added functionality to the textfields, the input is now written to the user.json file

src/main/java/SignUpGUI.java

@@ -5,6 +5,7 @@ import java.util.UUID;
 
 public class SignUpGUI extends JFrame implements ActionListener {
 	private JTextField usernameField, passwordField, birthdayField;
+    private JButton signUpButton;
     
     public SignUpGUI() {
         setTitle("Sign Up");
@@ -36,10 +37,29 @@ public class SignUpGUI extends JFrame implements ActionListener {
         birthdayField.setBounds(100, 80, 160, 25);
         add(birthdayField);
         
+        signUpButton = new JButton("Sign Up");
+        signUpButton.setBounds(100, 120, 100, 25);
+        signUpButton.addActionListener(this);
+        add(signUpButton);
+
     }
 
     @Override
     public void actionPerformed(ActionEvent e) {
+    	if (e.getSource() == signUpButton) {
+            String username = usernameField.getText();
+            String password = passwordField.getText();
+            String birthday = birthdayField.getText();
+
+            try {
+                UUID randomUUID = UUID.randomUUID();
+                CreateUser user = CreateUser.createUser(randomUUID.toString(), username, password, birthday);
+                user.saveToJsonFile("user.json");
+                JOptionPane.showMessageDialog(this, "User signed up successfully!");
+            } catch (IllegalArgumentException ex) {
+                JOptionPane.showMessageDialog(this, "Error: " + ex.getMessage(), "Sign Up Error", JOptionPane.ERROR_MESSAGE);
+            }
+        }
     }
 
     public static void main(String[] args) {

user.json

@@ -10,5 +10,11 @@
     "userName": "asd",
     "password": "test1234",
     "birthday": "01.01.2013"
+  },
+  {
+    "id": "4165bfe6-9851-480f-90e1-8229415fa44e",
+    "userName": "asdasdasd",
+    "password": "1231412512",
+    "birthday": "1111111"
   }
 ]