I can not change JTable Header value or data?

Question

I am using java swing to code an simple desktop application but when i config jtable, it's not working:

public MainFrame() {
    initComponents();
    try {
        //add header for jtable
        String column_names[] = {"Code", "Name", "Phone", "Email", "Address", "Department"};
        DefaultTableModel table_model = new DefaultTableModel(column_names, 0);
        tbEmployee = new JTable(table_model);
        //create fake data
        List<Employee> employees = new ArrayList<>();
        employees.add(new Employee("code 1", "name 1", "phone 1", "email 1", "address 1", "department 1"));
        employees.add(new Employee("code 2", "name 2", "phone 2", "email 2", "address 2", "department 2"));
        //add list data to table
        for (int i = 0; i < employees.size(); i++) {
            String code = employees.get(i).getCode();
            String name = employees.get(i).getName();
            String phone = employees.get(i).getPhone();
            String email = employees.get(i).getEmail();
            String address = employees.get(i).getAddress();
            String department = employees.get(i).getDepartment();

            Object[] data = {code, name, phone, email, address, department};

            table_model.addRow(data);

        }
        tbEmployee = new JTable(table_model);
        //setting header color`
        JTableHeader header = tbEmployee.getTableHeader();
        header.setBackground(Color.yellow);
    } catch (Exception e) {
        e.printStackTrace();
    }

}

This is my design java This is when i run file