public static void main(String[] args) throws IOException {

        BufferedWriter bw = new BufferedWriter(new FileWriter("c:/s1.txt"));
        BufferedReader br = new BufferedReader(new FileReader("c:/s1.txt"));
        Scanner input = new Scanner(System.in);

        while (true) {
            System.out
                    .println("Please input a sentence(input \"end\" to exit):");
            String str = input.nextLine();
            if (str.toUpperCase().equals("END")) {

                break;
            }
            bw.write(str.toUpperCase());
            bw.newLine();

        }

//到这里时，运行正确，文件写入内容也符合预期，但是下面的读取操作却没有返回任何结果，为什么？

        String str = "";
        while ((str = br.readLine()) != null) {
            System.out.println(str);        }
//或者使用如下代码
                  //while(br.ready()){
        //    System.out.println(br.readLine());
        //}
//这样写正确吗？
        bw.close();
        br.close();

    }