Wicket下拉选择

Question

我使用了带有预选值的下拉选项和另外两个选项，但是当我选择其他选项并打印相同的预选值时会被打印 这是我的代码

public class MainPanel extends Panel {

    public String Vehicle= "Bike";

    public MainPanel(String id)
    {
        super(id);
        final Form<?> form= new Form<Void>("userForm");
              add(form);
        final List<String> VEHICLES = Arrays.asList(new String[] {"Bike", "car", "Both" });

        final DropDownChoice<String> listSites = new DropDownChoice<String>(
            "dropdwn", new PropertyModel<String>(this,"Vehicle"),VEHICLES);

        listSites.setRequired(true);

        form.add(listSites);
    }

    public String getVehicle()
    {
            return Vehicle;
    }
    public void setVehicle(String Vehicle)
    {
        this.Vehicle= Vehicle;
    }
}

任何人都可以帮我解决这个问题。