JSTL and hashmap not working

问题

In a servlet I have:

HashMap eventsByDayNo = new HashMap();
eventsByDayNo.put (new Integer(12), "day 12 info");
eventsByDayNo.put (new Integer(11), "day 11 info");
eventsByDayNo.put (new Integer(15), "day 15 info");
eventsByDayNo.put (new Integer(16), "day 16 info");

request.setAttribute("eventsByDayNo", eventsByDayNo);
request.setAttribute("daysInMonth", new Integer(31));

And in a jsp I have:

<c:forEach var="dn" begin="1" end="${daysInMonth}" step="1" varStatus="status">
  Day Number=<c:out value="${dn}" /> Value=<c:out value="${eventsByDayNo[dn]}" /><br>
</c:forEach>

The above JSTL works fine, but if I try to offset the day number <c:out value="${eventsByDayNo[dn+3]}" /> none of the hashmap entries are not printed. Any answers as to why not?

The above is just a proof of concept for my real application.

回答1:

My guess is that dn+3 has type java.lang.Double, not java.lang.Integer (which you may expect).

<ul>
<c:forEach var="dn" begin="1" end="${daysInMonth}" step="1">
  <li>
    <c:set var="dnplus3" value="${dn+3}" />
    dn=<c:out value="${dn}" />
    dnplus3=<c:out value="${dnplus3}" />
    class=<c:out value="${dnplus3.class.name}" />
  </li>
</c:forEach>
</ul>

回答2:

Numbers (at least, whole numbers) in EL are implicitly treated as Long. So replace your Map<Integer, String> by a Map<Long, String> and it will work.

来源：https://stackoverflow.com/questions/3919025/jstl-and-hashmap-not-working

标签

java

jsp

jstl