Merge branch 'main' into fix/game-turn-and-action-bugs

documents/docs/networking/commands/protocol-document.md

@@ -1,8 +1,9 @@
 # Protocol Document
+
 This document describes the protocol for client-server communication in our application. It defines the structure of requests and responses, the supported commands along with their request parameters, response formats, and possible errors.
 
-
 # Table of Contents
+
 - [Protocol Document](#protocol-document)
 - [Table of Contents](#table-of-contents)
 - [General structure of requests](#general-structure-of-requests)
@@ -36,20 +37,27 @@ This document describes the protocol for client-server communication in our appl
     - [Error Response](#error-response-4)
     - [Example Request](#example-request-2)
     - [Example Response](#example-response-2)
-  - [LOGOUT command](#logout-command)
+  - [CHANGE\_USERNAME command](#change_username-command)
     - [Required pre-execution checks](#required-pre-execution-checks-3)
     - [Request Parameters](#request-parameters-3)
     - [Success Response](#success-response-3)
     - [Error Response](#error-response-5)
     - [Example Request](#example-request-3)
     - [Example Response](#example-response-3)
-  - [LIST\_USERS command](#list_users-command)
+  - [LOGOUT command](#logout-command)
     - [Required pre-execution checks](#required-pre-execution-checks-4)
     - [Request Parameters](#request-parameters-4)
     - [Success Response](#success-response-4)
     - [Error Response](#error-response-6)
     - [Example Request](#example-request-4)
     - [Example Response](#example-response-4)
+  - [LIST\_USERS command](#list_users-command)
+    - [Required pre-execution checks](#required-pre-execution-checks-5)
+    - [Request Parameters](#request-parameters-5)
+    - [Success Response](#success-response-5)
+    - [Error Response](#error-response-7)
+    - [Example Request](#example-request-5)
+    - [Example Response](#example-response-5)
   - [GET_LOBBY_LIST command](#get_lobby_list-command)
     - [Required pre-execution checks](#required-pre-execution-checks)
     - [Request Parameters](#request-parameters)
@@ -91,6 +99,7 @@ This document describes the protocol for client-server communication in our appl
 <!-- Please see the comments for copy ‚ n' paste ready examples -->
 
 # General structure of requests
+
 As mentioned before, our protocol is based on POP3.
 Each command is represented as a single line of text, starting with the command name followed by parameters. The server responds with a status line indicating success or failure, followed by the body.
 
@@ -99,41 +108,43 @@ Requests can have parameters that provide additional information for the command
 Responses are collections of key-value pairs, containing either a value or another collection, allowing for nested structures.
 Each collection is ended with the `END` keyword.
 
-
-
 # Preconditions
+
 The serverside pipeline to process incoming requests consists of multiple stages.
 Each of these stages can yield an error response if the request does not meet the requirements of that stage.
 
 ## Parsing
+
 One of these stages is the parsing. It is responsible for parsing the raw request into a structured format that can be easily processed by the command handlers. It validates the syntax of the request as well.
 
 ### Error Response
+
 | Code            | Description                                                                                     |
 | :-------------- | :---------------------------------------------------------------------------------------------- |
 | `PARSING_ERROR` | The body of the request contains syntax errors (see message field of response for more details) |
 
-
 ## Command dispatching
+
 After the request has been successfully parsed, the next stage is to dispatch the `PrimitiveRequest` to the appropriate `CommandParser`. This is done by the `CommandDispatcherDispatcher`, which uses the command name to determine which parser to use.
 
 ### Error Response
+
 | Code              | Description                                                       |
 | :---------------- | :---------------------------------------------------------------- |
 | `UNKNOWN_COMMAND` | The command is unknown to this server. No parser has been defined |
 
-
 ## Command parsing
+
 Once the `PrimitiveRequest` has been dispatched to the appropriate `CommandParser`, the parser is responsible for parsing the parameters of the request and creating a `Request` that can be executed by the responsible `CommandHandler`.
 
 ### Error Response
+
 | Code                | Description                                                                                       |
 | :------------------ | :------------------------------------------------------------------------------------------------ |
 | `MISSING_PARAMETER` | A required parameter is missing from the request (see message field of response for more details) |
 
-
-
 # Pre-execution checks
+
 Pre-execution checks are reusable validation steps that can be registered on command handlers.
 They are implemented as `HandlerCheck` instances and are executed by the `CommandHandlerExecutor` before the handler's main logic is invoked.
 
@@ -148,16 +159,17 @@ Description of the check, what it does and when it should be used.
 -->
 
 ## UserLoggedInCheck
+
 The `UserLoggedInCheck` is a common pre-execution check that verifies whether the user is logged in (i.e. has a user associated with his session).
 
 ### Error Response
+
 | Code                 | Description               |
 | :------------------- | :------------------------ |
 | `USER_NOT_LOGGED_IN` | The user is not logged in |
 
-
-
 # Commands
+
 Commands are the core of our protocol, representing the various actions that clients can request from the server. Each command has a unique name and may require specific parameters in addition to pre-execution checks.
 The server processes these commands and responds accordingly.
 
@@ -212,41 +224,50 @@ END
 -->
 
 ## PING command
+
 The `PING` command is a simple command that can be used to check if the server is responsive.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 No parameters.
 
 ### Success Response
+
 No response fields.
 
 ### Example Request
+
 ```
 PING
 ```
 
 ### Example Response
+
 ```
 +OK
 END
 ```
 
-
 ## CHECK_USERNAME command
+
 The `CHECK_USERNAME` command is used to check if a username is already taken by another user. Additional users can still log in with the same username, but their name will be substituted with a suffix.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 | Parameter Name | Type     | Optional | Description                            |
 | :------------- | :------- | :------- | :------------------------------------- |
 | `USERNAME`     | `String` | no       | The username to check for availability |
 
 ### Success Response
+
 | Field    | Type                         | Description                                                             |
 | :------- | :--------------------------- | :---------------------------------------------------------------------- |
 | `STATUS` | `Enum<UsernameAvailability>` | Member of enum indicating if the username is available or already taken |
@@ -257,11 +278,13 @@ None.
 | `TAKEN`                           | Username is already in use |
 
 ### Example Request
+
 ```
 CHECK_USERNAME USERNAME='Lars'
 ```
 
 ### Example Response
+
 ```
 +OK 
   STATUS=FREE
@@ -269,33 +292,40 @@ END
 ```
 
 ## LOGIN command
+
 The `LOGIN` command is used to log in a user with a specified username. If the username is already taken by another user, the server will append a suffix to the username to make it unique.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 | Parameter Name | Type     | Optional | Description                          |
 | :------------- | :------- | :------- | :----------------------------------- |
 | `USERNAME`     | `String` | no       | The username to create the user with |
 
