java我无法从充满Order对象的ArrayList打印
因此，当我试图从arraylist打印时，会出现空指针异常，我似乎无法理解为什么，因为我对编码还是相当陌生。请记住，我将列表存储为会话的一个属性，该属性是在您登录时创建的。下面是示例1，我在其中创建并尝试添加到列表中：
public static ArrayList<Order> getOrdersByUserId( int userid ) throws LoginSampleException {
try {
Connection con = Connector.connection();
String SQL = "SELECT * FROM `order` "
+ "WHERE fk_userid=?";
PreparedStatement ps = con.prepareStatement( SQL );
ps.setInt( 1, userid );
ResultSet rs = ps.executeQuery();
ArrayList<Order> orders = new ArrayList<>();
while ( rs.next() ) {
Order order = new Order(rs.getInt("length"), rs.getInt("width"), rs.getInt("height"));
orders.add(order);
}
return orders;
} catch ( ClassNotFoundException | SQLException ex ) {
throw new LoginSampleException(ex.getMessage());
}
}
这是示例2，我将列表存储在会话中
public class ViewOrders extends Command {
@Override
String execute(HttpServletRequest request, HttpServletResponse response) throws LoginSampleException {
ArrayList<Order> ol = new ArrayList();
HttpSession session = request.getSession();
if ( session.getAttribute("ordersByUserID") == null ) {
User user = (User) session.getAttribute( "user" );
ol = LogicFacade.getOrdersByUserID( user.getId() );
System.out.println( "session not found" );
request.getSession().setAttribute( "ordersByUserID", ol );
}
return "vieworders";
}
}
下面是示例3，我尝试从中打印，得到NullPointerException：
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>JSP Page</title>
</head>
<body>
<table>
<thead>
<td>This is your previous orders</td>
</thead>
<tr>
<td>
<%
if (request.getSession().getAttribute("user") != null) {
List<Order> orderlist = (List<Order>)request.getSession().getAttribute("ordersById");
for (int i = 0; i < orderlist.size(); i++) { %>
<a href="vieworderdetails.jsp?orderID=<%out.println(orderlist.get(i).getId());%>">
<%
out.println(orderlist.get(i).getId());
%>
</a> <%
}
}
%>
</td>
</tr>
</table>
</body>
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