code: @Override
  	protected void onComponentTag(ComponentTag tag) {
  		super.onComponentTag(tag);
  		
  		if(model.getObject().size() > 0) tag.append("class", "has-error", " ");
  	}
docs: /* (non-Javadoc)
  @see org.apache.wicket.Component#onComponentTag(org.apache.wicket.markup.ComponentTag)