Freemarker checkboxex selection to be put on jbpm4 contex

Question

Using JBPM and Freemarker... I need to put on the jbpm context the list of checkboxex that a user selects in a .ftl page. My code looks like:

<table border="0">
    <#list keyNames as key>
        <tr>        
            <td><input type="checkbox" name="selectedKeys" value=${key} /></td>
            <td>${key}</td>                                 
        </tr>
    </#list>
</table> 

When I make in jbpm

String<List> selectedKeys = ( String<List> ) execution.getVariable( "selectedKeys" );

it gives a cast exception: cannot cast String to List...

If I do:

String selectedKeys = ( String ) execution.getVariable( "selectedKeys" );

It only takes the latest checkbox that was selected, not all of them.

Please advice. Thanks in advance