XSL Transformation in Java with parameters

If you pass a parameter like:

transformer.setParameter("render_id", "1234");

the parameter can be picked up by the transform:

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform" >

<xsl:output method="xml" encoding="UTF-8" omit-xml-declaration="yes"/>

<!-- Receives the id of the menu being rendered. -->
<xsl:param name="render_id" />

rsp's answer was spot on. Thanks. Just want to add that you cannot pass a parameter to a variable in the same way (I am setting parameters via Java's TransformerFactory).

I made the mistake of thinking variables and params were interchangeable :)