how to add class from java script without removing class from css : im new in javascript, i got problem with javascript from callmenick

    $(document).ready(function() {
    $( '.dropdown' ).hover(
        function(){
            $(this).children('.sub-menu').slideDown(200);
        },
        function(){
            $(this).children('.sub-menu').slideUp(200);
        }
    ); });

to call 'dropdown' and 'sub-menu' must using class, but in my class already have css 

    <li class="button_a hvr-underline"> /* i want to put 'dropdown' in this class */
       <a href="#" class="button" style="width: 128px;">a</a>
	</li>
im not advance