EL access a map value by Integer key

Initial answer (EL 2.1, May 2009)

As mentioned in this java forum thread:

Basically autoboxing puts an Integer object into the Map. ie:

map.put(new Integer(0), "myValue")

EL (Expressions Languages) evaluates 0 as a Long and thus goes looking for a Long as the key in the map.ie it evaluates:

map.get(new Long(0))

As a Long is never equal to an Integer object, it does not find the entry in the map.
That's it in a nutshell.

Update since May 2009 (EL 2.2)

Dec 2009 saw the introduction of EL 2.2 with JSP 2.2 / Java EE 6, with a few differences compared to EL 2.1.
It seems ("EL Expression parsing integer as long") that:

you can call the method intValue on the Long object self inside EL 2.2:

<c:out value="${map[(1).intValue()]}"/>

That could be a good workaround here (also mentioned below in Tobias Liefke's answer)

Original answer:

EL uses the following wrappers:

Terms                  Description               Typenull                   null value.               -123                    int value.                java.lang.Long123.00                 real value.               java.lang.Double"string" ou 'string'   string.                   java.lang.Stringtrue or false          boolean.                  java.lang.Boolean

JSP page demonstrating this:

 <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%> <%@ page import="java.util.*" %> <h2> Server Info</h2>Server info = <%= application.getServerInfo() %> <br>Servlet engine version = <%=  application.getMajorVersion() %>.<%= application.getMinorVersion() %><br>Java version = <%= System.getProperty("java.vm.version") %><br><%  Map map = new LinkedHashMap();  map.put("2", "String(2)");  map.put(new Integer(2), "Integer(2)");  map.put(new Long(2), "Long(2)");  map.put(42, "AutoBoxedNumber");  pageContext.setAttribute("myMap", map);    Integer lifeInteger = new Integer(42);  Long lifeLong = new Long(42);  %>  <h3>Looking up map in JSTL - integer vs long </h3>  This page demonstrates how JSTL maps interact with different types used for keys in a map.  Specifically the issue relates to autoboxing by java using map.put(1, "MyValue") and attempting to display it as ${myMap[1]}  The map "myMap" consists of four entries with different keys: A String, an Integer, a Long and an entry put there by AutoBoxing Java 5 feature.         <table border="1">    <tr><th>Key</th><th>value</th><th>Key Class</th></tr>    <c:forEach var="entry" items="${myMap}" varStatus="status">    <tr>            <td>${entry.key}</td>      <td>${entry.value}</td>      <td>${entry.key.class}</td>    </tr>    </c:forEach></table>    <h4> Accessing the map</h4>        Evaluating: ${"${myMap['2']}"} = <c:out value="${myMap['2']}"/><br>    Evaluating: ${"${myMap[2]}"}   = <c:out value="${myMap[2]}"/><br>        Evaluating: ${"${myMap[42]}"}   = <c:out value="${myMap[42]}"/><br>        <p>    As you can see, the EL Expression for the literal number retrieves the value against the java.lang.Long entry in the map.    Attempting to access the entry created by autoboxing fails because a Long is never equal to an Integer    <p>    lifeInteger = <%= lifeInteger %><br/>    lifeLong = <%= lifeLong %><br/>    lifeInteger.equals(lifeLong) : <%= lifeInteger.equals(lifeLong) %> <br>

Just another helpful hint in addition to the above comment would be when you have a string value contained in some variable such as a request parameter.In this case, passing this in will also result in JSTL keying the value of say "1" as a sting and as such no match being found in a Map hashmap.

One way to get around this is to do something like this.

<c:set var="longKey" value="${param.selectedIndex + 0}"/>

This will now be treated as a Long object and then has a chance to match an object when it is contained withing the map Map or whatever.

Then, continue as usual with something like

${map[longKey]}

You can use all functions from Long, if you put the number into "(" ")". That way you can cast the long to an int:

<c:out value="${map[(1).intValue()]}"/>