How to put data into a list inside a object using Struts2 ModelDriven

I am doing my practice project and meet a problem. How can I encapsulate data i entered in jsp into a set which is inside a object using ModelDriven. Here's the sample code.

I have debugged my program and it showed null value. I knew the problem is inside the jsp but i don't how to solve it.

Order:

public class Order {
    private int id;
    private Date createDate;
    private Date requestDate;
    private double totalCost;
    private String status;
    private Vehicle vehicle;        //contains car info

    private Set<Part> parts = new HashSet<>();         
    //setters and getters
}

Part:

public class Part {
    private int id;
    private int quality;
    private double laborCost;
    private int status;

    //setters and getters
}

My action code:

public class OrderAction extends ActionSupport implements ModelDriven<Order>{

    private Order order = new Order();
    public Order getOrder() {return order;}
    public void setOrder(Order order) {this.order = order;}

    @Override
    public Order getModel() { return order;}

    public String addOrder(){
    try{
        orderSerive.addOrder(order);
    }catch (Exception e){
        return LOGIN;
    }
    return "home";
}

My jsp page sample:

<div class="form-group" id="part1">
    <s:label value="1" cssClass="col-md-1 col-sm-1 col-xs-4 col-md-offset 1"/>
    <div class="col-md-6 col-sm-6 col-xs-12">
        <s:textfield cssClass="form-control" id="partModel"/>
    </div>
    <div class="col-md-2 col-sm-2 col-xs-4 col-md-offset-1">
        <s:textfield name="parts[1].quality" cssClass="form-control"/>
    </div>
</div>

Your code is right for a List, that you can index with [n] because List are indexed:

<s:textfield name="parts[1].quality" cssClass="form-control"/>

it can't work for a Set, instead, because Sets don't have indexes.

You can index a Set by one of its properties (in this case id) like follows:

1. Specify the property in the file OrderAction-conversion.properties (in the same folder of the action) with the format KeyProperty_parts=id.
   Note: some tweak could be needed due to the fact that parts is not a Set of the action, but of an object of the action.

2. Use the (n) idiom:

    <s:textfield name="parts(1).quality" cssClass="form-control"/>
   

3. Override hashCode() and equals().

Read all the twisted story on the official documentation.