使用java编写网络客户/服务器程序，实现如下功能： (1)设计服务器程序，运行时等待客户端连接； (2)客户端设计如下界面，并在启动时与服务器建立连接；界面有一个输入消息，用户输入年份以后，点击发送按钮，下面的文本框中将会显示服务器返回的消息，即将小写转换为大写(3) 客户端发送消息， 服务器收到消息后， 将所有小写字符转换为大写字 符，发回给客户端； (4)客户端收到服务器发回的消息后，在界面上显示。写出完整代码给我

这里提供一个简单的Java网络客户端/服务器程序，实现将客户端输入的年份转换为大写字母并返回给客户端。

服务器端代码：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = null;
        Socket clientSocket = null;

        try {
            serverSocket = new ServerSocket(8080); // 监听8080端口
            System.out.println("Server started, listening on port 8080...");
        } catch (IOException e) {
            System.err.println("Could not listen on port: 8080.");
            System.exit(1);
        }

        while (true) {
            try {
                clientSocket = serverSocket.accept(); // 等待客户端连接
                System.out.println("Client connected: " + clientSocket.getInetAddress());
            } catch (IOException e) {
                System.err.println("Accept failed.");
                System.exit(1);
            }

            // 处理客户端请求
            new Thread(new ServerThread(clientSocket)).start();
        }
    }
}

class ServerThread implements Runnable {
    private Socket clientSocket;

    public ServerThread(Socket clientSocket) {
        this.clientSocket = clientSocket;
    }

    @Override
    public void run() {
        try {
            PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                System.out.println("Received from client: " + inputLine);

                // 转换为大写字母并返回给客户端
                String outputLine = inputLine.toUpperCase();
                out.println(outputLine);
                System.out.println("Sent to client: " + outputLine);
            }

            out.close();
            in.close();
            clientSocket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

客户端代码：

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 extends JFrame {
    private JTextField inputField;
    private JTextArea outputArea;
    private PrintWriter out;
    private BufferedReader in;

    public Client() {
        super("Client");

        // UI界面
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400, 300);
        setLayout(new BorderLayout());

        JPanel panel = new JPanel();
        panel.setLayout(new BorderLayout());
        add(panel, BorderLayout.NORTH);

        JLabel label = new JLabel("Enter a year:");
        panel.add(label, BorderLayout.WEST);

        inputField = new JTextField();
        panel.add(inputField, BorderLayout.CENTER);

        JButton sendButton = new JButton("Send");
        panel.add(sendButton, BorderLayout.EAST);

        outputArea = new JTextArea();
        outputArea.setEditable(false);
        JScrollPane scrollPane = new JScrollPane(outputArea);
        add(scrollPane, BorderLayout.CENTER);

        // 与服务器建立连接
        try {
            Socket socket = new Socket("localhost", 8080); // 连接服务器
            out = new PrintWriter(socket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 发送消息
        sendButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String year = inputField.getText();
                out.println(year);

                try {
                    String response = in.readLine();
                    outputArea.append(response + "\n");
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
            }
        });

        setVisible(true);
    }

    public static void main(String[] args) {
        new Client();
    }
}

在运行Server.java后，再运行Client.java即可看到UI界面。在输入框中输入年份，点击Send按钮，下面的文本框中将会显示服务器返回的消息（将小写字母转换为大写字母）。