src/main/java/ch/unibas/dmi/dbis/cs108/casono/Main.java

@@ -7,6 +7,12 @@ import org.apache.logging.log4j.Logger;
 
 /** Main entry point for Casono application. Handles client and server startup. */
 public final class Main {
+
+    private static final int MIN_ARGS_FOR_USERNAME = 3;
+    private static final int ARGS_COUNT_SERVER = 2;
+    private static final int ARGS_COUNT_CLIENT_MIN = 2;
+    private static final int ARGS_COUNT_CLIENT_WITH_USER = 3;
+
     /**
      * Main entry point for Casono.
      *
@@ -17,10 +23,13 @@ public final class Main {
             printUsage();
             System.exit(1);
         }
-
         switch (args[0]) {
             case "server" -> ServerApp.start(args[1]);
-            case "client" -> ClientApp.start(args[1]);
+            case "client" -> {
+                String address = args[1];
+                String username = args.length >= MIN_ARGS_FOR_USERNAME ? args[2] : null;
+                ClientApp.start(address, username);
+            }
             default -> {
                 printUsage();
                 System.exit(1);
@@ -29,12 +38,15 @@ public final class Main {
     }
 
     private static boolean isValid(String[] args) {
-        if (args.length != 2) {
+        if (args.length < ARGS_COUNT_SERVER) {
             return false;
         }
 
         return switch (args[0]) {
-            case "server", "client" -> true;
+            case "server" -> args.length == ARGS_COUNT_SERVER;
+            case "client" ->
+                    args.length == ARGS_COUNT_CLIENT_MIN
+                            || args.length == ARGS_COUNT_CLIENT_WITH_USER;
             default -> false;
         };
     }
@@ -45,7 +57,7 @@ public final class Main {
                 """
                 Usage:
                   java -jar xyz.jar server <listenPort>
-                  java -jar xyz.jar client <serverIp>:<serverPort>
+                  java -jar xyz.jar client <serverIp>:<serverPort> [<username>]
                 """);
     }
 }

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

@@ -1,33 +1,51 @@
 package ch.unibas.dmi.dbis.cs108.casono.client;
 
-/**
+import ch.unibas.dmi.dbis.cs108.casono.client.network.ClientService;
- * Entry point for the Casono client application. Handles client startup and connection parameters.
+import ch.unibas.dmi.dbis.cs108.casono.client.network.LobbyClient;
- */
+import ch.unibas.dmi.dbis.cs108.casono.client.network.LoginResult;
 import ch.unibas.dmi.dbis.cs108.casono.client.ui.Launcher;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
 /**
- * Entry point for the Casono client application. Handles client startup and connection parameters.
+ * Entry point and bootstrap helper for the Casono client application.
  *
- * <p>Default constructor for the application.
+ * <p>This class is responsible for two tasks: - performing an optional startup LOGIN when a
+ * username is supplied on the command line, and - providing a shared {@link ClientService} instance
+ * that the UI can reuse so the initial connection remains active.
  */
 public class ClientApp {
 
     private static final Logger LOGGER = LogManager.getLogger(ClientApp.class);
 
+    /** Shared client connection used when a username is provided at startup. */
+    private static volatile ClientService sharedClientService;
+
     /** Default constructor. */
     public ClientApp() {
         // Default constructor
     }
 
+    /**
+     * Returns the shared {@link ClientService} instance created during startup, or {@code null} if
+     * none exists. The UI may call this to reuse the connection that already performed the initial
+     * LOGIN.
+     */
+    public static ClientService getSharedClientService() {
+        return sharedClientService;
+    }
+
+    private static void setSharedClientService(ClientService cs) {
+        sharedClientService = cs;
+    }
+
     /**
      * Starts the client application with the given address.
      *
      * @param arg Address in the format "ip:port".
      * @throws IllegalArgumentException if the address format is invalid.
      */
-    public static void start(String arg) {
+    public static void start(String arg, String username) {
         String[] parts = arg.split(":", 2);
         if (parts.length != 2) {
             throw new IllegalArgumentException("Address must be in format <ip>:<port>.");
@@ -36,16 +54,69 @@ public class ClientApp {
         int port = Integer.parseInt(parts[1]);
 
