java如何删除文件的前几行数据（不使用临时文件）

    private List<String> readAndRemoveFirstLines(File file, int lineNum) throws IOException{         List<String> strList = new ArrayList<String>();        RandomAccessFile raf = null;        try{            raf = new RandomAccessFile(file, "rw");                      //Initial write position                                                         long writePosition = raf.getFilePointer();            for (int i = 0 ; i < lineNum ; i++){                String line = raf.readLine();                if(line == null){                    break;                }                strList.add(line);                                 }            // Shift the next lines upwards.                                                  long readPosition = raf.getFilePointer();                                             byte[] buff = new byte[1024];                                                     int n;                                                                            while (-1 != (n = raf.read(buff))) {                                                  raf.seek(writePosition);                                                          raf.write(buff, 0, n);                                                            readPosition += n;                                                                writePosition += n;                                                               raf.seek(readPosition);                                                       }                                                                                 raf.setLength(writePosition);        } catch(IOException e){            LOG.error("readAndRemoveFirstLines error", e);            throw e;        } finally{            try{                if(raf != null){                    raf.close();                   }            }catch(IOException e){                LOG.error("close RandomAccessFile error", e);                throw e;            }        }                return strList;    }