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为什么原代码要先removeClass呢

<!DOCTYPE html><html xmlns=" 
<head>        
    <title>unbind()移除绑定的事件</title>        
    <script src="http://libs.baidu.com/jquery/1.9.0/jquery.js" type="text/javascript"></script>        
    <link href="style.css" rel="stylesheet" type="text/css" />    
</head>        
<body>        
    <h3>unbind()移除绑定的事件</h3>        
    <input id="btntest" type="button" value="移除事件" />        
    <div>土豪，咱们交个朋友吧</div>                
    <script type="text/javascript">            
        $(function () {                
            $("div").bind("click",                
                function () {                    
                    $(this).addClass("color");                
                }).bind("dblclick", function () {                    
                    $(this).addClass("backcolor");                
                })                
            $("#btntest").bind("click", function () {                    
                $("div").unbind("dbclick");                    
                    $(this).attr("disabled", "true");                
            });            
        });        
    </script>    
</body>
</html>