Alternatives

Here is the list of other implementations of JTable with groupable header.

JRubik is an open source set of tools for navigating and reporting over OLAP Datasources. It has a package supporting groupable table headers and spanned table UI. Package name: es.aeat.eett.rubik.tableRubik.

JIDE is a rich library of Swing components. It has an open source layer but grids aren't included there.

Other variants may include embedding JavaFX TableView in a Swing application.