The listener interface for receiving WindowEvents, including WINDOW_GAINED_FOCUS and WINDOW_LOST_FOCUS events. The class that is interested in processing a WindowEvent either implements this interface (and all the methods it contains) or extends the abstract WindowAdapter class (overriding only the methods of interest). The listener object created from that class is then registered with a Window using the Window's addWindowFocusListener method. When the Window's status changes by virtue of it being opened, closed, activated, deactivated, iconified, or deiconified, or by focus being transfered into or out of the Window, the relevant method in the listener object is invoked, and the WindowEvent is passed to it.