如何用Java 的 IO 改写txt文件中第四行的数据

怎么用Java 的 IO 改写txt文件中第四行的数据，
帮我实现一下吧！我做不出来

[解决办法]
for example

Java code

BufferedReader br = new BufferedReader(new FileReader("test.txt"));PrintStream ps = new PrintStream(new FileOutputStream("test.txt.new"));String buf;int count = 0;while ((buf=br.readLine()) != null) {    count++;    if (count == 4) {        buf = buf.replaceAll("xxx", "yyy");    }    ps.println(buf);}br.close();ps.close();File f1 = new File("test.txt");f1.delete();File f2 = new File("test.txt.new");f2.renameTo(f1);