         LOGGER.info("You've selected the client. It will connect port {} at host {}", port, host);
-        // Expose the chosen host/port to the UI via system properties so controllers
-        // (which read System.getProperty("casono.server.host"/"casono.server.port"))
-        // can obtain the correct connection information.
         System.setProperty("casono.server.host", host);
-        System.setProperty("casono.server.port", Integer.toString(port));
+        System.setProperty("casono.server.port", String.valueOf(port));
-        // Forward the original address argument to the launcher as well.
+        // If a username was provided, create a shared connection and perform
-        Launcher.main(new String[] {arg});
+        // the initial LOGIN asynchronously on that connection. The connection
+        // is intentionally NOT closed so the UI can reuse it.
+        if (username != null && !username.isBlank()) {
+            try {
+                ClientService clientService = new ClientService(host, port);
+                setSharedClientService(clientService);
+
+                java.util.concurrent.ExecutorService bg =
+                        java.util.concurrent.Executors.newSingleThreadExecutor(
+                                r -> {
+                                    Thread t = new Thread(r);
+                                    t.setDaemon(true);
+                                    t.setName("casono-startup-login");
+                                    return t;
+                                });
+                bg.submit(
+                        () -> {
+                            try {
+                                LobbyClient lobbyClient = new LobbyClient(clientService);
+                                LoginResult res = lobbyClient.login(username);
+                                LOGGER.info(
+                                        "Assigned username='{}' id={}",
+                                        res.getUsername(),
+                                        res.getId());
+                            } catch (RuntimeException e) {
+                                LOGGER.warn("Startup login failed: {}", e.getMessage());
+                            } catch (Exception e) {
+                                LOGGER.warn("Unexpected error during startup login", e);
+                            } finally {
+                                bg.shutdown();
+                            }
+                        });
+            } catch (RuntimeException e) {
+                LOGGER.warn(
+                        "Could not establish initial connection for startup login: {}",
+                        e.getMessage());
+                // UI will create its own connection when it initializes
+            }
+        }
+
+        if (username != null && !username.isBlank()) {
+            Launcher.main(new String[] {arg, username});
+        } else {
+            Launcher.main(new String[] {arg});
+        }
     }
 
+    /**
+     * Main entry point. {@code <host:port>} (e.g. {@code 127.0.0.1:1234})
+     *
+     * <p>If a username is provided the application attempts a startup LOGIN on the shared
+     * connection; the UI will still start and will reuse the connection when possible. The username
+     * is not propagated via a system property for UI display.
+     */
     public static void main(String[] args) {
-        start(args[0]);
+        if (args == null || args.length == 0) {
+            throw new IllegalArgumentException("Address argument required: <host:port>");
+        }
+        // Optional username argument
+        String username = args.length > 1 && args[1] != null && !args[1].isBlank() ? args[1] : null;
+        start(args[0], username);
     }
 }

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

@@ -138,33 +138,64 @@ public class ClientService {
                 () -> {
                     try {
                         writeToTransport(message);
-                        String responseText = null;
+                        String responseText = clienttcptransport.read().payload();
+                        logger.info("Raw message '{}'", responseText);
 
