Retrieve value of a map using a dynamic key in XSLT

Question

I am forming an xsl where I am forming a map with keys and values.

     <java:map>
        <entry key="10001">2.1</entry>
        <entry key="48201">3.1</entry>
        <entry key="11215">4.1</entry>
        <entry key="28201">5.1</entry>
        <entry key="60601">6.1</entry>
    </java:map>
<xsl:template match="/">
<!--    <output>
         <xsl:value-of select="document('')/*/java:map/entry[@key='10001']"/>
    </output> -->
 <xsl:variable name="zipCodeString" select="./PersonInfoShipTo/@ZipCode"/
      <xsl:value-of select='$zipCodeString' /> // giving value of 10001

       <xsl:value-of select="document('')/*/java:map/entry[@key='$zipCodeString']"/> // not displaying any thing.

    </xsl:template>

As mentioned in the comments, I was using a variable which will get a value of 10001, I would like to use that to retrieve value in the next statement i.e., dynamically

Not able to understand , the syntax how to do that . Please advice

using exclude-result-prefixes="java"

Regards Suresh Midde