added functionality checking if a username already exists

src/main/java/SignUpGUI.java

@@ -1,6 +1,7 @@
 import javax.swing.*;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
+import java.util.List;
 import java.util.UUID;
 
 public class SignUpGUI extends JFrame implements ActionListener {
@@ -64,6 +65,11 @@ public class SignUpGUI extends JFrame implements ActionListener {
                 JOptionPane.showMessageDialog(this, "Passwords do not match!", "Sign Up Error", JOptionPane.ERROR_MESSAGE);
                 return;
             }
+            
+            if (!isUsernameAvailable("user.json", username)) {
+                JOptionPane.showMessageDialog(this, "Username already exists!", "Sign Up Error", JOptionPane.ERROR_MESSAGE);
+                return;
+            }
             try {
                 UUID randomUUID = UUID.randomUUID();
                 CreateUser user = CreateUser.createUser(randomUUID.toString(), username, password, birthday);
@@ -75,6 +81,18 @@ 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; // Username already exists
+                }
+            }
+        }
+        return true; // Username is available
+    }
 
     public static void main(String[] args) {
         SwingUtilities.invokeLater(() -> {

user.json

@@ -1,32 +1,14 @@
 [
   {
-    "id": "f9cd70c1-57cd-4943-a00b-0de521347aff",
+    "id": "961ca202-ecbd-4dfc-ac0b-28f367618aa1",
     "userName": "asd",
-    "password": "test1234",
-    "birthday": "01.01.2000"
-  },
-  {
-    "id": "69e4b1d3-3089-4f98-936f-865dfe40e6a0",
-    "userName": "asd",
-    "password": "test1234",
-    "birthday": "01.01.2013"
-  },
-  {
-    "id": "4165bfe6-9851-480f-90e1-8229415fa44e",
-    "userName": "asdasdasd",
-    "password": "1231412512",
-    "birthday": "1111111"
-  },
-  {
-    "id": "f2775f81-564b-4101-8e86-ce879a9e7737",
-    "userName": "asd",
-    "password": "asdasdasd",
-    "birthday": "asd"
+    "password": "123456",
+    "birthday": "1"
   },
   {
-    "id": "1e035bce-37b5-4f1e-894e-8136b8021c4d",
-    "userName": "asd",
+    "id": "d563a466-753b-4a5e-8b6c-e7e4756c7397",
+    "userName": "asd1",
     "password": "123456",
-    "birthday": "1234"
+    "birthday": "1"
   }
 ]