 ### Success Response
+
 | Field      | Type                                                                    | Description                                                           |
 | :--------- | :---------------------------------------------------------------------- | :-------------------------------------------------------------------- |
 | `USERNAME` | `String`                                                                | Username of the newly created user, can differ from the requested one |
 | `ID`       | [`UUID`](https://docs.oracle.com/javase/8/docs/api/java/util/UUID.html) | The ID of the created user                                            |
 
 ### Error Response
+
 | Code                | Description                                                                    |
 | :------------------ | :----------------------------------------------------------------------------- |
 | `ALREADY_LOGGED_IN` | The session is already associated with a user, logging in again is prohibited. |
 
 ### Example Request
+
 ```
 LOGIN USERNAME='Lars'
 ```
 
 ### Example Response
+
 ```
 +OK
   USERNAME='Lars_1234'
@@ -303,45 +333,100 @@ LOGIN USERNAME='Lars'
 END
 ```
 
+## CHANGE_USERNAME command
 
-## LOGOUT command
+The `CHANGE_USERNAME` command is used to change the username of an already logged-in user. The request is tied to the current session and updates all affected server-side mappings.
-Description of the command, what it does, and when it should be used.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
+| Parameter Name | Type     | Optional | Description                           |
+| :------------- | :------- | :------- | :------------------------------------ |
+| `USERNAME`     | `String` | no       | The new username for the active user  |
+
+### Success Response
+
+| Field      | Type                                                                    | Description                             |
+| :--------- | :---------------------------------------------------------------------- | :-------------------------------------- |
+| `USERNAME` | `String`                                                                | Effective username after rename         |
+| `ID`       | [`UUID`](https://docs.oracle.com/javase/8/docs/api/java/util/UUID.html) | The ID of the renamed user              |
+
+### Error Response
+
+| Code                 | Description                                                           |
+| :------------------- | :-------------------------------------------------------------------- |
+| `USER_NOT_LOGGED_IN` | No active user is associated with this session.                       |
+| `INVALID_USERNAME`   | Username contains disallowed characters or is empty.                  |
+| `USERNAME_TAKEN`     | Requested username is already used by another user.                   |
+| `RENAME_CONFLICT`    | Rename could not be propagated to all current lobby/game structures.  |
+
+### Example Request
+
+```
+CHANGE_USERNAME USERNAME='Lars_New'
+```
+
+### Example Response
+
+```
++OK
+  USERNAME='Lars_New'
+  ID=e47a671e-2b2a-42df-bb82-953fe2ebd307
+END
+```
+
+## LOGOUT command
+
+Description of the command, what it does, and when it should be used.
+
+### Required pre-execution checks
+
+None.
+
+### Request Parameters
+
 No parameters.
 
 ### Success Response
+
 No response fields.
 
 ### Error Response
+
 | Code                 | Description                                                      |
 | :------------------- | :--------------------------------------------------------------- |
 | `NO_USER_ASSOCIATED` | The session has no user associated, logging out is not possible. |
 
 ### Example Request
+
 ```
 LOGOUT
 ```
 
 ### Example Response
+
 ```
 +OK
 END
 ```
 
 ## LIST_USERS command
+
 The `LIST_USERS` command is used to retrieve a list of all currently logged-in users.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 No parameters.
 
 ### Success Response
+
 | Field   | Type               | Description                              |
 | :------ | :----------------- | :--------------------------------------- |
 | `USERS` | `Collection<User>` | Collection of all users currently online |
@@ -351,16 +436,18 @@ No parameters.
 | `USERNAME`       | `String`                                                                | Username of the newly created user, can differ from the requested one |
 | `ID`             | [`UUID`](https://docs.oracle.com/javase/8/docs/api/java/util/UUID.html) | The ID of the created user                                            |
 
-
 ### Error Response
+
 None.
 
 ### Example Request
+
 ```
 LIST_USERS
 ```
 
 ### Example Response
+
 ```
 +OK
   USERS
@@ -381,8 +468,11 @@ END
 ```
 
 ## SEND_MESSAGE command
+
 The `SEND_MESSAGE` command is used to transfer the chat message sent by a user to the server.
+
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
@@ -403,28 +493,36 @@ None.
 | `WHISPER`             | Message is for the whisper chat |
 
 ### Success Response
+
 No response fields.
 
 ### Error Response
+
 None.
 
 ### Example Request
+
 ```
 SEND_MESSAGE TYPE=GLOBAL GAME=1 USER=player1 TARGET=null TIME='10:30' TEXT='Hello World'
 ```
 
 ### Example Response
+
 ```
 +OK
 END
 ```
 
 ## GET_MESSAGE_COUNT command
+
 The `GET_MESSAGE_COUNT` is used to get the current number of messages that are stored in the queue for a client.
+
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 No parameters.
 
 ### Success Response
@@ -434,17 +532,19 @@ No parameters.
 | `COUNT` | `int` | The current number of messages |
 
 ### Error Response
+
 | Code                 | Description                                                                      |
 | :------------------- |:---------------------------------------------------------------------------------|
 | `NO_USER_ASSOCIATED` | The session has no user associated, there is no queue of messages for the client |
 
-
 ### Example Request
+
 ```
 GET_MESSAGE_COUNT
 ```
 
 ### Example Response
+
 ```
 +OK
 COUNT=10
@@ -452,14 +552,19 @@ END
 ```
 
 ## GET_NEXT_MESSAGE command
+
 The `GET_NEXT_MESSAGE` command is used to get the next message stored in a queue for the client.
+
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 No parameters.
 
 ### Success Response
+
 | Field    | Type            | Description                                                        |
 |:---------|:----------------|:-------------------------------------------------------------------|
 | `TYPE`   | `Enum<ChatType` | Member of Enum, indicating with chat type is used                  |
@@ -476,16 +581,19 @@ No parameters.
 | `WHISPER`             | Message is for the whisper chat |
 
 ### Error Response
+
 | Code                 | Description                                                                      |
 | :------------------- |:---------------------------------------------------------------------------------|
 | `NO_USER_ASSOCIATED` | The session has no user associated, there is no queue of messages for the client |
 
