refactoring: isUsernameAvailable in SignUpGUI

src/main/java/SignUpGUI.java

@@ -149,14 +149,7 @@ public class SignUpGUI extends JFrame implements ActionListener {
     // Function to check if the input username doesn't already exist in the JSON file
     private boolean isUsernameAvailable(String filename, String username) {
         List<CreateUser> userList = CreateUser.readUserListFromJsonFile(filename);
-        if (userList != null) {
-            for (CreateUser user : userList) {
-                if (user.getUserName().equals(username)) {
-                    return false; 
-                }
-            }
-        }
-        return true; 
+        return userList == null || userList.stream().noneMatch(user -> user.getUserName().equals(username));
     }
 
     public static void main(String[] args) {