客户端发送一串小写字符串给服务端，服务端接受后将字符串转为大写，然后发给客户端，写出这个过程的java的程序，包括客户端的程序和服务端的程序

以下是一个可能的Java程序，实现了客户端发送小写字符串给服务端，服务端将其转换为大写，然后发送回客户端的过程。这个程序使用TCP套接字通信，并且假设客户端和服务端在同一台计算机上运行。

客户端程序：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;

public class Client {
    public static void main(String[] args) {
        try (Socket socket = new Socket("localhost", 8080);
             Scanner scanner = new Scanner(System.in);
             BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
             PrintWriter writer = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()), true)) {
            System.out.println("Connected to server.");
            while (true) {
                System.out.print("Enter lowercase string (or 'exit' to quit): ");
                String input = scanner.nextLine();
                if (input.equals("exit")) {
                    break;
                }
                writer.println(input);
                String output = reader.readLine();
                System.out.println("Server response: " + output);
            }
        } catch (Exception e) {
            System.err.println("Client error: " + e.getMessage());
        }
    }
}

服务端程序：

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

public class Server {
    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(8080)) {
            System.out.println("Server started.");
            while (true) {
                try (Socket socket = serverSocket.accept();
                     BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                     PrintWriter writer = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()), true)) {
                    System.out.println("Client connected: " + socket.getInetAddress().getHostAddress());
                    String input = reader.readLine();
                    String output = input.toUpperCase();
                    writer.println(output);
                }
            }
        } catch (Exception e) {
            System.err.println("Server error: " + e.getMessage());
        }
    }
}

注意：这个程序仅仅是一个演示。在实际应用中，需要进行异常处理、输入验证、注册多个客户端等等。