Java使用套接字连接编写一个网络程序，用户在客户端输入三角形的3条边长，客户端将他们发送给服务器端，服务器吧计算出的三角形的面积返回给客户端，客户端命令行窗口中显示面积值。

以下是一个简单的Java网络程序，实现了客户和服务器之间的通信，以计算三角形面积为例：

服务器端代码：

import java.io.*;
import java.net.*;

public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = null;
        try {
            serverSocket = new ServerSocket(8888);
        } catch (IOException e) {
            System.err.println("Could not listen on port: 8888.");
            System.exit(1);
        }

        Socket clientSocket = null;
        try {
            System.out.println("Waiting for connection...");
            clientSocket = serverSocket.accept();
            System.out.println("Connection successful.");
        } catch (IOException e) {
            System.err.println("Accept failed.");
            System.exit(1);
        }

        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 input: " + inputLine);
            String[] sides = inputLine.split(",");
            double a = Double.parseDouble(sides[0]);
            double b = Double.parseDouble(sides[1]);
            double c = Double.parseDouble(sides[2]);

            // 计算三角形面积
            double p = (a + b + c) / 2;
            double area = Math.sqrt(p * (p - a) * (p - b) * (p - c));

            out.println(area);
            System.out.println("Sent output: " + area);
        }

        out.close();
        in.close();
        clientSocket.close();
        serverSocket.close();
    }
}

客户端代码：

import java.io.*;
import java.net.*;

public class Client {
    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.err.println("Usage: java Client <host name>");
            System.exit(1);
        }

        String hostName = args[0];
        int portNumber = 8888;

        try (
            Socket socket = new Socket(hostName, portNumber);
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in))
        ) {
            String userInput;
            while ((userInput = stdIn.readLine()) != null) {
                out.println(userInput);
                String response = in.readLine();
                System.out.println("Server response: " + response);
            }
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host " + hostName);
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to " +
                hostName);
            System.exit(1);
        }
    }
}

在客户端命令行窗口中输入三角形的3条边长，以逗号分隔，例如：

3,4,5

然后客户端将它们发送给服务器端。服务器端计算出三角形的面积，并将结果返回给客户端，客户端在命令行窗口中显示面积值。