使用Java语言，利用多线程、网络编程设计一个聊天程序。要求通过监听指定的端口号、目标IP地址和目标端口号，实现消息的发送和接收功能，并把聊天内容显示出来。

好的，以下是一个简单的聊天程序设计思路：

1. 创建一个客户端和一个服务端，客户端用于发送消息，服务端用于接收消息并转发给其他客户端。
2. 客户端与服务端之间通过网络编程通信，使用Socket类连接服务端。
3. 客户端和服务端都需要实现多线程，以便能够同时处理多个消息。
4. 客户端通过读取用户输入，将消息发送给服务端，服务端将收到的消息广播给其他客户端。
5. 在客户端和服务端中都需要实现界面，用于显示聊天内容和输入框。

以下是示例代码：

服务端：

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;

public class Server {

    private ArrayList<ClientHandler> clients = new ArrayList<>();
    private ServerSocket serverSocket;

    public void start(int port) throws IOException {
        serverSocket = new ServerSocket(port);
        System.out.println("Server started on port " + port);

        while (true) {
            Socket clientSocket = serverSocket.accept();
            ClientHandler client = new ClientHandler(clientSocket, this);
            clients.add(client);
            client.start();
            System.out.println("New client connected: " + clientSocket.getInetAddress());
        }
    }

    public void broadcast(String message, ClientHandler sender) {
        for (ClientHandler client : clients) {
            if (client != sender) {
                client.sendMessage(sender.getClientName() + ": " + message);
            }
        }
    }

    public void removeClient(ClientHandler client) {
        clients.remove(client);
    }

    public static void main(String[] args) {
        Server server = new Server();
        try {
            server.start(12345);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

客户端：

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class Client {

    private String serverAddress;
    private int serverPort;
    private String clientName;

    private BufferedReader in;
    private PrintWriter out;

    private JFrame frame = new JFrame("Chat");
    private JTextField textField = new JTextField(40);
    private JTextArea messageArea = new JTextArea(8, 40);

    public Client(String serverAddress, int serverPort, String clientName) {
        this.serverAddress = serverAddress;
        this.serverPort = serverPort;
        this.clientName = clientName;

        textField.setEditable(true);
        messageArea.setEditable(false);
        frame.getContentPane().add(textField, BorderLayout.SOUTH);
        frame.getContentPane().add(new JScrollPane(messageArea), BorderLayout.CENTER);
        frame.pack();

        textField.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                out.println(textField.getText());
                textField.setText("");
            }
        });
    }

    public void start() {
        try {
            Socket socket = new Socket(serverAddress, serverPort);
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out = new PrintWriter(socket.getOutputStream(), true);

            out.println(clientName);

            while (true) {
                String line = in.readLine();
                if (line.startsWith("SUBMITNAME")) {
                    out.println(clientName);
                } else if (line.startsWith("NAMEACCEPTED")) {
                    textField.setEditable(true);
                } else if (line.startsWith("MESSAGE")) {
                    messageArea.append(line.substring(8) + "\n");
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String serverAddress = "localhost";
        int serverPort = 12345;
        String clientName = JOptionPane.showInputDialog(null, "Enter your name:", "Name selection", JOptionPane.PLAIN_MESSAGE);

        Client client = new Client(serverAddress, serverPort, clientName);
        client.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        client.frame.setVisible(true);
        client.start();
    }
}

以上代码仅供参考，不一定完全正确，也不一定满足所有需求。需要根据具体情况进行修改和完善。