public String intercept(ActionInvocation actionInvocation) throws Exception 
{
        Map parameters = actionInvocation.getInvocationContext().getParameters();
        Map< String, Object > newParams = new HashMap< String, Object >();
        Set< String > keys = parameters.keySet();
        for (Iterator< String > iterator = keys.iterator(); iterator.hasNext();) {
            String key = iterator.next();
            if (key.startsWith("__multiselect_")) {
                String name = key.substring("__multiselect_".length());
                iterator.remove();
                // is this multi-select box submitted?
                if (!parameters.containsKey(name)) {
                    // if not, let's be sure to default the value to an empty string array
                    newParams.put(name, new String[0]);
                }
            }
        }
        parameters.putAll(newParams);
        return actionInvocation.invoke();
}

Just as the CheckboxInterceptor checks that if only the hidden field is present, so too does this interceptor. If the "__multiselect_" request parameter is present and its visible counterpart is not, set a new request parameter to an empty Sting.