Overview (java-gnome 4.1.3 API Documentation)

Packages
Package	Description
org.freedesktop.bindings
org.freedesktop.cairo	The Cairo Graphics library is the high performance drawing suite used to do custom drawing in GTK.
org.freedesktop.enchant
org.freedesktop.icons
org.gnome.atk	ATK is the toolkit that GNOME uses to enable accessibility for users needing extra support to make the most of their computers.
org.gnome.gdk	GDK is the low level drawing and events library used by GTK to wrap the underlying diplsay hardware and to react to actions by the user.
org.gnome.glib	GLib is the general-purpose base library in GNOME; most of the its elements are just implementation infrastructure.
org.gnome.gtk	GTK is the widget toolkit which is used by applications to create rich user interfaces on the screen.
org.gnome.notify
org.gnome.pango	Pango is the text layout and rending engine used to power font rendering and internationalization in GNOME.
org.gnome.rsvg
org.gnome.screenshot
org.gnome.sourceview
org.gnome.unixprint

This is the java-gnome language bindings project! The libraries here are as follows:

GTK: GTK, the widget toolkit that us used to create amazing desktop applications for the GNOME desktop and other platforms that support it. GTK was originally created as the GIMP's widget Tool Kit), and has since become the backbone of the entire GNOME project.
GDK: GDK contains the low level Drawing primitives (elements on the screen like points, images, cursors, etc) along with facilities for handling events coming back from the user (such as mouse clicks and keystrokes). It has implementations of various backends allowing GTK to be used on a number of different platforms. Most of the actual drawing is done via Cairo.
GLib and GObject: GLib and GObject are the base libraries in GNOME, providing general purpose utilities and an extensible object-oriented type system. These are largely transparent to the Java developer, but nevertheless the infrastructure needed to support the other libraries resides here.
Pango: Pango, the text layout and rending engine used to power font rendering and internationalization in GNOME.
ATK: ATK, the Accessibility toolkit that enables GNOME to be used by people needing extra support to make the most of their computers. ATK is used by tools such as screen readers, magnifiers, and input devices to permit a rich interaction with the desktop through alternative means. It is also, amazingly enough, used by testing frameworks to introspect widgets and text on the screen.
GtkSourceView: A variant of TextView that does source code highlighting.
LibNotify: Popup warnings to the user.
GtkSpell: Spell checking for TextView Widgets.
LibUnique: A small library to facilitate the IPC necessary to enable you to maintain one instance (only) of an application.

Getting started

The first call you will make in any java-gnome program is going to be Gtk.init(), so you might try starting there.

External resources

For further reference, you might try

http://java-gnome.sourceforge.net: The java-gnome website
http://www.gtk.org: The GTK+ project page
http://library.gnome.org: The GNOME developers site, with low level API documentation

The java-gnome project have an IRC channel, #java-gnome, and a mailing list, java-gnome-developer that are great places to ask questions. See the contact us page for details.