init project

pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.example</groupId>
+    <artifactId>chat-app</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+</project>

src/main/java/chat/ChatClient.java

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+package chat;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.PrintWriter;
+import java.net.Socket;
+
+public class ChatClient {
+
+    private final String host;
+    private final int port;
+    private Socket socket;
+    private PrintWriter out;
+    private BufferedReader in;
+    private BufferedReader consoleReader;
+
+    public ChatClient(String host, int port) {
+        this.host = host;
+        this.port = port;
+        this.consoleReader = new BufferedReader(new InputStreamReader(System.in));
+    }
+
+    public void start() {
+        try {
+            connectToServer();
+            MessageListener messageListener = new MessageListener(in);
+            Thread listenerThread = new Thread(messageListener);
+            listenerThread.setDaemon(true);
+            listenerThread.start();
+            readUserInputAndSend();
+        } catch (IOException e) {
+            System.err.println("Client error: " + e.getMessage());
+        } finally {
+            cleanup();
+        }
+    }
+
+    private void connectToServer() throws IOException {
+        // TODO: create socket connection to host:port
+        // TODO: initialize output stream (PrintWriter)
+        // TODO: initialize input stream (BufferedReader)
+    }
+
+    private void readUserInputAndSend() throws IOException {
+        // TODO: read continuously from console
+        // TODO: send each input line to server
+    }
+
+    private void cleanup() {
+        // TODO: close socket safely
+        // TODO: release resources
+    }
+
+    public static void main(String[] args) {
+        String host = "127.0.0.1";
+        int port = 12345;
+
+        if (args.length > 0) {
+            host = args[0];
+        }
+        if (args.length > 1) {
+            try {
+                port = Integer.parseInt(args[1]);
+            } catch (NumberFormatException e) {
+                System.err.println("Invalid port. Using default: " + port);
+            }
+        }
+
+        new ChatClient(host, port).start();
+    }
+}

src/main/java/chat/ChatServer.java

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+package chat;
+
+import java.io.IOException;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+public class ChatServer {
+
+    private final int port;
+    private final Set<ClientHandler> connectedClients;
+    private final ExecutorService threadPool;
+
+    public ChatServer(int port) {
+        this.port = port;
+        this.connectedClients = Collections.synchronizedSet(new HashSet<>());
+        this.threadPool = Executors.newCachedThreadPool();
+    }
+
+    public void start() {
+        try (ServerSocket serverSocket = new ServerSocket(port)) {
+            System.out.println("Chat server started on port " + port);
+
+            while (true) {
+                Socket clientSocket = serverSocket.accept();
+                ClientHandler clientHandler = new ClientHandler(clientSocket, this);
+                connectedClients.add(clientHandler);
+                threadPool.execute(clientHandler);
+            }
+        } catch (IOException e) {
+            System.err.println("Server error: " + e.getMessage());
+        } finally {
+            threadPool.shutdown();
+        }
+    }
+
+    public void broadcast(String message, ClientHandler sender) {
+        // step 1: iterate through connectedClients safely
+        // step 2: skip the sender client
+        // step 3: send message to remaining clients using sendMessage
+    }
+
+    public void broadcastSystemMessage(String message) {
+        // step 1: iterate through all connected clients
+        // step 2: send system message to each client
+    }
+
+    public void removeClient(ClientHandler clientHandler) {
+        // step 1: remove client from connectedClients set
+        // step 2: retrieve username from clientHandler
+        // step 3: broadcast system message about user leaving
+    }
+
+    public static void main(String[] args) {
+        int port = 12345;
+        if (args.length > 0) {
+            try {
+                port = Integer.parseInt(args[0]);
+            } catch (NumberFormatException e) {
+                System.err.println("Invalid port. Using default: " + port);
+            }
+        }
+        new ChatServer(port).start();
+    }
+}

src/main/java/chat/ClientHandler.java

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+package chat;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.PrintWriter;
+import java.net.Socket;
+
+public class ClientHandler implements Runnable {
+
+    private final Socket clientSocket;
+    private final ChatServer chatServer;
+    private PrintWriter out;
+    private BufferedReader in;
+    private String username;
+
+    public ClientHandler(Socket clientSocket, ChatServer chatServer) {
+        this.clientSocket = clientSocket;
+        this.chatServer = chatServer;
+    }
+
+    @Override
+    public void run() {
+        try {
+            setupStreams();
+            requestUsername();
+            announceJoin();
+            listenForMessages();
+        } catch (IOException e) {
+            System.err.println("Client handler error: " + e.getMessage());
+        } finally {
+            cleanup();
+        }
+    }
+
+    private void setupStreams() throws IOException {
+        out = new PrintWriter(clientSocket.getOutputStream(), true);
+        in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
+    }
+
+    private void requestUsername() throws IOException {
+        out.println("Enter your username:");
+        username = in.readLine();
+        if (username == null || username.trim().isEmpty()) {
+            username = "Anonymous";
+        }
+    }
+
+    private void announceJoin() {
+        chatServer.broadcastSystemMessage("[System] " + username + " joined the chat.");
+    }
+
+    private void listenForMessages() throws IOException {
+        String message;
+        while ((message = in.readLine()) != null) {
+            chatServer.broadcast(username + ": " + message, this);
+        }
+    }
+
+    private void cleanup() {
+        chatServer.removeClient(this);
+        try {
+            if (clientSocket != null && !clientSocket.isClosed()) {
+                clientSocket.close();
+            }
+        } catch (IOException e) {
+            System.err.println("Error closing socket: " + e.getMessage());
+        }
+    }
+
+    public void sendMessage(String message) {
+        out.println(message);
+    }
+
+    public String getUsername() {
+        return username;
+    }
+}

src/main/java/chat/MessageListener.java

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+package chat;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+
+public class MessageListener implements Runnable {
+
+    private final BufferedReader inputReader;
+
+    public MessageListener(BufferedReader inputReader) {
+        this.inputReader = inputReader;
+    }
+
+    @Override
+    public void run() {
+        // TODO: continuously read messages from server
+        // TODO: print received messages immediately
+    }
+}