How to use Virtualizers in iReport

[mulcamd]

I'm running a report in iReport and it runs out of memory. It reads 150K records and they have to be sorted.

The solution is to use Virtualizers.

My problem is that the examples given in the documentation start from using a Java program.

Also the iReport-Ultimate-Guide-4 does not give a glue. It tells that there is a REPORT_VIRTUALIZER parameters:
This defines the class for the report filler that implements the JRVirtualizer interface for filling the report.

I want to run this report in iReport.

Question: what do I have to do to use a Virtualizer in iReport. Should I add some code in a Scriptlet?

Could anyone give a small example?