Final Code

src/chat/Chat.java

+package chat;
+
+import java.rmi.Remote;
+import java.rmi.RemoteException;
+import java.util.List;
+
+public interface Chat extends Remote {
+	public void sendMessage(ClientInterface client, String message) throws RemoteException;
+	public void saveMessageTofile( MessageList message) throws RemoteException;
+	public void exitChat(ClientInterface client) throws RemoteException;
+	void registerUsers(ClientInterface client,String username) throws RemoteException;
+	public void broadcasteMessages(Message message) throws RemoteException;
+	public MessageList loadHistory() throws RemoteException;
+}
\ No newline at end of file

src/chat/ClientInterface.java

+package chat;
+
+import java.rmi.Remote;
+import java.rmi.RemoteException;
+
+public interface ClientInterface extends Remote {
+    public void showMessages(String message) throws RemoteException;  // Correct name
+}

src/chat/Message.java

+package chat;
+
+import java.io.Serializable;
+
+public class Message implements Serializable{
+    private static final long serialVersionUID = 1234567L;
+    String username;
+    String message;
+
+    public Message(String username, String message){
+        this.username=username;
+        this.message=message;
+    }
+
+    public String getUsername(String username){
+        return username;
+    }
+
+    public String getMessage(String message){
+        return message;
+    }
+
+    public String toString(){
+        return username + ": " + message;
+    }
+    
+}

src/chat/MessageList.java

+package chat;
+
+import java.io.Serializable;
+import java.util.*;
+
+public class MessageList implements Serializable{
+    private static final long serialVersionUID = 1234569L;
+   private List<Message> msgList=new ArrayList<>();
+
+public List<Message> getMsgList() {
+    return msgList;
+}
+
+public void setMsgList(List<Message> msgList) {
+    this.msgList = msgList;
+}
+
+    
+}

src/client/Client.java

+package client;
+
+import java.rmi.registry.LocateRegistry;
+import java.rmi.registry.Registry;
+import java.util.Scanner;
+
+import chat.Chat;
+
+public class Client {
+	public static void main(String [] args) {
+		
+		try {
+		  /*if (args.length < 2) {
+		   System.out.println("Usage: java HelloClient <rmiregistry host> <rmiregistry port>");
+		   return;}*/
+
+		//String host = args[0];
+		//int port = Integer.parseInt(args[1]);
+		String host = "localhost";
+		int port = 2099;
+		Registry registry = LocateRegistry.getRegistry(host, port); 
+		Chat chat = (Chat) registry.lookup("ChatService");
+		
+		Scanner scanner = new Scanner(System.in);
+		System.out.print("Enter your username: ");
+		String username = scanner.nextLine();
+		
+		ClientSide client = new ClientSide();
+
+		// Remote method invocation
+		chat.registerUsers(client, username);
+		
+        System.out.println("Welcome to the chat, " + username + "! Type your messages below:");
+
+        while (true) {
+            String message = scanner.nextLine();
+
+            // Exit command
+            if (message.equalsIgnoreCase("/exit")) {
+                chat.exitChat(client);  // Notify the server about exiting
+                System.out.println("You have left the chat.");
+                break;
+            }
+
+            // Send message to the server
+            chat.sendMessage(client, message);
+        }
+
+		} catch (Exception e)  {
+//			System.err.println("Error on client: " + e);
+			e.printStackTrace();
+		}
+	  }
+}

src/client/ClientSide.java

+package client;
+
+import java.rmi.RemoteException;
+import java.rmi.server.UnicastRemoteObject;
+
+import chat.ClientInterface;
+import chat.Message;
+
+public class ClientSide extends UnicastRemoteObject implements ClientInterface {
+
+    protected ClientSide() throws RemoteException {
+        super();
+    }
+
+    @Override
+    public void showMessages(String message) {
+        System.out.println(message);
+    }
+}
+

