code: @Override
  	public String getValueAsString() {
  		Set<String> selected = getValue();
  		if (selected == null || selected.isEmpty()) {
  			return null;
  		}
  
  		StringBuilder stringValues = new StringBuilder();
  		boolean first = true;
  
  		for (String item : selected) {
  			if (!first) {
  				stringValues.append(", ");
  			}
  			stringValues.append(item);
  			first = false;
  		}
  
  		return stringValues.toString();
  	}
docs: Returns a string value of the selected item for this users session. If multiple selections have been made, this
  will be a comma separated list of string values. If no value is selected, null is returned.
  
  @return the selected item value as a rendered String