-                        responseText = clienttcptransport.read().payload();
+                        boolean hasStatus = false;
-                        logger.info("Raw message '" + responseText + "'");
+                        boolean success = false;
-                        Boolean success = null;
+                        for (String rawLine : responseText.split("\n")) {
-                        int count = 0;
+                            String line = rawLine;
-                        for (String line : responseText.split("\n")) {
+                            if (!hasStatus) {
-                            if (success == null) {
                                 if ("+OK".equals(line)) {
                                     success = true;
+                                    hasStatus = true;
                                     continue;
-
-                                } else if (("-ERROR").equals(responseText)) {
-                                    success = false;
                                 }
+                                if (line.startsWith("-ERR") || line.startsWith("-ERROR")) {
+                                    success = false;
+                                    hasStatus = true;
+                                    continue;
+                                }
+                                // ignore any lines before the status indicator
                                 continue;
-                            } else if ("END".equals(line)) {
+                            }
+
+                            if ("END".equals(line)) {
                                 break;
                             }
-                            line = line.replaceFirst("^\t", "");
+
+                            // strip a single leading tab if present (protocol formatting)
+                            if (line.startsWith("\t")) {
+                                line = line.substring(1);
+                            }
                             response.add(line);
                         }
-                        if (success != null && success) {
+
-                            return;
+                        if (!hasStatus) {
-                        } else {
+                            // Fallback for servers that do not place the status on a
-                            throw new RuntimeException("Error in " + message + ": " + response);
+                            // dedicated line: try to detect status markers anywhere
+                            // in the payload to remain compatible with older servers.
+                            if (responseText.contains("+OK")) {
+                                success = true;
+                                hasStatus = true;
+                            } else if (responseText.contains("-ERR")
+                                    || responseText.contains("-ERROR")) {
+                                success = false;
+                                hasStatus = true;
+                            } else {
+                                throw new RuntimeException(
+                                        "No status line in response for '"
+                                                + message
+                                                + "': "
+                                                + responseText);
+                            }
                         }
+
+                        if (success) {
+                            return;
+                        }
+
+                        throw new RuntimeException("Error in " + message + ": " + response);
                     } catch (Exception e) {
                         throw getRuntimeException(e);
                     }
@@ -200,15 +231,14 @@ public class ClientService {
      * @return A RuntimeException representing the cause of the original exception.
      */
     private static RuntimeException getRuntimeException(Exception e) {
-        Throwable reason = e.getCause();
+        Throwable cause = e.getCause();
-        RuntimeException re;
+        if (cause == null) {
-        if (reason == null) {
+            return e instanceof RuntimeException ? (RuntimeException) e : new RuntimeException(e);
-            reason = e;
-        } else if (reason instanceof RuntimeException rte) {
-            re = rte;
         }
-        re = new RuntimeException(reason);
+        if (cause instanceof RuntimeException) {
-        return re;
+            return (RuntimeException) cause;
+        }
+        return new RuntimeException(cause);
     }
 
     /**

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

@@ -1,5 +1,8 @@
 package ch.unibas.dmi.dbis.cs108.casono.client.network;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.RequestParameter;
+import java.util.List;
+
 /**
  * The LobbyClient class is responsible for communicating with the server to manage game lobbies. It
  * provides methods to create a lobby, join a lobby, and fetch the current status of a lobby by
@@ -65,8 +68,23 @@ public class LobbyClient {
      * Logs in to the server with the given username by sending a "LOGIN" command.
      *
      * @param user The username to log in with.
+     * @return a {@link LoginResult} containing the assigned username and id as returned by the
+     *     server
      */
-    public void login(String user) {
+    public LoginResult login(String user) {
-        client.processCommand("LOGIN USERNAME=" + user);
+        List<String> lines = client.processCommand("LOGIN USERNAME=" + user);
+
+        List<RequestParameter> params = ClientService.convertToRequestParameters(lines);
+
+        String assigned = user;
+        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);
     }
 }

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

@@ -0,0 +1,38 @@
+package ch.unibas.dmi.dbis.cs108.casono.client.network;
+
+/**
+ * Result of a successful login request.
+ *
+ * <p>Holds the username assigned by the server and the server-side id as a string (UUID). The
+ * client previously attempted to parse the id as an integer which failed when the server returned a
+ * UUID; this class therefore stores the id as a {@link String}.
+ */
+public class LoginResult {
+    private final String username;
+    private final String id;
+
+    /**
+     * Create a login result.
+     *
+     * @param username the assigned username
+     * @param id the assigned id (UUID string) returned by the server
+     */
+    public LoginResult(String username, String id) {
+        this.username = username;
+        this.id = id;
+    }
+
+    /**
+     * @return the assigned username
+     */
+    public String getUsername() {
+        return username;
+    }
+
+    /**
+     * @return the assigned id as returned by the server (UUID string)
+     */
+    public String getId() {
+        return id;
+    }
+}

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

@@ -1,5 +1,6 @@
 package ch.unibas.dmi.dbis.cs108.casono.client.ui.lobbyui;
 
+import ch.unibas.dmi.dbis.cs108.casono.client.ClientApp;
 import ch.unibas.dmi.dbis.cs108.casono.client.network.ClientService;
 import ch.unibas.dmi.dbis.cs108.casono.client.network.LobbyClient;
 import javafx.application.Platform;
@@ -36,12 +37,16 @@ public class CasinomainuiController {
     private int nextButtonId = 1;
     private LobbyClient lobbyClient;
 