 ### Example Request
+
 ```
 GET_NEXT_MESSAGE
 ```
 
 ### Example Response
+
 ```
 +OK
 TYPE=GLOBAL GAME=-1 USER=player1 TARGET=null TIME=9:30 TEXT="Guten Tag"
@@ -613,9 +721,11 @@ END
 The `GET_LOBBY_LIST` command requests the server to return a list of currently available lobbies with basic metadata.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 No parameters.
 
 ### Implementation notes
@@ -667,9 +777,11 @@ END
 The `GET_GAME_STATE` command returns a snapshot of the current game for a lobby. It includes global fields (`PHASE`, `POT`, `CURRENT_BET`, `DEALER`, `ACTIVE_PLAYER`), repeated `CARD` blocks for community cards and repeated `PLAYER` blocks for per-player information. If the requester is a player in the game, their hole cards are included inside their `PLAYER` block as nested `CARD` blocks.
 
 ### Required pre-execution checks
+
 - [`UserLoggedInCheck`](#userloggedincheck)
 
 ### Request Parameters
+
 | Parameter Name | Type   | Optional | Description |
 | :------------- | :----- | :------: | :---------- |
 | `GAME_ID`      | `int`  | yes      | Numeric id of the lobby/game to query. If omitted the server will resolve the lobby by the requesting session's associated user.
@@ -1114,9 +1226,11 @@ END
 The `GET_LOBBY_STATUS` command requests the server to return the current state of a lobby, including the list of players and their ready state.
 
 ### Required pre-execution checks
+
 None.
 
 ### Request Parameters
+
 | Parameter Name | Type | Optional | Description |
 | :------------- | :--- | :------: | :---------- |
 | `ID` | `int` | no | Numeric id of the target lobby |
@@ -1151,6 +1265,7 @@ END
 ```
 
 ### Error Response
+
 | Code | Description |
 | :--- | :---------- |
 | `LOBBY_NOT_FOUND` | The specified lobby id does not exist |

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/ClientApp.java

@@ -39,6 +39,10 @@ public class ClientApp {
         return sharedUsername;
     }
 
+    public static void updateSharedUsername(String username) {
+        setSharedUsername(username != null && !username.isBlank() ? username.trim() : null);
+    }
+
     private static void setSharedUsername(String username) {
         sharedUsername = username;
         LOGGER.info("sharedUsername set to '{}'", getSharedUsername());

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/chat/ChatController.java

@@ -1,15 +1,20 @@
 package ch.unibas.dmi.dbis.cs108.casono.client.chat;
 
+import ch.unibas.dmi.dbis.cs108.casono.client.ClientApp;
 import ch.unibas.dmi.dbis.cs108.casono.client.network.ChatClient;
 import ch.unibas.dmi.dbis.cs108.casono.client.network.ClientService;
 import ch.unibas.dmi.dbis.cs108.casono.client.ui.chatui.ChatBoxController;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.RequestParameter;
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.Timer;
 import java.util.TimerTask;
 import java.util.WeakHashMap;
+import java.util.function.Consumer;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.jspecify.annotations.Nullable;
@@ -24,7 +29,7 @@ public class ChatController {
     private static final Map<ClientService, ChatController> ACTIVE_CONTROLLERS =
             new WeakHashMap<>();
 
-    private final String username;
+    private volatile String username;
 
     private final ClientService clientService;
 
@@ -37,6 +42,7 @@ public class ChatController {
     private final ChatBoxController chatBoxController;
     private int lobbyId = -1;
     private final Timer timer;
+    private final Consumer<List<String>> serverEventListener;
 
     public record ChatKey(ChatType type, @Nullable String targetUser) {
         public ChatKey(ChatType type) {
@@ -71,8 +77,10 @@ public class ChatController {
         localUserList = new ArrayList<>();
         this.chatBoxController = new ChatBoxController(username, this);
         this.logger = LogManager.getLogger(ChatController.class);
+        this.serverEventListener = this::handleServerEvent;
 
         registerAsActiveController(clientService);
+        clientService.addEventListener(serverEventListener);
 
         this.timer = new Timer(true);
         timer.schedule(
@@ -131,6 +139,7 @@ public class ChatController {
      * <p>All Messages get added to a particular {@link ChatModel}, if the checks passed.
      */
     public void receiveMessage() {
+        String currentUsername = getCurrentUsername();
         List<Message> newMessages = chatClient.getMessages();
         if (!newMessages.isEmpty()) {
             for (Message msg : newMessages) {
@@ -146,16 +155,17 @@ public class ChatController {
                                             (_key) ->
                                                     new ChatModel(
                                                             ChatType.LOBBY,
-                                                            username,
+                                                            currentUsername,
                                                             msg.lobbyId,
                                                             null))
                                     .addMessage(msg);
                         }
                         break;
                     case ChatType.WHISPER:
-                        if (msg.target.equals(username) || msg.sender.equals(username)) {
+                        if (msg.target.equals(currentUsername)
+                                || msg.sender.equals(currentUsername)) {
                             ChatKey key;
-                            if (msg.target.equals(username)) {
+                            if (msg.target.equals(currentUsername)) {
                                 key = new ChatKey(ChatType.WHISPER, msg.sender);
                             } else {
                                 key = new ChatKey(ChatType.WHISPER, msg.target);
@@ -166,7 +176,7 @@ public class ChatController {
                                 ChatModel chatModel =
                                         new ChatModel(
                                                 ChatType.WHISPER,
-                                                username,
+                                                currentUsername,
                                                 lobbyId,
                                                 key.targetUser());
                                 chatBoxController.addWhisperChat(key.targetUser(), chatModel);
@@ -203,13 +213,29 @@ public class ChatController {
      */
     public synchronized void checkWhisperUsers() {
         List<String> users = chatClient.getUsers();
-        logger.info(users);
+        Set<String> remoteUsers = new HashSet<>();
-        if (!users.isEmpty()) {
+        String currentUsername = getCurrentUsername();
-            for (String user : users) {
+
-                String value = user.split("\\=")[1];
+        for (String user : users) {
-                logger.info(value);
+            if (user == null || !user.contains("=")) {
-                addWhisperUser(value);
+                continue;
             }
+            String value = user.split("\\=", 2)[1].trim();
+            if (value.isBlank() || value.equals(currentUsername)) {
+                continue;
+            }
+            remoteUsers.add(value);
+        }
+
+        for (String known : new ArrayList<>(localUserList)) {
+            if (!remoteUsers.contains(known)) {
+                localUserList.remove(known);
+                chatBoxController.removeWhisperUser(known);
+            }
+        }
+
+        for (String remote : remoteUsers) {
+            addWhisperUser(remote);
         }
     }
 
@@ -222,16 +248,110 @@ public class ChatController {
         if (user == null || user.isBlank()) {
             return;
         }
-        if (!(localUserList.contains(user) || user.equals(username))) {
+        if (!(localUserList.contains(user) || user.equals(getCurrentUsername()))) {
             localUserList.add(user);
             logger.info("adding new whisper user");
             chatBoxController.addWhisperUser(user);
         }
     }
 
