.txt