-    /** Default constructor for dependency injection by FXMLLoader. */
+    /** Default constructor used by FXMLLoader. */
     public CasinomainuiController() {
         // Default constructor
     }
 
-    /** Initializes the UI components and sets default values. */
+    /**
+     * Initializes the UI components and sets default values. If a shared {@link
+     * ch.unibas.dmi.dbis.cs108.casono.client.network.ClientService} exists (created at application
+     * start), the controller reuses it so the connection remains open and already-logged-in.
+     */
     @FXML
     public void initialize() {
         titleLabel.setText("Casono");
@@ -51,16 +56,18 @@ public class CasinomainuiController {
         translationManager = LobbyButtonTranslationManager.getInstance();
         String host = System.getProperty("casono.server.host");
         int port = Integer.parseInt(System.getProperty("casono.server.port"));
-        ClientService clientService;
+        ClientService clientService = ClientApp.getSharedClientService();
-        try {
+        if (clientService == null) {
-            clientService = new ClientService(host, port);
+            try {
-        } catch (RuntimeException e) {
+                clientService = new ClientService(host, port);
-            LOGGER.warn(
+            } catch (RuntimeException e) {
-                    "Could not connect to server {}:{} — starting in offline mode: {}",
+                LOGGER.warn(
-                    host,
+                        "Could not connect to server {}:{} — starting in offline mode: {}",
-                    port,
+                        host,
-                    e.getMessage());
+                        port,
-            clientService = new ClientService(true); // offline mode
+                        e.getMessage());
+                clientService = new ClientService(true); // offline mode
+            }
         }
         gridManager =
                 new LobbyButtonGridManager(
@@ -117,7 +124,11 @@ public class CasinomainuiController {
         Platform.exit();
     }
 
-    /** Handles creation of a new lobby button. */
+    /**
+     * Handles creation of a new lobby button. Attempts to create a lobby on the server via the
+     * {@link LobbyButtonGridManager} and registers the new button in the local translation manager.
+     * Errors are logged and displayed as an informational alert.
+     */
     @FXML
     public void handleCreateLobbyButton() {
         if (translationManager.isFull()) {

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

@@ -13,7 +13,7 @@ import java.util.concurrent.ConcurrentLinkedQueue;
 /** Represents an authenticated user on the server. */
 public class User {
     private final UserId id;
-    private final String name;
+    private String name;
     private SessionId sessionId;
     private Instant disconnectedAt;
     private final Queue<Message> messages;
@@ -47,10 +47,20 @@ public class User {
      *
      * @return the user name
      */
-    public String getName() {
+    public synchronized String getName() {
         return name;
     }
 
+    /**
+     * Sets a new display name for this user. Thread-safe; callers must ensure the registry is
+     * updated to maintain uniqueness when needed.
+     *
+     * @param newName the new display name
+     */
+    public synchronized void setName(String newName) {
+        this.name = newName;
+    }
+
     /**
      * Returns the session currently associated with this user, if any.
      *

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

@@ -1,10 +1,15 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
 
 import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 
 /** Creates new users, resolving name conflicts automatically. */
 public class UserFactory {
     private final UserRegistry registry;
+    private final AtomicInteger anonymousCounter = new AtomicInteger(1);
+    private static final Logger LOGGER = LogManager.getLogger(UserFactory.class);
 
     /**
      * Creates a new UserFactory backed by the given registry.
@@ -18,16 +23,40 @@ public class UserFactory {
     /**
      * Creates and registers a new user with the given name and session. If the name is already
      * taken, a numeric suffix is appended and incremented until a free name is found (e.g.
-     * Lars_001, Lars_002, ...).
+     * Lars_001, Lars_002, ...). If the desiredName is null or empty, an automatic name of the form
+     * "playerN" is assigned, incrementing N until a free name is found.
      *
      * @param desiredName the preferred display name
-     * @param sessionId the session to associate with the new user
+     * @param sessionId the session to associate with the new userwas
      * @return the newly created and registered user
      */
     public User create(String desiredName, SessionId sessionId) {
+        if (desiredName == null || desiredName.isBlank()) {
+            // assign playerN names for anonymous logins
+            while (true) {
+                String candidate = "player" + anonymousCounter.getAndIncrement();
+                var result = registry.registerIfAvailable(candidate, sessionId);
+                if (result.isPresent()) {
+                    var user = result.get();
+                    LOGGER.info(
+                            "Registered user '{}' with id {} for session {}",
+                            user.getName(),
+                            user.getId().value(),
+                            sessionId == null ? "null" : sessionId.value());
+                    return user;
+                }
+            }
+        }
+
         var result = registry.registerIfAvailable(desiredName, sessionId);
         if (result.isPresent()) {
-            return result.get();
+            var user = result.get();
+            LOGGER.info(
+                    "Registered user '{}' with id {} for session {}",
+                    user.getName(),
+                    user.getId().value(),
+                    sessionId == null ? "null" : sessionId.value());
+            return user;
         }
 
