SysTray for Java is an API written to enable Java applications access the system tray. Originally it was running on Windows only. In the meantime it is also available for KDE 3.It supports the creation of submenus, checkable menus, extended ASCII characters as menu item labels and the modification of everything during runtime.
Couldn't you already do that in Java? No, because it's platform dependant, not all OSes/window managers have a tray.