+    public synchronized void updateUsername(String newUsername) {
+        if (newUsername == null || newUsername.isBlank()) {
+            return;
+        }
+        String oldUsername = this.username;
+        this.username = newUsername.trim();
+        chatBoxController.setUsername(this.username);
+
+        if (oldUsername != null && !oldUsername.equals(this.username)) {
+            localUserList.remove(oldUsername);
+            chatBoxController.removeWhisperUser(oldUsername);
+        }
+    }
+
+    private void handleServerEvent(List<String> lines) {
+        if (lines == null || lines.isEmpty()) {
+            return;
+        }
+
+        List<RequestParameter> params;
+        try {
+            params = ClientService.convertToRequestParameters(lines);
+        } catch (RuntimeException e) {
+            return;
+        }
+        String event = null;
+        String oldUsername = null;
+        String newUsername = null;
+
+        for (RequestParameter p : params) {
+            if ("EVENT".equalsIgnoreCase(p.key())) {
+                event = p.value();
+            } else if ("OLD_USERNAME".equalsIgnoreCase(p.key())) {
+                oldUsername = p.value();
+            } else if ("NEW_USERNAME".equalsIgnoreCase(p.key())) {
+                newUsername = p.value();
+            }
+        }
+
+        if (!"USERNAME_CHANGED".equalsIgnoreCase(event)) {
+            return;
+        }
+
+        applyUsernameMigration(oldUsername, newUsername);
+    }
+
+    private synchronized void applyUsernameMigration(String oldUsername, String newUsername) {
+        if (oldUsername == null
+                || newUsername == null
+                || oldUsername.isBlank()
+                || newUsername.isBlank()
+                || oldUsername.equals(newUsername)) {
+            return;
+        }
+
+        String currentUsername = getCurrentUsername();
+        if (oldUsername.equals(currentUsername)) {
+            updateUsername(newUsername);
+        }
+
+        ChatKey oldKey = new ChatKey(ChatType.WHISPER, oldUsername);
+        ChatKey newKey = new ChatKey(ChatType.WHISPER, newUsername);
+
+        ChatModel oldModel = chatModelMap.remove(oldKey);
+        ChatModel existingNewModel = chatModelMap.get(newKey);
+        if (oldModel != null) {
+            oldModel.setTarget(newUsername);
+            if (existingNewModel == null) {
+                chatModelMap.put(newKey, oldModel);
+            } else {
+                // Merge possible parallel history into the already existing new-key model.
+                for (Message msg : oldModel.messages) {
+                    existingNewModel.addMessage(msg);
+                }
+            }
+            chatBoxController.renameWhisperUser(oldUsername, newUsername);
+        }
+
+        localUserList.remove(oldUsername);
+        chatBoxController.removeWhisperUser(oldUsername);
+        if (!newUsername.equals(getCurrentUsername())) {
+            addWhisperUser(newUsername);
+        }
+    }
+
+    public String getCurrentUsername() {
+        String shared = ClientApp.getSharedUsername();
+        if (shared != null && !shared.isBlank()) {
+            return shared.trim();
+        }
+        return username;
+    }
+
     /** Stops polling background tasks for this chat controller instance. */
     public void shutdown() {
         timer.cancel();
+        clientService.removeEventListener(serverEventListener);
         synchronized (ACTIVE_CONTROLLERS) {
             if (ACTIVE_CONTROLLERS.get(clientService) == this) {
                 ACTIVE_CONTROLLERS.remove(clientService);

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/chat/ChatModel.java

@@ -22,7 +22,7 @@ public class ChatModel {
     public final String username;
 
     /** The person to send the message to If the chat is a whisper chat */
-    private final String target;
+    private String target;
 
     private final IntegerProperty count;
 
@@ -82,4 +82,8 @@ public class ChatModel {
     public String getTarget() {
         return target;
     }
+
+    public void setTarget(String target) {
+        this.target = target;
+    }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/network/LobbyClient.java

@@ -131,6 +131,29 @@ public class LobbyClient {
         return new LoginResult(assigned, id);
     }
 
+    /**
+     * Changes the username for the currently logged-in session.
+     *
+     * @param newUsername desired new username
+     * @return a {@link LoginResult} containing assigned username and id as returned by the server
+     */
+    public LoginResult changeUsername(String newUsername) {
+        List<String> lines = client.processCommand("CHANGE_USERNAME USERNAME=" + newUsername);
+
+        List<RequestParameter> params = ClientService.convertToRequestParameters(lines);
+
+        String assigned = newUsername;
+        String id = null;
+        for (RequestParameter p : params) {
+            if ("USERNAME".equalsIgnoreCase(p.key())) {
+                assigned = p.value();
+            } else if ("ID".equalsIgnoreCase(p.key())) {
+                id = p.value();
+            }
+        }
+        return new LoginResult(assigned, id);
+    }
+
     /**
      * Request the server for the list of available lobbies.
      *

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/ui/chatui/ChatBoxController.java

@@ -60,6 +60,12 @@ public class ChatBoxController {
         usernameTabMap = new HashMap<>();
     }
 
+    public void setUsername(String username) {
+        if (username != null && !username.isBlank()) {
+            this.username = username.trim();
+        }
+    }
+
     /**
      * Initializes the chat interface by creating the global chat model and adding the corresponding
      * "GLOBAL" tab to the interface. It also registers the global chat in the {@link
@@ -84,6 +90,11 @@ public class ChatBoxController {
     public void addWhisperUser(String targetUserName) {
         Platform.runLater(
                 () -> {
+                    for (MenuItem existing : addWhisperChatButton.getItems()) {
+                        if (targetUserName.equals(existing.getText())) {
+                            return;
+                        }
+                    }
                     MenuItem menuItem = new MenuItem(targetUserName);
                     addWhisperChatButton.getItems().add(menuItem);
                     menuItem.setOnAction(
@@ -98,6 +109,54 @@ public class ChatBoxController {
                 });
     }
 
+    public void removeWhisperUser(String targetUserName) {
+        Platform.runLater(
+                () ->
+                        addWhisperChatButton
+                                .getItems()
+                                .removeIf(item -> targetUserName.equals(item.getText())));
+    }
+
+    public void renameWhisperUser(String oldUsername, String newUsername) {
+        if (oldUsername == null
+                || newUsername == null
+                || oldUsername.isBlank()
+                || newUsername.isBlank()
+                || oldUsername.equals(newUsername)) {
+            return;
+        }
+
+        Platform.runLater(
+                () -> {
+                    for (MenuItem item : addWhisperChatButton.getItems()) {
+                        if (oldUsername.equals(item.getText())) {
+                            item.setText(newUsername);
+                            item.setOnAction(
+                                    event ->
+                                            addWhisperChat(
+                                                    newUsername,
+                                                    new ChatModel(
+                                                            ChatType.WHISPER,
+                                                            username,
+                                                            -1,
+                                                            newUsername)));
+                            break;
+                        }
+                    }
+
+                    Tab tab = usernameTabMap.remove(oldUsername);
+                    if (tab != null) {
+                        tab.setText(newUsername);
+                        usernameTabMap.put(newUsername, tab);
+                    }
+
+                    int idx = activeWhisperChats.indexOf(oldUsername);
+                    if (idx >= 0) {
+                        activeWhisperChats.set(idx, newUsername);
+                    }
+                });
+    }
+
     /**
      * Creates a new private (whisper) chat model for a specific target user, registers it within
      * the chat system, and opens a new chat tab.

