Jdbc Java.lang.NullPointerException

Question

下面是我的jsp页面的代码部分，当我打印时，rs.getString（1）打印完美但是只要我把rs.getString（1）放在if块中就会显示空指针异常。请帮忙

String nam=request.getParameter("n");
String query = "select name from ques where username='"+userid+"'";

Statement st=con.prepareStatement(query);

ResultSet rs = st.executeQuery(query);


while(rs.next()){

    out.println(rs.getString(1));

    if(rs.getString(1).equals(nam)){

        out.print("hello");

    }

}