This video shows how to number the rows in a Jasper Reports crosstab. A custom Java class 'Counter' is used in a Text Expression. The expression retrieves the current value and increments the counter.
Developing a class in the Eclipse IDE for use in iReport Designer is also featured.
The code referenced in the video is available here. It's compiled using Java 6 into a JAR file here which is ready to use in iReport Designer 4.7.0.