returns a validation error

Question

**Hi everybody, I've look around in this and in other forums but i can't resolve that problem: i want to select some elements of a list e put the selected elements in another list. This should be a simple thing but i got an error and i don't know why and how to resolve it. this is my controller:

@ManagedBean
@ViewScoped
public class Controller{

    private ArrayList<String> keyColumns;
    private List<String> keyColumnsAvailables;
    //getters and setters
}

and this is my xhtml page:

 <div>
    <f:view>
        <h:selectManyListbox value="#{controller.keyColumns}">
          <f:selectItems value="#{controller.keyColumnsAvailables}"/>
        </h:selectManyListbox>
   </f:view>
 </div>

now, the list of availables columns is printed normally but when i select some items and press de button, the page says : Validation Error.

I've tried to use Arrays instead Lists (same problem),to select items from a list of static elements (using ) and it works, but the list is set in another table of the page, so it cannot be static.

keyColumnsAvailable is an ArrayList filled up from a facade method who takes some information from a MongoCollection. If I add some elements in a method of controller like :

public List<String> lista(){

    this.keyColumns1 = new ArrayList<>();
    this.keyColumns1.addAll(this.eFacade.getKeyColumnsFile(String name));


    return keyColumns1;
}

and i use :

 <div>
    <f:view>
        <h:selectManyListbox value="#{controller.keyColumns}">
          <f:selectItems value="#{controller.lista()}"/>
        </h:selectManyListbox>
   </f:view>
 </div>

it works. But i have to select the Name from an uploaded file, who gives the List of available columns, and in this case i got a validation error,even thought i've verified that the list isn't null or empty.

To reassume, i select a file e with the name of this file i retrieve some information from mongoDB (return type is a list) in the Facade.This List is filled up correctly from the Facade method, but when i go to select some informations from the list --> validation error.

Others answers doesn't help me, do you have ideas?

Thanks