public function addJavaScript($java_script)
    {
        if ($this->html_head_entry_set === null) {
            throw new SwatException(
                sprintf(
                    "Child class '%s' did not " .
                        'instantiate a HTML head entry set. This should be done in  ' .
                        'the constructor either by calling parent::__construct() or ' .
                        'by creating a new HTML head entry set.',
                    get_class($this)
                )
            );
        }

        $this->html_head_entry_set->addEntry(
            new SwatJavaScriptHtmlHeadEntry($java_script)
        );
    }