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/ui/chatui/ChatViewController.java

@@ -68,11 +68,12 @@ public class ChatViewController implements Initializable {
         String message = inputField.getText().trim();
         if (!message.isEmpty()) {
             inputField.clear();
+            String currentUsername = controller.getCurrentUsername();
             Message msg =
                     new Message(
                             chatModel.getChattype(),
                             chatModel.lobbyId,
-                            username,
+                            currentUsername,
                             chatModel.getTarget(),
                             message);
             controller.onSendToNetwork(msg);

src/main/java/ch/unibas/dmi/dbis/cs108/casono/client/ui/lobbyui/CasinomainuiController.java

@@ -104,6 +104,13 @@ public class CasinomainuiController {
         casinoTable.getChildren().add(gridManager.getGridPane());
         gridManager.renderLobbyButtons();
 
+        String sharedUsername = ClientApp.getSharedUsername();
+        updateUsernameFieldPresentation(sharedUsername);
+        if (loginButton != null) {
+            loginButton.setText(
+                    sharedUsername != null && !sharedUsername.isBlank() ? "Change Name" : "Login");
+        }
+
         initializeChat(clientService);
     }
 
@@ -156,7 +163,7 @@ public class CasinomainuiController {
     /** Handles the login button action. Validates input and calls LobbyClient.login(). */
     @FXML
     public void handleLoginButton() {
-        String username = usernameField.getText();
+        String username = usernameField == null ? null : usernameField.getText();
         if (username == null || username.isBlank()) {
             showAlert("Please enter a username.");
             return;
@@ -170,12 +177,63 @@ public class CasinomainuiController {
             showAlert("Offline mode: cannot send login to server.");
             return;
         }
+
+        String trimmed = username.trim();
         try {
-            lobbyClient.login(username);
+            var result = lobbyClient.login(trimmed);
-            showAlert("Login sent: " + username);
+            String assigned = result != null ? result.getUsername() : trimmed;
+            ClientApp.updateSharedUsername(assigned);
+            if (chatController != null) {
+                chatController.updateUsername(assigned);
+            }
+            updateUsernameFieldPresentation(assigned);
+            if (loginButton != null) {
+                loginButton.setText("Change Name");
+            }
+            return;
+        } catch (RuntimeException loginError) {
+            if (!containsProtocolError(loginError, "ALREADY_LOGGED_IN")) {
+                LOGGER.error("Login failed: {}", loginError.getMessage());
+                showAlert("Login failed: " + loginError.getMessage());
+                return;
+            }
+            LOGGER.info("Session already logged in, trying username change");
+        }
+
+        try {
+            var result = lobbyClient.changeUsername(trimmed);
+            String assigned = result != null ? result.getUsername() : trimmed;
+            ClientApp.updateSharedUsername(assigned);
+            if (chatController != null) {
+                chatController.updateUsername(assigned);
+            }
+            updateUsernameFieldPresentation(assigned);
+            if (loginButton != null) {
+                loginButton.setText("Change Name");
+            }
         } catch (RuntimeException e) {
-            LOGGER.error("Login failed: {}", e.getMessage());
+            LOGGER.error("Change username failed: {}", e.getMessage());
-            showAlert("Login failed: " + e.getMessage());
+            showAlert("Change username failed: " + e.getMessage());
+        }
+    }
+
+    private boolean containsProtocolError(RuntimeException error, String code) {
+        String msg = error.getMessage();
+        return msg != null && msg.contains(code);
+    }
+
+    private void updateUsernameFieldPresentation(String username) {
+        if (usernameField == null) {
+            return;
+        }
+
+        if (username != null && !username.isBlank()) {
+            usernameField.setText(username.trim());
+            usernameField.positionCaret(usernameField.getText().length());
+            usernameField.setPromptText("Username");
+        } else {
+            usernameField.clear();
+            usernameField.setPromptText("Username");
         }
     }
 

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/ServerApp.java

@@ -1,8 +1,26 @@
 package ch.unibas.dmi.dbis.cs108.casono.server;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username.ChangeUsernameHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username.ChangeUsernameParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username.ChangeUsernameRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet.PlayerBetHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet.PlayerBetParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet.PlayerBetRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call.PlayerCallHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call.PlayerCallParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call.PlayerCallRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold.PlayerFoldHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold.PlayerFoldParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold.PlayerFoldRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state.GetGameStateHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state.GetGameStateParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state.GetGameStateRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise.PlayerRaiseHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise.PlayerRaiseParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise.PlayerRaiseRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.get_message_count.GetMessageCountHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.get_message_count.GetMessageCountParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.get_message_count.GetMessageCountRequest;
@@ -12,6 +30,21 @@ import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.get_next_message.GetN
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.list_users.ListUsersHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.list_users.ListUsersParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.list_users.ListUsersRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby.CreateLobbyHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby.CreateLobbyParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby.CreateLobbyRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list.GetLobbyListHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list.GetLobbyListParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list.GetLobbyListRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_status.GetLobbyStatusHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_status.GetLobbyStatusParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_status.GetLobbyStatusRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby.JoinLobbyHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby.JoinLobbyParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby.JoinLobbyRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game.StartGameHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game.StartGameParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game.StartGameRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginRequest;
@@ -28,6 +61,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby.LobbyManager;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserCleanupJob;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.NetworkManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandlerExecutor;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandRouter;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParserDispatcher;
@@ -168,6 +202,15 @@ public class ServerApp {
         commandRouter.register(
                 LoginRequest.class, new LoginHandler(responseDispatcher, userRegistry));
 
+        parserDispatcher.register("CHANGE_USERNAME", new ChangeUsernameParser());
+        commandRouter.register(
+                ChangeUsernameRequest.class,
+                new ChangeUsernameHandler(
+                        responseDispatcher,
+                        userRegistry,
+                        context.lobbyManager(),
+                        context.sessionManager()));
+
         parserDispatcher.register("LOGOUT", new LogoutParser());
         commandRouter.register(
                 LogoutRequest.class, new LogoutHandler(responseDispatcher, userRegistry));
@@ -192,152 +235,84 @@ public class ServerApp {
                 ListUsersRequest.class, new ListUsersHandler(responseDispatcher, userRegistry));
 
