Delegates and Events - in COBOL and Java

delegate-id MessageHandler (str as string).
end delegate.

class-id DelegatesEvents.

01 MessageArrived     event type MessageHandler static.

method-id Main static.
  *> explicit delegate construction
  invoke new MessageHandler(MyHandler)

  *> implicit delegate construction
  declare handler as type MessageHandler = method MyHandler

  *> subscribe to an event
  attach method MyHandler to MessageArrived

  *> raise the event
  invoke MessageArrived("Test message")

  *> unsubscribe from the event
  detach method MyHandler from MessageArrived

  *> Throws a null reference exception as there are no subscribers
  invoke MessageArrived("Test message 2")

  *> Safely raising an #event
  declare handler2 as type MessageHandler = MessageArrived
  if handler2 not equals null
     invoke handler1("Safe message")

end method.

method-id MyHandler static (str as string).
  display str
end method.

end class.
// Java has no concept of delegates or events.
// Similar behavior is typically achieved by using
// anonymous inner classes instead.

Portions of these examples were produced by Dr. Frank McCown, Harding University Computer Science Dept, and are licensed under a Creative Commons License.