This method will be invoked once when/if a Future that this callback is registered on becomes completed with a failure or a success.
This method will be invoked once when/if a Future that this callback is registered on becomes completed with a failure or a success. In the case of success then "failure" will be null, and in the case of failure the "success" will be null.
Callback for when a Future is completed with either failure or a success SAM (Single Abstract Method) class
Java API