A DESCRIPTION OF THE PROBLEM :
The documentation at https://openjfx.io/javadoc/22/javafx.controls/javafx/scene/control/TreeTableView.html contains a typo for the TreeTableColumns:

 TreeTableColumns<File, String> fileNameCol = new TreeTableColumn<>("Filename");
 TreeTableColumns<File, Long> sizeCol = new TreeTableColumn<>("Size");

The type is clearly TreeTableColumn, not TreeTableColumns.


FREQUENCY : always