用Java多线程，集合，网络通信写一个租车App

好的，下面是一个简单的租车App的Java代码示例：

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

public class RentCarAppServer {
    private static final int PORT = 8888;
    private static List<ClientHandler> clients = new ArrayList<>();

    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(PORT);
        System.out.println("Server started...");

        while(true) {
            Socket clientSocket = serverSocket.accept();
            System.out.println("Client connected: " + clientSocket);

            ClientHandler clientHandler = new ClientHandler(clientSocket, clients);
            clients.add(clientHandler);
            clientHandler.start();
        }
    }
}

class ClientHandler extends Thread {
    private Socket clientSocket;
    private List<ClientHandler> clients;
    private String name;
    private boolean isRented = false;

    public ClientHandler(Socket clientSocket, List<ClientHandler> clients) {
        this.clientSocket = clientSocket;
        this.clients = clients;
    }

    @Override
    public void run() {
        try {
            handleClient();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                clientSocket.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            clients.remove(this);
            System.out.println("Client disconnected: " + clientSocket);
        }
    }

    private void handleClient() throws IOException {
        String inputLine;
        while ((inputLine = CommunicationUtils.receiveMessage(clientSocket)) != null) {
            if (name == null) {
                name = inputLine;
                CommunicationUtils.sendMessage(clientSocket, "Welcome " + name + "!");
                broadcastMessage(name + " has joined the chat!");
            } else {
                if (inputLine.startsWith("rent")) {
                    if (isRented) {
                        CommunicationUtils.sendMessage(clientSocket, "You have already rented a car.");
                    } else {
                        isRented = true;
                        broadcastMessage(name + " has rented a car.");
                    }
                } else if (inputLine.startsWith("return")) {
                    if (!isRented) {
                        CommunicationUtils.sendMessage(clientSocket, "You have not rented any car yet.");
                    } else {
                        isRented = false;
                        broadcastMessage(name + " has returned a car.");
                    }
                } else {
                    broadcastMessage(name + ": " + inputLine);
                }
            }
        }
    }

    private void broadcastMessage(String message) {
        for (ClientHandler client : clients) {
            if (client != this) {
                CommunicationUtils.sendMessage(client.clientSocket, message);
            }
        }
    }
}

class CommunicationUtils {
    public static String receiveMessage(Socket socket) throws IOException {
        return new String(socket.getInputStream().readAllBytes()).trim();
    }

    public static void sendMessage(Socket socket, String message) throws IOException {
        socket.getOutputStream().write((message + "\n").getBytes());
    }
}

这个租车App使用了Java的多线程机制，每当有一个客户端连接时，就会启动一个新的线程处理该客户端的请求。同时，该App使用了Java集合来存储所有客户端的信息，并使用了Java网络通信来实现客户端与服务器之间的通信。

客户端可以发送 "rent" 和 "return" 命令来租借和归还车辆，同时还可以发送普通文本消息来与其他客户端进行聊天。

需要注意的是，这只是一个简单的示例代码，实际的租车App需要更加完善的功能和更严谨的代码。