//得到表单提交的值
        String receiveUname=request.getParameter("sendName");
        String message=request.getParameter("message");
        String note=request.getParameter("note");
        HttpSession session=request.getSession();
        String userName=(String)session.getAttribute("userName");
        UserinfoDAO udao=new UserinfoDAO();
        //从数据库中查出数据
            ArrayList list=udao.findAll();
            //for循环比较和当前字符串是否相等
            for(int i=0;i<list.size();i++){
            Userinfo uf=(Userinfo)list.get(i);
            if(receiveUname.equals(uf.getUserName()));
            {
                MessageinfoDAO mdao=new MessageinfoDAO();
                int count=mdao.sendMessage(note, userName, receiveUname, message);
                if(count>0){
                    System.out.print("发送成功！");
                    response.sendRedirect("/shortmessage1/index.jsp");
                    break;
                }
                ｝
                //请问为执行这段代码receiveUname和uf.getUserName()总是相等的，控制台总是提示发送成功！该怎么判断这两个值是否相等，用什么方法

你先用System.out.println()在控制台输出receiveUname和uf.getUserName()，看一下它们两个值是不是相等，是相等就没问题，不是的话就是程序有问题!

boolean flag = false;
 if(eceiveUname == getUserName()){
    flag = true;
}

  判断 flag  当它为true 相等 为 false 不相等