         // GET_LOBBY_LIST registration
-        parserDispatcher.register(
+        parserDispatcher.register("GET_LOBBY_LIST", new GetLobbyListParser());
-                "GET_LOBBY_LIST",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list
-                        .GetLobbyListParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list
+                GetLobbyListRequest.class,
-                        .GetLobbyListRequest.class,
+                (CommandHandler<GetLobbyListRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new GetLobbyListHandler(responseDispatcher, context.lobbyManager()));
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby
-                                        .get_lobby_list.GetLobbyListRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_list
-                                .GetLobbyListHandler(responseDispatcher, context.lobbyManager()));
 
         // GET_GAME_STATE registration
-        parserDispatcher.register(
+        parserDispatcher.register("GET_GAME_STATE", new GetGameStateParser());
-                "GET_GAME_STATE",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state
-                        .GetGameStateParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state
+                GetGameStateRequest.class,
-                        .GetGameStateRequest.class,
+                (CommandHandler<GetGameStateRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new GetGameStateHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game
-                                        .get_game_state.GetGameStateRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.get_game_state
-                                .GetGameStateHandler(
                                 responseDispatcher, context.lobbyManager(), userRegistry));
 
         // BET registration
-        parserDispatcher.register(
+        parserDispatcher.register("BET", new PlayerBetParser());
-                "BET",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet.PlayerBetParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet.PlayerBetRequest.class,
+                PlayerBetRequest.class,
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                (CommandHandler<PlayerBetRequest>)
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet
+                        new PlayerBetHandler(
-                                        .PlayerBetRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.bet
-                                .PlayerBetHandler(
                                 responseDispatcher, userRegistry, context.lobbyManager()));
 
         // RAISE registration
-        parserDispatcher.register(
+        parserDispatcher.register("RAISE", new PlayerRaiseParser());
-                "RAISE",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise
-                        .PlayerRaiseParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise.PlayerRaiseRequest
+                PlayerRaiseRequest.class,
-                        .class,
+                (CommandHandler<PlayerRaiseRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new PlayerRaiseHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise
-                                        .PlayerRaiseRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.raise
-                                .PlayerRaiseHandler(
                                 responseDispatcher, userRegistry, context.lobbyManager()));
 
         // CALL registration
-        parserDispatcher.register(
+        parserDispatcher.register("CALL", new PlayerCallParser());
-                "CALL",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call
-                        .PlayerCallParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call.PlayerCallRequest
+                PlayerCallRequest.class,
-                        .class,
+                (CommandHandler<PlayerCallRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new PlayerCallHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call
-                                        .PlayerCallRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.call
-                                .PlayerCallHandler(
                                 responseDispatcher, userRegistry, context.lobbyManager()));
 
         // FOLD registration
-        parserDispatcher.register(
+        parserDispatcher.register("FOLD", new PlayerFoldParser());
-                "FOLD",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold
-                        .PlayerFoldParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold.PlayerFoldRequest
+                PlayerFoldRequest.class,
-                        .class,
+                (CommandHandler<PlayerFoldRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new PlayerFoldHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold
-                                        .PlayerFoldRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.game.fold
-                                .PlayerFoldHandler(
                                 responseDispatcher, userRegistry, context.lobbyManager()));
 
         // GET_LOBBY_STATUS registration
-        parserDispatcher.register(
+        parserDispatcher.register("GET_LOBBY_STATUS", new GetLobbyStatusParser());
-                "GET_LOBBY_STATUS",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_status
-                        .GetLobbyStatusParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.get_lobby_status
+                GetLobbyStatusRequest.class,
-                        .GetLobbyStatusRequest.class,
+                (CommandHandler<GetLobbyStatusRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new GetLobbyStatusHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby
-                                        .get_lobby_status.GetLobbyStatusRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby
-                                .get_lobby_status.GetLobbyStatusHandler(
                                 responseDispatcher, context.lobbyManager(), userRegistry));
 
         // CREATE_LOBBY registration
-        parserDispatcher.register(
+        parserDispatcher.register("CREATE_LOBBY", new CreateLobbyParser());
-                "CREATE_LOBBY",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby
-                        .CreateLobbyParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby
+                CreateLobbyRequest.class,
-                        .CreateLobbyRequest.class,
+                (CommandHandler<CreateLobbyRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new CreateLobbyHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby
-                                        .create_lobby.CreateLobbyRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.create_lobby
-                                .CreateLobbyHandler(
                                 responseDispatcher,
                                 context.lobbyManager(),
                                 context.sessionManager()));
 
         // JOIN_LOBBY registration
-        parserDispatcher.register(
+        parserDispatcher.register("JOIN_LOBBY", new JoinLobbyParser());
-                "JOIN_LOBBY",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby
-                        .JoinLobbyParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby
+                JoinLobbyRequest.class,
-                        .JoinLobbyRequest.class,
+                (CommandHandler<JoinLobbyRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new JoinLobbyHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby
-                                        .JoinLobbyRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby
-                                .JoinLobbyHandler(
                                 responseDispatcher, context.lobbyManager(), userRegistry));
 
         // START_GAME registration
-        parserDispatcher.register(
+        parserDispatcher.register("START_GAME", new StartGameParser());
-                "START_GAME",
-                new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game
-                        .StartGameParser());
         commandRouter.register(
-                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game
+                StartGameRequest.class,
-                        .StartGameRequest.class,
+                (CommandHandler<StartGameRequest>)
-                (ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler<
+                        new StartGameHandler(
-                                ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game
-                                        .StartGameRequest>)
-                        new ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.start_game
-                                .StartGameHandler(
                                 responseDispatcher, context.lobbyManager(), userRegistry));
     }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/change_username/ChangeUsernameHandler.java

