TreeTableHeader is a specialized JTableHeader capable of keeping a hierarchical (tree) structure of table columns. The columns that have children contain an expand/collapse icon on the left, which can be used to expand/collapse the column respectively. The column tree structure is defined in a TreeTableColumnModel, which defines methods for retrieving, adding and removing table columns. Finally, TreeTableHeader's column model is a TreeTableColumnModelAdapter that provides an interface from the visible columns in the header to the actual column data.