The listener interface for receiving keyboard focus events on a component.

  The listener interface for receiving keyboard focus events on
a component.
The class that is interested in processing a focus event
either implements this interface (and all the methods it
contains) or extends the abstract FocusAdapter class
(overriding only the methods of interest).
The listener object created from that class is then registered with a
component using the component's addFocusListener
method. When the component gains or loses the keyboard focus,
the relevant method in the listener object
is invoked, and the FocusEvent is passed to it.