Ok so JasperReports Java API is still very bad.

This is how to add content:

((JRDesignSection) jasperDesign.getDetailSection()).addBand(designBand);

It is shocking that after all these years you have to cast to add a band. If someone has documentation about this trick I would appreciate it.

again:

jasperDesign.getDetailSection().addBand(designBand)  // compile error.