public class User {
   private int id;
   private String firstname;
   private String lastname;
   private List<Order> orders;
   // ... some other properties and getters and setters
}

public class Order {
   private int id;
   private User owner;
}

public UserDTO userToDto(User user) {
   UserDTO dto = new UserDTO();

   dto.setIs(user.getId());
   dto.setFirstname(user.getFirsname());
   dto.setLastname(user.getLastname());

   List<OrderDTO> ordersDto = new List<OrderDTO>();

   for (Order order : user.getOrders()) {
      OrderDTO orderDto = new OrderDTO();
      orderDto.setId(order.getId());
      orderDto.setUser(...) // what I should I set here?

      ordersDto.add(orderDto);
   }

   dto.setOrders(ordersDto);

   return dto;
}