@@ -0,0 +1,112 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby.LobbyManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.SuccessResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.builder.ResponseBodyBuilder;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.Session;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionManager;
+import java.util.Optional;
+import java.util.regex.Pattern;
+
+/** Handles CHANGE_USERNAME requests for already logged-in users. */
+public class ChangeUsernameHandler extends CommandHandler<ChangeUsernameRequest> {
+    private static final Pattern VALID_USERNAME = Pattern.compile("[a-zA-Z0-9_-]+");
+    private final UserRegistry userRegistry;
+    private final LobbyManager lobbyManager;
+    private final SessionManager sessionManager;
+
+    /**
+     * @param responseDispatcher dispatcher used for responses
+     * @param userRegistry registry containing all users
+     * @param lobbyManager lobby manager used to keep lobby/game mappings in sync
+     * @param sessionManager session manager used to broadcast rename events
+     */
+    public ChangeUsernameHandler(
+            ResponseDispatcher responseDispatcher,
+            UserRegistry userRegistry,
+            LobbyManager lobbyManager,
+            SessionManager sessionManager) {
+        super(responseDispatcher);
+        this.userRegistry = userRegistry;
+        this.lobbyManager = lobbyManager;
+        this.sessionManager = sessionManager;
+    }
+
+    @Override
+    public void execute(ChangeUsernameRequest request) {
+        Optional<User> user = userRegistry.getBySessionId(request.getSessionId());
+        if (user.isEmpty()) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "USER_NOT_LOGGED_IN",
+                            "This session is not associated with an active user."));
+            return;
+        }
+
+        String newUsername = request.getUsername() == null ? "" : request.getUsername().trim();
+        if (newUsername.isEmpty() || !VALID_USERNAME.matcher(newUsername).matches()) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "INVALID_USERNAME",
+                            "Only letters, numbers, '_' and '-' are allowed."));
+            return;
+        }
+
+        User currentUser = user.get();
+        String oldUsername = currentUser.getName();
+        boolean changed = userRegistry.changeUsername(currentUser.getId(), newUsername);
+        if (!changed) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "USERNAME_TAKEN",
+                            "The requested username is already taken."));
+            return;
+        }
+
+        boolean lobbySynced =
+                lobbyManager == null || lobbyManager.renamePlayer(oldUsername, newUsername);
+        if (!lobbySynced) {
+            userRegistry.changeUsername(currentUser.getId(), oldUsername);
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "RENAME_CONFLICT",
+                            "Could not update username in current lobby/game state."));
+            return;
+        }
+
+        responseDispatcher.dispatch(
+                new ChangeUsernameResponse(
+                        request.getContext(), currentUser.getName(), currentUser.getId()));
+
+        broadcastUsernameChanged(oldUsername, currentUser.getName());
+    }
+
+    private void broadcastUsernameChanged(String oldUsername, String newUsername) {
+        if (sessionManager == null) {
+            return;
+        }
+
+        for (Session session : sessionManager.getAllSessions()) {
+            RequestContext ctx = new RequestContext(session.getId(), 0);
+            SuccessResponse ev =
+                    new SuccessResponse(
+                            ctx,
+                            new ResponseBodyBuilder()
+                                    .param("EVENT", "USERNAME_CHANGED")
+                                    .param("OLD_USERNAME", oldUsername)
+                                    .param("NEW_USERNAME", newUsername)
+                                    .build()) {};
+            responseDispatcher.dispatch(ev);
+        }
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/change_username/ChangeUsernameParser.java

@@ -0,0 +1,15 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.PrimitiveRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.accessor.RequestParameterAccessor;
+
+/** Parses CHANGE_USERNAME requests. */
+public class ChangeUsernameParser implements CommandParser<ChangeUsernameRequest> {
+    @Override
+    public ChangeUsernameRequest parse(PrimitiveRequest primitiveRequest) {
+        RequestParameterAccessor accessor =
+                new RequestParameterAccessor(primitiveRequest.parameters());
+        return new ChangeUsernameRequest(primitiveRequest.context(), accessor.require("USERNAME"));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/change_username/ChangeUsernameRequest.java

@@ -0,0 +1,25 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+
+/** Request used to change the username of the current session user. */
+public class ChangeUsernameRequest extends Request {
+    private final String username;
+
+    /**
+     * @param context request context for responses
+     * @param username desired new username
+     */
+    public ChangeUsernameRequest(RequestContext context, String username) {
+        super(context);
+        this.username = username;
+    }
+
+    /**
+     * @return desired new username
+     */
+    public String getUsername() {
+        return username;
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/change_username/ChangeUsernameResponse.java

@@ -0,0 +1,23 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.change_username;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserId;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.SuccessResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.builder.ResponseBodyBuilder;
+
+/** Response for successful username changes. */
+public class ChangeUsernameResponse extends SuccessResponse {
+    /**
+     * @param context request context
+     * @param username current username after the rename operation
+     * @param id user id of renamed user
+     */
+    public ChangeUsernameResponse(RequestContext context, String username, UserId id) {
+        super(
+                context,
+                new ResponseBodyBuilder()
+                        .param("USERNAME", username)
+                        .param("ID", id.value())
+                        .build());
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/send_message/SendMessageHandler.java

@@ -9,6 +9,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkResponse;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Optional;
 
 public class SendMessageHandler extends CommandHandler<SendMessageRequest> {
     private final UserRegistry userRegistry;
@@ -39,6 +40,11 @@ public class SendMessageHandler extends CommandHandler<SendMessageRequest> {
     @Override
     public void execute(SendMessageRequest request) {
         Message message = request.getMessage();
+        Optional<ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User> senderUser =
+                userRegistry.getBySessionId(request.getSessionId());
+        if (senderUser.isPresent()) {
+            message.sender = senderUser.get().getName();
+        }
         broadcast(request, message);
         OkResponse response = new OkResponse(request.getContext());
         responseDispatcher.dispatch(response);

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java

@@ -61,6 +61,35 @@ public class GameController {
         engine.getState().addPlayer(name, chips);
     }
 
+    /**
+     * Renames a player id in the controller list and underlying game state.
+     *
+     * @param oldId old player id
+     * @param newId new player id
+     * @return true if rename succeeded
+     */
+    public boolean renamePlayer(PlayerId oldId, PlayerId newId) {
+        if (oldId == null || newId == null) {
+            return false;
+        }
+        if (oldId.equals(newId)) {
+            return true;
+        }
+
+        int idx = players.indexOf(oldId);
+        if (idx < 0 || players.contains(newId)) {
+            return false;
+        }
+
+        boolean stateRenamed = engine.getState().renamePlayerId(oldId, newId);
+        if (!stateRenamed) {
+            return false;
+        }
+
+        players.set(idx, newId);
+        return true;
+    }
+
     /**
      * Initializes a new hand by preparing the deck, setting the phase to PREFLOP, rotating the
      * dealer, dealing hole cards, posting blinds, and setting the first active player.

