如何在JSP中显示列表? [英] How to show a list in JSP?

问题描述

这个Servlet，我在彩色车上做GROUP BY.插入请求并在jsp中做广告但不会转换.

 查询查询= session.createQuery(从cars组中按carColor按carColor排序，从cars组中选择count(carColor)，carColor");List<汽车>列表= query.list();迭代器iter = list.iterator();while(iter.hasNext()){Object [] obj =(Object [])iter.next();System.out.println(obj [0] +" + obj [1]);}request.setAttribute("list"，list);RequestDispatcher rd = request.getRequestDispatcher("test.jsp");rd.forward(请求，响应)； 

控制台:2白10黑色5蓝色

JSP:[[Ljava.lang.Object; @ 1f3b536，[Ljava.lang.Object; @ fdffb1，]]

解决方案

您的代码确实没有任何意义:

  List< Cars>列表= query.list(); 

在上一行中，您声明您的列表是Cars列表(不是).

  Iterator iter = list.iterator();while(iter.hasNext()){Object [] obj =(Object [])iter.next(); 

然后在列表上进行迭代，并将每个元素转换为 Object [] .Cars实例如何成为 Object [] ?该列表应声明为 List< Object []> ，并且您不应使用原始类型.您的循环应写为

  Iterator< Object []>iter = list.iterator();while(iter.hasNext()){Object [] obj = iter.next(); 

或者，甚至更简单:

  for(Object [] obj:list){ 

现在，在JSP中，我怀疑您只是在使用 $ {list} 来显示列表.这只是在列表上调用toString()方法，而列表本身会调用每个元素的toString()方法.由于每个元素都是一个 Object [] ，因此结果字符串为 [Ljava.lang.Object; @ 1f3b536 "，表示具有hashCode 1f3b536的对象数组".

要显示列表的元素，应遍历该列表，就像必须在Java代码中进行的操作一样:

 < c:forEach var ="array" items ="$ {list}">计数:$ {array [0]}-颜色:< c:out value ="$ {array [1]}"//>< br/></c:forEach> 

This Servlet, I do GROUP BY on color car. Inserted into the request and advertise in jsp but it does not convert.

    Query query = session.createQuery("select count(carColor), carColor from Cars group by carColor order by carColor");
    List<Cars> list = query.list();
    Iterator iter = list.iterator();
    while (iter.hasNext()) {
        Object[] obj = (Object[]) iter.next();
        System.out.println(obj[0] + "  " + obj[1]);

    }

    request.setAttribute("list", list); 

    RequestDispatcher rd = request.getRequestDispatcher("test.jsp");
    rd.forward(request, response);

Console: 2 White 10 Black 5 Blue

JSP: [[Ljava.lang.Object;@1f3b536, [Ljava.lang.Object;@fdffb1,]]

解决方案

Your code really makes no sense:

List<Cars> list = query.list();

In the above line, you declare that your list is a list of Cars (which it is not)

Iterator iter = list.iterator();
while (iter.hasNext()) {
    Object[] obj = (Object[]) iter.next();

Then you iterate on the list, and cast each element to Object[]. How could a Cars instance ever be an Object[]? The list should be declared as List<Object[]>, and you shouldn't use raw types. Your loop should be written as

Iterator<Object[]> iter = list.iterator();
while (iter.hasNext()) {
    Object[] obj = iter.next();

Or, even simpler:

for (Object[] obj : list) {

Now, in the JSP, I suspect you're just using ${list} to display the list. This simply calls the toString() method on the list, which itself calls the toString() method of each element. Since each element is an Object[], the result string is [Ljava.lang.Object;@1f3b536, which means "array of Object with hashCode 1f3b536".

To display the elements of the list, you should iterate over the list, as you must do it in the Java code:

<c:forEach var="array" items="${list}">
    Count: ${array[0]} - Color: <c:out value="${array[1]}"/> <br/>
</c:forEach>

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