如何把集合修改的数据保存到文件里去

   通过io流可以把文件里的东西读到集合里，但是把集合数据修改下 或者增加些数据到集合里 却不能用输出流写到文件里去，因为write();这个函数只能写String类型的数据 假如集合是 s  用writer(s);这么是错误的 ，实在不知道怎么解决 还望高手提点 秒数采纳  这是我的原代码

import java.io.*;
import java.util.*;

class One
{
 private String name;
 private String age;

 public One(String name, String age) {
  this.name = name;
  this.age = age;
 }

 public String toString() {
  return name + " " + age;
 }
}

public class Demo {
 public static void main(String[] args) throws IOException {
  List<One> lt = new ArrayList<One>();
  BufferedReader br = new BufferedReader(new FileReader("D:\\ziYuan\\vb.txt"));
  String s = null;
  while ((s = br.readLine()) != null)
  {
   String[] s1 = s.split(",");
   One one = new One(s1[0],s1[1]);
   lt.add(one);
  }
  br.close();
  
  BufferedReader br1 = new BufferedReader(new InputStreamReader(System.in));
  BufferedWriter bw = new BufferedWriter(new FileWriter("D:\\ziYuan\\vb.txt",true));
  String s2 = br1.readLine();
  String[] s3 = s2.split(" ");
  lt.add(new One(s3[0],s3[1]));
  
   bw.write(lt);
  
 }
}

class One
{
 private String name;
 private String age;

 public One(String name, String age) {
  this.name = name;
  this.age = age;
 }

 public String toString() {
  return name + " " + age;
 }
}

public class Demo {
 public static void main(String[] args) throws IOException {
  List<One> lt = new ArrayList<One>();
  BufferedReader br = new BufferedReader(new FileReader("D:\\ziYuan\\vb.txt"));
  String s = null;
  while ((s = br.readLine()) != null)
  {
   String[] s1 = s.split(",");
   One one = new One(s1[0],s1[1]);
   lt.add(one);
  }
  br.close();
 
  BufferedReader br1 = new BufferedReader(new InputStreamReader(System.in));
  String s2 = br1.readLine();
  String[] s3 = s2.split(" ");
  lt.add(new One(s3[0],s3[1]));
  BufferedWriter bw = new BufferedWriter(new FileWriter("D:\\ziYuan\\vb.txt",true));
  bw.write(s3[0].toCharArray());
  bw.write(",");
  bw.write(s3[1].toCharArray());
  bw.write((char)13);
  bw.write((char)10);
  bw.close();
 }
}