try {
            FileReader fr=new FileReader("a.txt");
            BufferedReader br=new BufferedReader(fr);
            
            String s="";
            List list=new ArrayList();
            while((s=br.readLine())!=null)list.add(s);
            Collections.shuffle(list);
        
            FileWriter fw=new FileWriter("b.txt");
            for(int i=0;i<list.size();i++)fw.write(list.get(i)+"");
            fw.close();
        } catch (Exception e) {
            e.printStackTrace();
        }