为什么这在带有散列表的jstl中起作用？

Question

why does display what i want:

   <c:forEach var="temp" items="${AvailableLessonBean.lessons['description']}">
                    <form action="" method="POST">
                        <tr>
                            <td>
                                <c:out value="${temp}"/>
                            </td>

which displays in a html table:

Description Start Date Start Time End Time Level Make Booking Snowboarding for dummies
Advanced Carving Techniques
How to not fall off the draglift
Gnarliness Extreeeeeeeme
Parallel turns
How to splint a broken leg with as
Cross-country techniques
Aerobatics
Intermediate Slalom

and this does not display anything but an exception:

    <c:forEach var="temp" items="${AvailableLessonBean.lessons}">
                    <form action="" method="POST">
                        <tr>
                            <td>
                                <c:out value="${temp['description']}"/>
                            </td>

I have no hair left because of this!!

Answer 1

Once again, you didn't tell us what these objects are, which makes it impossible to answer. Hopefully for you, I remember the data structure from a previous question.

So, AvailableLessonBean.getLessons() returns a Map<String, List<String>> .

In your first snippet, you iterate over AvailableLessonBean.lessons['description'] . What this EL code does is that it gets the value associated with the key "description" in the map:

map.get("description")

that works fine. It returns a List<String> . It iterates over that list, and stores the current element of the list in a variable named temp . And at each iteration, it prints temp . This is thus equivalent to the following Java code:

List<String> list = availableLessonBean.getLessons().get("description");
for (String temp : list) {
    out.println(temp);
}

In the second snippet, you're iterating over the map itself. The JSTL specifies that when you iterate over a map, you iterate over all the entries of the map. Each entry has a key and a value. So the iteration is equivalent to doing, in Java:

for (Map.Entry<String, List<String>> temp : availableLessonBean.getLessons().entrySet()) {
    ...
}

But then you're trying to access temp["description"] . That doesn't make any sense. temp is a Map Entry. A Map Entry has a key and a value. That's all. In this case, the key is a String and the value is a List<String> .