ATMClient.java

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

public class ATMClient {
    private static final String DEFAULT_HOST = "127.0.0.1";
    private static final int DEFAULT_PORT = 2525;

    public static void main(String[] args) {
        String serverHost = DEFAULT_HOST;
        int serverPort = DEFAULT_PORT;
        
        if (args.length >= 1) {
            serverHost = args[0];
        }
        if (args.length >= 2) {
            try {
                serverPort = Integer.parseInt(args[1]);
                if (serverPort < 1024 || serverPort > 65535) {
                    System.err.println("Port range 1024-65535, using default " + DEFAULT_PORT);
                    serverPort = DEFAULT_PORT;
                }
            } catch (NumberFormatException e) {
                System.err.println("Invalid port, using default " + DEFAULT_PORT);
            }
        }
        
        System.out.println("Connecting to " + serverHost + ":" + serverPort);
        
        try (Socket socket = new Socket(serverHost, serverPort);
             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
             BufferedReader console = new BufferedReader(new InputStreamReader(System.in))) {
            
            System.out.println("Connected to ATM Server");
            System.out.println("========== ATM System ==========");
            
            System.out.print("Enter card number: ");
            String cardNo = console.readLine();
            out.println("HELO " + cardNo);
            
            String response = in.readLine();
            System.out.println("Server: " + response);
            
            if (!response.startsWith("500")) {
                System.out.println("Auth failed, exit");
                return;
            }
            
            System.out.print("Enter password: ");
            String passwd = console.readLine();
            out.println("PASS " + passwd);
            
            response = in.readLine();
            System.out.println("Server: " + response);
            
            if (!"525 OK!".equals(response)) {
                System.out.println("Wrong password, exit");
                return;
            }
            
            System.out.println("Login success!");
            System.out.println("========================================");
            
            while (true) {
                System.out.println("\nChoose operation:");
                System.out.println("  1. Check balance (BALA)");
                System.out.println("  2. Withdraw (WDRA <amount>)");
                System.out.println("  3. Quit (QUIT)");
                System.out.print("> ");
                
                String input = console.readLine();
                if (input == null) break;
                
                out.println(input);
                String resp = in.readLine();
                if (resp == null) break;
                
                if (resp.startsWith("AMNT:")) {
                    System.out.println("Balance: " + resp.substring(5) + " yuan");
                } else if ("525 OK!".equals(resp)) {
                    System.out.println("Success!");
                } else if ("401 ERROR!".equals(resp)) {
                    System.out.println("Failed! Please check input or balance.");
                } else if ("BYE".equals(resp)) {
                    System.out.println("Session ended, thank you!");
                    break;
                } else {
                    System.out.println("Server response: " + resp);
                }
                
                if ("QUIT".equalsIgnoreCase(input.trim())) {
                    break;
                }
            }
            
            System.out.println("========== Goodbye! ==========");
            
        } catch (IOException e) {
            System.err.println("Connection failed: " + e.getMessage());
            System.out.println("Make sure server is running");
        }
    }
}