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/player/Player.java

@@ -49,6 +49,16 @@ public class Player {
         return id;
     }
 
+    /**
+     * Updates the player's id. This is used when a username change is propagated into an already
+     * running game.
+     *
+     * @param id new player id
+     */
+    public void setId(PlayerId id) {
+        this.id = id;
+    }
+
     /**
      * Returns the display name of the player. Currently identical to the player ID.
      *

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/state/GameState.java

@@ -174,6 +174,51 @@ public class GameState {
         holeCards.computeIfAbsent(id, k -> new ArrayList<>());
     }
 
+    /**
+     * Renames a player id across all game-state structures.
+     *
+     * @param oldId existing player id
+     * @param newId new player id
+     * @return true if the rename was applied, false otherwise
+     */
+    public synchronized boolean renamePlayerId(PlayerId oldId, PlayerId newId) {
+        if (oldId == null || newId == null) {
+            return false;
+        }
+        if (!players.containsKey(oldId)) {
+            return false;
+        }
+        if (oldId.equals(newId)) {
+            return true;
+        }
+        if (players.containsKey(newId)) {
+            return false;
+        }
+
+        Player player = players.remove(oldId);
+        if (player == null) {
+            return false;
+        }
+        player.setId(newId);
+        players.put(newId, player);
+
+        int idx = playerOrder.indexOf(oldId);
+        if (idx >= 0) {
+            playerOrder.set(idx, newId);
+        }
+
+        moveMapEntry(currentBets, oldId, newId, 0);
+        moveMapEntry(playerBetCommitments, oldId, newId, 0);
+        moveMapEntry(holeCards, oldId, newId, new ArrayList<>());
+        return true;
+    }
+
+    private <T> void moveMapEntry(
+            Map<PlayerId, T> map, PlayerId oldId, PlayerId newId, T fallback) {
+        T value = map.remove(oldId);
+        map.put(newId, value != null ? value : fallback);
+    }
+
     // Betting
     public int getCurrentBet(PlayerId playerId) {
         return currentBets.getOrDefault(playerId, 0);

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/Lobby.java

@@ -70,6 +70,30 @@ public class Lobby {
         return playerNames.remove(playerName);
     }
 
+    /**
+     * Renames a player in this lobby's player list.
+     *
+     * @param oldName old username
+     * @param newName new username
+     * @return true if renamed successfully
+     */
+    public boolean renamePlayer(String oldName, String newName) {
+        if (oldName == null || newName == null) {
+            return false;
+        }
+        synchronized (playerNames) {
+            if (oldName.equals(newName)) {
+                return playerNames.contains(oldName);
+            }
+            int idx = playerNames.indexOf(oldName);
+            if (idx < 0 || playerNames.contains(newName)) {
+                return false;
+            }
+            playerNames.set(idx, newName);
+            return true;
+        }
+    }
+
     public void initGame(GameController controller) {
         this.gameController = controller;
     }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/lobby/LobbyManager.java

@@ -1,5 +1,6 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.game.player.PlayerId;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby.Lobby.AddResult;
 import java.time.Duration;
 import java.time.Instant;
@@ -170,6 +171,56 @@ public class LobbyManager {
         }
     }
 
+    /**
+     * Renames a player across lobby mapping, lobby player list and running game ids.
+     *
+     * @param oldUsername old username
+     * @param newUsername new username
+     * @return true if rename was applied
+     */
+    public synchronized boolean renamePlayer(String oldUsername, String newUsername) {
+        if (oldUsername == null || newUsername == null) {
+            return false;
+        }
+        if (oldUsername.equals(newUsername)) {
+            return true;
+        }
+
+        LobbyId lobbyId = playerToLobby.get(oldUsername);
+        if (lobbyId == null) {
+            return true;
+        }
+        if (playerToLobby.containsKey(newUsername)) {
+            return false;
+        }
+
+        Lobby lobby = activeLobbies.get(lobbyId);
+        if (lobby == null) {
+            playerToLobby.remove(oldUsername);
+            return true;
+        }
+
+        boolean lobbyRenamed = lobby.renamePlayer(oldUsername, newUsername);
+        if (!lobbyRenamed) {
+            return false;
+        }
+
+        if (lobby.getGameController() != null) {
+            boolean gameRenamed =
+                    lobby.getGameController()
+                            .renamePlayer(PlayerId.of(oldUsername), PlayerId.of(newUsername));
+            if (!gameRenamed) {
+                // Best-effort rollback to keep structures consistent.
+                lobby.renamePlayer(newUsername, oldUsername);
+                return false;
+            }
+        }
+
+        playerToLobby.remove(oldUsername);
+        playerToLobby.put(newUsername, lobbyId);
+        return true;
+    }
+
     /**
      * Apply the given action to every player username in the lobby identified by {@code lobbyId}.
      * This is a small helper that keeps iteration logic centralized and avoids leaking internal

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserRegistry.java

@@ -200,6 +200,10 @@ public class UserRegistry {
             return false;
         }
 
+        if (user.getName().equals(newName)) {
+            return true;
+        }
+
         if (byName.containsKey(newName)) {
             return false;
         }

src/main/resources/ui-structure/Casinomainui.fxml

@@ -81,25 +81,24 @@
                         </GridPane.margin>
                     </Button>
 
-                        <!-- Username input and login button -->
+                        <!-- Username input and login/change button -->
-                            <!-- <HBox alignment="CENTER_LEFT" spacing="10"-->
+                        <HBox alignment="CENTER_LEFT" spacing="10"
-                            <!--             GridPane.rowIndex="0"-->
+                              GridPane.rowIndex="0"
-                            <!--             GridPane.columnIndex="0"-->
+                              GridPane.columnIndex="0"
-                            <!--             GridPane.halignment="LEFT"-->
+                              GridPane.halignment="LEFT"
-                            <!--             GridPane.valignment="TOP">-->
+                              GridPane.valignment="TOP">
-                            <!--         <GridPane.margin>-->
+                            <GridPane.margin>
-                            <!--                 &lt;!&ndash; place below the 'CREATE A LOBBY' button &ndash;&gt;-->
+                                <Insets top="100" left="20" />
-                            <!--                 <Insets top="100" left="20" />-->
+                            </GridPane.margin>
-                            <!--         </GridPane.margin>-->
+                            <TextField fx:id="usernameField"
-                            <!--         <TextField fx:id="usernameField"-->
+                                       promptText="Username"
-                            <!--                    promptText="Username"-->
+                                       styleClass="gray-input-field"
-                            <!--                    styleClass="gray-input-field"-->
+                                       maxWidth="180" />
-                            <!--                    maxWidth="180" />-->
+                            <Button text="Apply Name"
-                            <!--         <Button text="Login"-->
+                                    fx:id="loginButton"
-                            <!--                         fx:id="loginButton"-->
+                                    onAction="#handleLoginButton"
-                            <!--                         onAction="#handleLoginButton"-->
+                                    styleClass="button-create-lobby" />
-                            <!--                         styleClass="button-create-lobby" />-->
+                        </HBox>
-                            <!-- </HBox>-->
 
                     <VBox fx:id="casinoTable"
                           alignment="CENTER"