如何设置xhtml面板的默认切换？

<div class="panel-group" id="accordion">
  <div class="panel panel-default">
    <div class="panel-heading">
      <h4 class="panel-title">
        <a data-toggle="collapse" data-parent="#accordion" href="#collapse1">面板标题1</a>
      </h4>
    </div>
    <div id="collapse1" class="panel-collapse collapse in">
      <div class="panel-body">
        面板内容1
      </div>
    </div>
  </div>
  <div class="panel panel-default">
    <div class="panel-heading">
      <h4 class="panel-title">
        <a data-toggle="collapse" data-parent="#accordion" href="#collapse2">面板标题2</a>
      </h4>
    </div>
    <div id="collapse2" class="panel-collapse collapse">
      <div class="panel-body">
        面板内容2
      </div>
    </div>
  </div>
</div>

$(document).ready(function(){
  // 设置默认展开的面板
  $('#collapse1').addClass('in');
});