首先是从一个指定的文件中读出数据.
    public String reader(){
  StringBuffer sb = new StringBuffer();
  try{
      BufferedReader br = new BufferedReader(new FileReader("D:\\log.txt"));
      String temp ;
      while((temp=br.readLine())!=null){
       sb.append(temp) ;
       br.close();
      }
     }catch(Exception e){
      e.printStackTrace() ;
     }
     return sb.toString();
 }

然后是把指定的字符串写入文件中(追加形式).
public void writer(String code) {
File tofile=new File("D:\\log.txt");
    FileWriter fw=new FileWriter(tofile,true);
    BufferedWriter bw=new BufferedWriter(fw);
    PrintWriter pw=new PrintWriter(bw);
    pw.println(code);
    pw.close();
    bw.close();
    fw.close();
}