Using nested classes in overloaded methods in java

Question

I am trying to use nested classes in overloaded methods, however I get a message saying my IDE (IntelliJ) cannot resolve reference to the overloaded class. I do not have access to any book about java and the Oracle documentation is not really helpful. I would like to avoid creating seperate files for these classes. How can I solve the problem?

Here is my Connection class that is using the overloaded methods:

public class Connection {
    //(...)
    private Order order;
    //(...)
    public void receiveData(){
        downloadData(order.getOrderFromList()); //error message here
    }

    //overloaded methods:
    private void downloadData(Order.OrderWelcomeMessage order){

    }

    private void downloadData(Order.OrderSendFile order){

    }
}

And here the Order class which nests other classes. The instance of 'orderFromList' is created in another method I've ommited here.

public class Order {
    private OrderFromList orderFromList;
    //(...)
    public OrderFromList getOrderFromList(){
        return orderFromList;
    }

    public abstract class OrderFromList{}

    public class OrderWelcomeMessage
            extends OrderFromList{
    //(...)
    }

    public class OrderSendFile
            extends OrderFromList{
    //(...)
    }
}