Added logic for checking if username / password exist

src/main/java/LoginGUI.java

@@ -43,11 +43,28 @@ public class LoginGUI extends JFrame implements ActionListener {
             String username = usernameField.getText();
             String password = new String(passwordField.getPassword());
 
-            	//Add login functionality
-                JOptionPane.showMessageDialog(this, "Login");
+            if (authenticateUser(username, password)) {
+            JOptionPane.showMessageDialog(this, "Login successful!");
+            // Perform actions after successful login
+            } else {
+            JOptionPane.showMessageDialog(this, "Invalid username or password", "Login Error", JOptionPane.ERROR_MESSAGE);
+            }
+                
             
+        }
+    }
     
+ // Function to authenticate the user by comparing the entered username and password with the saved user data
+    private boolean authenticateUser(String username, String password) {
+        List<CreateUser> userList = CreateUser.readUserListFromJsonFile("user.json");
+        if (userList != null) {
+            for (CreateUser user : userList) {
+            if (user.getUserName().equals(username) && user.getPassword().equals(password)) {
+            return true; //Success
+           }
+            }
         }
+        return false; // Fail
     }