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