JSTL forEach reverse order

Question

Using JSTL\'s forEach tag, is it possible to iterate in reverse order?

Answer 1

When you are using forEach to create an integer loop, you can go forward or backward, but it requires some work. It turns out you cannot do this, for example:

<c:forEach var="i" begin="10" end="0" step="-1">
    ....
</c:forEach>

because the spec requires the step is positive. But you can always loop in forward order and then use <c:var to convert the incrementing number into a decrementing number:

<c:forEach var="i" begin="0" end="10" step="1">
   <c:var var="decr" value="${10-i}"/>
    ....
</c:forEach>

However, when you are doing a forEach over a Collection of any sort, I am not aware of any way to have the objects in reverse order. At least, not without first sorting the elements into reverse order and then using forEach.

I have successfully navigated a forEach loop in a desired order by doing something like the following in a JSP:

<%
List list = (List)session.getAttribute("list");
Comparator comp = ....
Collections.sort(list, comp);
%>


<c:forEach var="bean" items="<%=list%>">
     ...
</c:forEach>

With a suitable Comparator, you can loop over the items in any desired order. This works. But I am not aware of a way to say, very simply, iterate in reverse order the collection provided.

Answer 2

You could reverse the Collection before adding it to your model.

This also means you would not need to do anything in the view layer to achieve your requirement.

Answer 3

We can do this with a little bit trick:-

<c:forEach begin="1" end="10" var="i" step="1">
     <c:set var="j" value="${10-i+1}" scope="page"></c:set>
     <c:out value="${j}"/>
</c:forEach>

OutPut will be :- 10 9 8 7 6 5 4 3 2 1

Answer 4

You could also consider rolling a custom JSTL function that returned a reversed copy of your list, backed by something like this:

public static <T extends Object> List<T> reverse(List<T> list) {
    List<T> copy = Collections.emptyList();
    Collections.copy(copy, list);
    Collections.reverse(copy);
    return copy;
}

Doesn't work for Collections, but as mentioned in another answer, the concept of ordering is a bit vague for some collections.

Answer 5

Building on the answer given by Eddie, I used the following code to iterate over a collection in reverse order.

Given a collection called "list", which stores a list of people.

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn"%>

<%-- Keep a reference to the size of the collection --%>
<c:set var="num_people" value="${fn:length(list)}" />

<%-- Iterate through items. Start at 1 to avoid array index out of bounds --%>
<c:forEach var="i" begin="1" end="${num_people}" step="1">
    <c:set var="person" value="${list[num_people-i]}" />

    ${person.name}<br />
</c:forEach>