Good design that uses inner class in class?

Question

I have a class called SalesOrder(SO) that allows users to buy multiple items in the same order. SO has an order number.

class SalesOrder {
   public String orderNumber;
}

Each SO has many elements, so I created a new class OrderItemthat has the name and price of the element.

class OrderItem {
    public String name;

    public double price;
}

Each SO has an order header, including username and address. It also has a field called the total price, which contains the sum of all the prices of the goods

class OrderHeader {
    public String username;

    public String address;

    public double totalPrice;
}

After that, I added two fields to SO:

class SalesOrder {
    ...

    public List<OrderItem> items;

    public OrderHeader header;
}

Since OrderItem and OrderHeader are always used with SalesOrder, and the header should return the prices of all products, I turned them into SalesOrder inner classes.

class SalesOrder {


    ...

    public  SalesOrder() {
        this.items = new ArrayList<>();
        this.header = new OrderHeader();
    }   

    public class OrderItem {
       ...
    }

    public class OrderHeader {
        ...

        public double getTotalPrice() {
             double total = 0.0;
             // loop SalesOrder.items
             total += items[i].price;          
             return total;
        }
    }
}

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construe private, SalesOrder.

SalesOrder factory

class SalesOrder {
    ...

    public SalesOrder parseOrder(Xml xml) {
        //init header and items from xml
        this.header = new OrderHeader(valueFromXml, valueFromXml);
    }

    public class OrderHeader {
        ....
        private OrderHeader(username, address) { ... }
    }

    public Class OrderItem {
       ...

        private OrderItem(name, price) { ... }
    }
}

Xml xml = orderXmlData;
SalesOrder order = SalesOrder.parseOrder(orderXmlData);
OrderItem item = order.item;
OrderHeader header = order.header;

Answer 1

, , . -, , totalPrice . , , . -, , , . interface, . -, , Header - , , .

, :

class Order {

    interface Item {
        String getName();
        double getPrice();
    }

    interface Header {
        String getName(); 
        Address getAddress();
    }

    public Order(Header header) {
        ...
    }

    public double getTotalPrice() {
        return streamItems().mapToDouble(Item::getPrice).sum();
    }

    public void addItem(String name, double price) {
        ...
    }

    public Stream<Item> streamItems() {
        ...
    }
}

Answer 2

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