         int suffix = 1;
@@ -35,7 +64,13 @@ public class UserFactory {
             String candidate = desiredName + "_" + String.format("%03d", suffix);
             result = registry.registerIfAvailable(candidate, sessionId);
             if (result.isPresent()) {
-                return result.get();
+                var user = result.get();
+                LOGGER.info(
+                        "Registered user '{}' with id {} for session {}",
+                        user.getName(),
+                        user.getId().value(),
+                        sessionId == null ? "null" : sessionId.value());
+                return user;
             }
             suffix++;
         }

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

@@ -185,4 +185,29 @@ public class UserRegistry {
     public Collection<User> getAllUsers() {
         return byId.values();
     }
+
+    /**
+     * Attempts to change the username of an existing user. Ensures the new name is not already
+     * taken and updates internal indices atomically.
+     *
+     * @param userId the id of the user to rename
+     * @param newName the desired new name
+     * @return true if the rename succeeded, false if the name was already taken or user not found
+     */
+    public synchronized boolean changeUsername(UserId userId, String newName) {
+        User user = byId.get(userId);
+        if (user == null) {
+            return false;
+        }
+
+        if (byName.containsKey(newName)) {
+            return false;
+        }
+
+        // remove old mapping and put new mapping
+        byName.remove(user.getName());
+        user.setName(newName);
+        byName.put(newName, user);
+        return true;
+    }
 }

src/test/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserFactoryTest.java

@@ -0,0 +1,64 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.domain.user;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionId;
+import org.junit.jupiter.api.Test;
+
+/** Tests for {@link UserFactory} and {@link UserRegistry} name assignment behavior. */
+public class UserFactoryTest {
+
+    // Logger removed as per patch requirement
+
+    @Test
+    public void createUsersWithDuplicateDesiredNamesAssignsUniqueNames() {
+        UserRegistry registry = new UserRegistry();
+        UserFactory factory = new UserFactory(registry);
+
+        SessionId s1 = new SessionId();
+        SessionId s2 = new SessionId();
+        SessionId s3 = new SessionId();
+
+        // Create first user with desired name "Alice"
+        User u1 = factory.create("Alice", s1);
+        // Create second user requesting the same name
+        User u2 = factory.create("Alice", s2);
+        // Create third user requesting the same name again
+        User u3 = factory.create("Alice", s3);
+
+        assertNotNull(u1);
+        assertNotNull(u2);
+        assertNotNull(u3);
+
+        // All assigned names must be distinct
+        String n1 = u1.getName();
+        String n2 = u2.getName();
+        String n3 = u3.getName();
+
+        // assigned names: n1, n2, n3
+
+        assertNotEquals(n1, n2);
+        assertNotEquals(n1, n3);
+        assertNotEquals(n2, n3);
+
+        // Registry should contain all assigned names
+        assertTrue(registry.getByUsername(n1).isPresent());
+        assertTrue(registry.getByUsername(n2).isPresent());
+        assertTrue(registry.getByUsername(n3).isPresent());
+    }
+
+    @Test
+    public void createUserWithNullDesiredAssignsPlayerN() {
+        UserRegistry registry = new UserRegistry();
+        UserFactory factory = new UserFactory(registry);
+
+        SessionId s1 = new SessionId();
+
+        User u = factory.create(null, s1);
+        assertNotNull(u);
+        String name = u.getName();
+        // assigned anonymous name: name
+        assertTrue(name.startsWith("player"), "expected automatic name starting with 'player'");
+        assertTrue(registry.getByUsername(name).isPresent());
+    }
+}