An object which is representable as a collection of other
objects may have a Folder
Facet (Figure 4.7) which enables it to be
displayed in a generic "folder" UI element.
A Folder provides a
list of
Column objects,
each of which represents some property of all the elements of the
Folder. The
Columns can be used by
UI elements to provide a tabular view of the
contents of the
Folder,
and to sort the contents of the
Folder based on the
property represented by the
Column.