所学知识点：

https://www.cnblogs.com/yxdmoodoo/p/9243951.html 1、文件内容追加       // 在已有的文件后面追加信息       public static void appendInfoToFile(String fileName, String info) {         File file =new File(fileName);         try {             if(!file.exists()){                 file.createNewFile();             }             FileWriter fileWriter =new FileWriter(file, true);             info =info +System.getProperty("line.separator");             fileWriter.write(info);             fileWriter.flush();             fileWriter.close();         } catch (IOException e) {             e.printStackTrace();         }     }   2、文件内容清空       // 清空已有的文件内容，以便下次重新写入新的内容     public static void clearInfoForFile(String fileName) {         File file =new File(fileName);         try {             if(!file.exists()) {                 file.createNewFile();             }             FileWriter fileWriter =new FileWriter(file);             fileWriter.write("");             fileWriter.flush();             fileWriter.close();         } catch (IOException e) {             e.printStackTrace();         }     }   3、读取文件内容，文件中的内容为String      public static List<String> readInfoFromFile(String fileName) {         File file =new File(fileName);         if(!file.exists()) {             return null;         }         List<String> resultStr =new ArrayList<String>();         try {             BufferedReader bufferedReader =new BufferedReader(new FileReader(file));             String str =null;             while(null !=(str=bufferedReader.readLine())) {                 resultStr.add(str);             }         }catch (IOException e) {             e.printStackTrace();         }         return resultStr;     }