src/main/java/chat/ChatServer.java

@@ -42,11 +42,11 @@ public class ChatServer {
         // step 1: iterate through connectedClients safely
         // step 2: skip the sender client
         // step 3: send message to remaining clients using sendMessage
-        for(ClientHandler client: connectedClients)
-        {
-            if(!client.getUsername().equals(sender.getUsername()))
-            {
-                client.sendMessage(message);
+        synchronized (connectedClients) {
+            for (ClientHandler client : connectedClients) {
+                if (client != sender) {
+                    client.sendMessage(message);
+                }
             }
         }
     }
@@ -54,9 +54,10 @@ public class ChatServer {
     public void broadcastSystemMessage(String message) {
         // step 1: iterate through all connected clients
         // step 2: send system message to each client
-        for(ClientHandler client: connectedClients)
-        {
-                client.sendMessage(message);
+        synchronized (connectedClients) {
+            for (ClientHandler client : connectedClients) {
+                    client.sendMessage(message);
+            }
         }
     }
 
@@ -65,7 +66,7 @@ public class ChatServer {
         // step 2: retrieve username from clientHandler
         // step 3: broadcast system message about user leaving
         connectedClients.remove(clientHandler);
-        broadcastSystemMessage(clientHandler.getUsername() + " left");
+        broadcastSystemMessage("[System] " + clientHandler.getUsername() + " left the chat");
     }
 
     public static void main(String[] args) {

src/main/java/chat/MessageListener.java

@@ -23,7 +23,7 @@ public class MessageListener implements Runnable {
             }
 
         } catch (IOException e) {
-            throw new RuntimeException(e);
+            System.err.println(e.getMessage());
         }
     }
 }