java 面向网络编程

这是服务器端的代码

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.net.Socket;
import java.util.List;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
/*这个类是服务器端的UI*/
public class ServerUI extends JFrame {
    public static void main(String[] args) {
        ServerUI serverUI = new ServerUI();
    }

    public JButton btStart;//启动服务器
    public JButton btSend;//