1、需求：将键盘录入的数据写入到一个文件中

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

        BufferedReader bufr = new BufferedReader(new InputStreamReader(System.in));

        BufferedWriter bufw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream("demo.txt")));

        String line = null;
        while((line=bufr.readLine()) != null) {
            if("over".equals(line))
                break;
            bufw.write(line.toUpperCase());
            bufw.newLine();
            bufw.flush();
        }
    }

2、需求：将一个文本文件内容显示在控制台上

BufferedReader bufr = new BufferedReader(new InputStreamReader(new FileInputStream("demo.txt")));

BufferedWriter bufw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(System.out)));

String line = null;
while((line=bufr.readLine()) != null) {
    if("over".equals(line))
        break;
    bufw.write(line.toUpperCase());
    bufw.newLine();
    bufw.flush();
}

3、需求：将一个文本文件中的内容复制到另一个文件中

BufferedReader bufr = new BufferedReader(new InputStreamReader(new FileInputStream("a.txt")));

BufferedWriter bufw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream("b.txt")));

String line = null;
while((line=bufr.readLine()) != null) {
    if("over".equals(line))
        break;
    bufw.write(line.toUpperCase());
    bufw.newLine();
    bufw.flush();
}