java.lang.ClassCastException: java.util.Vector cannot be cast to java.util.ArrayList

I'm learning JavaEE, and looking for some help. I'm trying to get all record from a Derby DB table to display on a xhtml page (dataTable). I received this error "java.lang.ClassCastException: java.util.Vector cannot be cast to java.util.ArrayList".

1. Can someone explain the error and how I need to fix it?
2. How do I trigger the data table to be shown only when the "Print" button is clicked? Thanks in advance

My named query:

NamedQueries({ @NamedQuery(name="getAllItemActivities", query="select c from LibraryItemActivity c") })

My bean manager:

private ArrayList activities;

public ArrayList getActivities() {

  return control.getActivites();

}

My control bean:

public ArrayList getActivites() {

  ArrayList<LibraryItemActivity> temp = (ArrayList<LibraryItemActivity>) em.createNamedQuery("getAllItemActivities").getResultList();

  return temp;

}

My html data table:

<h:form>
    <h:commandButton value = "Print"></h:commandButton>
    <h:dataTable value = "#{libraryBeanManager.activities}" var = "a">
        <h:column>
            <f:facet name="header">Book Title:</f:facet>
            #{a.libraryitemtitle}
        </h:column>
        <h:column>
            <f:facet name="header">Author:</f:facet>
            #{a.patron}
        </h:column>
        <h:column>
            <f:facet name="header">Publisher:</f:facet>
            #{a.activitytype}
        </h:column>
    </h:dataTable>  
</h:form>