I have a Map keyed by Integer. Using EL, how can I access a value by its key?
Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
I thought this would work but it doesn't (where map is already in the request's attributes):
<c:out value="${map[1]}"/>
Follow up: I tracked down the problem. Apparently ${name[1]} does a map lookup with the number as a Long. I figured this out when I changed HashMap to TreeMap and received the error:
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Long
If I change my map to be:
Map<Long, String> map = new HashMap<Long, String>();
map.put(1L, "One");
then ${name[1]} returns "One". What's with that? Why does <c:out> treat a number as a long. Seems counterintuitive to me (as int is more commonly used than long).
So my new question is, is there a EL notation to access a map by an Integer value?
Initial answer (EL 2.1, May 2009)
As mentioned in this java forum thread:
Basically autoboxing puts an Integer object into the Map. ie:
map.put(new Integer(0), "myValue")
EL (Expressions Languages) evaluates 0 as a Long and thus goes looking for a Long as the key in the map. ie it evaluates:
map.get(new Long(0))
As a Long is never equal to an Integer object, it does not find the entry in the map.
That's it in a nutshell.
Dec 2009 saw the introduction of EL 2.2 with JSP 2.2 / Java EE 6, with a few differences compared to EL 2.1.
It seems ("EL Expression parsing integer as long") that:
you can call the method
intValueon theLongobject self inside EL 2.2:
<c:out value="${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 Type
null null value. -
123 int value. java.lang.Long
123.00 real value. java.lang.Double
"string" ou 'string' string. java.lang.String
true 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="${myMap[42]}"/><br>
<p>
As you can see, the EL Expression for the literal number retrieves the value against the java.lang.Long entry in the map.
Attempting to access the entry created by autoboxing fails because a Long is never equal to an Integer
<p>
lifeInteger = <%= lifeInteger %><br/>
lifeLong = <%= lifeLong %><br/>
lifeInteger.equals(lifeLong) : <%= lifeInteger.equals(lifeLong) %> <br>
Based on the above post i tried this and this worked fine I wanted to use the value of Map B as keys for Map A:
<c:if test="${not empty activityCodeMap and not empty activityDescMap}">
<c:forEach var="valueMap" items="${auditMap}">
<tr>
<td class="activity_white"><c:out value="${activityCodeMap[valueMap.value.activityCode]}"/></td>
<td class="activity_white"><c:out value="${activityDescMap[valueMap.value.activityDescCode]}"/></td>
<td class="activity_white">${valueMap.value.dateTime}</td>
</tr>
</c:forEach>
</c:if>
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
${map[longKey]}
If you just happen to have a Map with Integer keys you cannot change, you could write a custom EL function to convert a Long to Integer. This would allow you to do something like:
<c:out value="${map[myLib:longToInteger(1)]}"/>
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()]}"/>
Source: Stackoverflow.com