Add base files

src/main/java/com/university/chat/Server/ChatServer.java

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+package com.university.chat.Server;
+
+public class ChatServer {
+    // TODO: declare a single shared UserManager instance (static final)
+    //         This MUST be shared by all ClientSession threads so that
+    //         broadcasting and private messaging work correctly.
+
+    public static void main(String[] args) {
+        // TODO: Create a ServerSocket
+
+        // TODO: In an infinite loop:
+        //  accept an incoming client connection
+        //  make a new thread running ClientSession for each user.
+    }
+}

src/main/java/com/university/chat/Server/ClientSession.java

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+package com.university.chat.Server;
+
+import com.university.chat.Common.ChatMessage;
+import com.university.chat.Common.FileMessage;
+
+import java.io.IOException;
+import java.net.Socket;
+import java.nio.file.Files;
+
+public class ClientSession implements Runnable {
+
+    private String username;
+
+    public ClientSession(Socket socket, UserManager userManager) {
+        // TODO : Create an ObjectOutputStream from socket.getOutputStream()
+        //         and an ObjectInputStream from socket.getInputStream().
+    }
+
+    @Override
+    public void run() {
+        try {
+
+            // TODO: Welcome the user (login step)
+            //  1. Read the first object sent by the client.
+            //  2. Check it's a ChatMessage with type LOGIN.
+            //  3. Extract the username.
+            //  4. Try to register the user via userManager.addUser(...).
+            //  5. If the username is taken, send back LOGIN_FAILED and close the socket.
+            //  6. Otherwise, create the user's folders with FileManager.createUserFolders(...)
+            //      and send back LOGIN_SUCCESS.
+
+            // TODO: Main message loop
+            //   In a loop, call in.readObject(), you can separate messages by their type:
+            //     - if it's a ChatMessage      -> call handleChatMessage(msg)
+            //     - if it's a FileMessage      -> call handleFileMessage(fileMsg)
+            //   Keep looping until the connection is closed (an exception will be thrown).
+
+        } catch (Exception e) {
+            System.out.println("Disconnected: " + username);
+        } finally {
+            // TODO: Remove the user from UserManager so they no longer
+            //        receive broadcasts or appear in users list
+        }
+    }
+
+
+    private void handleChatMessage(ChatMessage msg) throws IOException {
+        switch (msg.getType()) {
+            case PUBLIC_MESSAGE -> {
+                // TODO: Broadcast this message to every connected client.
+            }
+            case PRIVATE_MESSAGE -> {
+                // TODO: Forward this message to the receiver user.
+            }
+            case USER_LIST -> {
+                // TODO: Reply to the requester with the list of online users.
+            }
+        }
+    }
+
+    private void handleFileMessage(FileMessage fileMsg) throws IOException {
+        // Storing the file
+        var sentPath = FileManager.getSentPath(fileMsg.getSender(), fileMsg.getFilename());
+        var recvPath = FileManager.getReceivedPath(fileMsg.getReceiver(), fileMsg.getFilename());
+
+        Files.write(sentPath, fileMsg.getData());
+        Files.write(recvPath, fileMsg.getData());
+
+        // TODO: Forward the received file-message to the destination user.
+    }
+}

src/main/java/com/university/chat/Server/FileManager.java

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+package com.university.chat.Server;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+public class FileManager {
+
+    private static final String BASE_DIR = "server_data";
+
+    public static void createUserFolders(String username) throws IOException {
+        Path userPath = Paths.get(BASE_DIR, username);
+        Files.createDirectories(userPath.resolve("received"));
+        Files.createDirectories(userPath.resolve("sent"));
+    }
+
+    public static Path getReceivedPath(String username, String filename) {
+        return Paths.get(BASE_DIR, username, "received", filename);
+    }
+
+    public static Path getSentPath(String username, String filename) {
+        return Paths.get(BASE_DIR, username, "sent", filename);
+    }
+}

src/main/java/com/university/chat/Server/UserManager.java

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+package com.university.chat.Server;
+
+import java.util.concurrent.ConcurrentHashMap;
+
+public class UserManager {
+
+    private final ConcurrentHashMap<String, ClientSession> users = new ConcurrentHashMap<>();
+
+    public boolean addUser(String username, ClientSession session) {
+        return users.putIfAbsent(username, session) == null;
+    }
+
+    public void removeUser(String username) {
+        users.remove(username);
+    }
+
+    public ClientSession getUser(String username) {
+        return users.get(username);
+    }
+
+    public String listUsers() {
+        return users.keySet().toString();
+    }
+
+    public Iterable<ClientSession> getAllSessions() {
+        return users.values();
+    }
+}