src/server/ChatApp.java

+package server;
+
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.rmi.RemoteException;
+import java.util.*;
+import java.util.Map.Entry;
+
+import chat.Chat;
+import chat.ClientInterface;
+import chat.Message;
+import chat.MessageList;
+
+public class ChatApp implements Chat{
+
+	Map<ClientInterface, String> activeUsers = new HashMap<>();
+	private List <Message> messages=new ArrayList<>();
+	@Override
+	public void registerUsers(ClientInterface client, String username) throws RemoteException {
+		MessageList history=loadHistory();    
+		if (activeUsers.containsValue(username)) {
+		        throw new RemoteException("Username already taken! Please choose another one.");
+		    } else {
+		        activeUsers.put(client,username);
+				if(history!=null){
+				for(Message msg: history.getMsgList()){
+					try{
+						client.showMessages(msg.toString());
+					}catch(Exception e){
+						e.printStackTrace();
+					}
+				}
+			}
+		        System.out.println(username + " has joined the chat.");
+		    }
+	}
+		
+
+	@Override
+	public void sendMessage(ClientInterface client, String message) throws RemoteException {
+		String username = activeUsers.get(client);
+		
+		if(username==null) {
+			System.out.println("Please register first");
+		}
+		
+		Message msg= new Message(username, message);
+		messages.add(msg);
+		saveMessage(msg);
+		broadcasteMessages(msg);
+	}
+
+	private void saveMessage(Message msg) throws RemoteException{
+		MessageList messageList= loadHistory();
+
+		if(messageList==null){
+			System.out.println("No previous history found \n");
+			MessageList messageListNew= new MessageList();
+			messageListNew.getMsgList().add(msg);
+			saveMessageTofile(messageListNew);
+
+		}else{
+			messageList.getMsgList().add(msg);
+			saveMessageTofile(messageList);
+
+		}
+	}
+
+	@Override
+	public void exitChat(ClientInterface client) throws RemoteException {
+		activeUsers.remove(client);
+	}
+
+
+	@Override
+	public void broadcasteMessages(Message message) throws RemoteException {
+		for (Entry<ClientInterface, String> entry : activeUsers.entrySet()) {
+			try {
+			ClientInterface client = entry.getKey();
+			client.showMessages(message.toString());
+			}catch (RemoteException e) {
+	            System.out.println("Error sending message to " + entry.getKey() + ": " + e.getMessage());
+	        }
+			
+		}
+	}
+
+	@Override
+	public void saveMessageTofile( MessageList message) throws RemoteException {
+		 try (FileOutputStream file= new FileOutputStream("chat_history.ser");
+			ObjectOutputStream out= new ObjectOutputStream(file)){
+				out.writeObject(message);
+		 // Append message to the file
+		    } catch (IOException e) {
+		        System.out.println("Error saving message: " + e.getMessage());
+		    }
+		
+	}
+
+	@Override
+public MessageList loadHistory() throws RemoteException {
+	MessageList msg=null;
+    try (FileInputStream file = new FileInputStream("chat_history.ser");
+         ObjectInputStream input = new ObjectInputStream(file)) {
+
+        
+            try {
+                msg= (MessageList) input.readObject();
+                if(msg!=null){
+					for(Message message : msg.getMsgList()){
+
+					}
+				}
+            } catch (IOException e) {
+               e.printStackTrace();
+            }
+        
+
+    } catch (Exception e) {
+    }
+
+    return msg;
+}
+
+}

src/server/Server.java

+package server;
+
+import java.rmi.registry.LocateRegistry;
+import java.rmi.registry.Registry;
+import java.rmi.server.UnicastRemoteObject;
+
+import chat.Chat;
+
+public class Server {
+	public static void  main(String [] args) {
+		try {
+            ChatApp ca = new ChatApp();
+            Chat h_stub = (Chat) UnicastRemoteObject.exportObject(ca, 0);
+
+            Registry registry;
+            if (args.length > 0) {
+                int port = Integer.parseInt(args[1]);
+                registry = LocateRegistry.createRegistry(port);  // Create registry if not already running
+                System.out.println("RMI Registry started on port " + port);
+            } else {
+                registry = LocateRegistry.createRegistry(2099);  // Default port
+                System.out.println("RMI Registry started on port 2099");
+            }
+
+            registry.rebind("ChatService", h_stub);
+            System.out.println("Server ready");
+
+        } catch (Exception e) {
+			  System.err.println("Error on server :" + e) ;
+			  e.printStackTrace();
+		  }
+	  }
+}