Java多线程聊天程序的问题 - Java / 非技术区

界面代码
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;

public class ChatJFrame extends JFrame implements ActionListener
{
  private JTextArea text_receiver; //显示对话内容的文本区
  private JTextField text_sender; //输入发送内容的文本行
  private PrintWriter cout; //字符输出流对象
  private String name; //网名
   
  public ChatJFrame(String name, String title, PrintWriter cout) //构造方法
  {
  super("聊天室 "+name+" "+title);
  this.setSize(320,240);
  this.setLocation(300,240);
  this.setDefaultCloseOperation(EXIT_ON_CLOSE);
   
  this.text_receiver = new JTextArea();
  this.text_receiver.setEditable(false); //不可编辑
  this.add(this.text_receiver);
   
  JPanel panel = new JPanel();
  this.add(panel,"South");
  this.text_sender = new JTextField(12);
  panel.add(this.text_sender);
  this.text_sender.addActionListener(this); //注册单击事件监听器

  JButton button_send = new JButton(&