Re: Custom Listeners, yes, but why custom Events?

Lew <>
Wed, 31 Mar 2010 18:49:19 -0400
Leo Breebaart writes:

Am I missing some very obvious advantage to the FooEvent

(Stefan Ram) wrote:

  Posted events get queued in the time-ordered event queue,
  while e direct call might be out of order.

  When Swing calls a listener, the listener can be sure to
  run in the EDT, which is not obvious, when a listener is
  called directly.

  A GUI object usually accepts an arbitrary number of
  listeners for certain kinds of events at run-time and then
  notifies them of the events. This can not be accomplished
  with hard-coded calls of specific methods. The binding is
  more loose with events than with calls.

  Swing might merge certain events in the event queue into
  a single event IIRC, so posted events are not mapped 1:1
  to calls of event listeners.

  Other event processors might possibly be added, such as
  macro recorders / macro replayers. They will only record
  events, but miss direct calls.

  Those were just things that immediatly came into my mind,
  I might have missed others.

John B. Matthews wrote:

This comports with my understanding. Using the approach described
in the API for EventListenerList offers additional benefits:


In addition to those mentioned, the use of a type-token (e.g.
FooListener.class) affords an opportunity for type checking.

In addition to what these fine folks have said, that all Events are in the
same type hierarchy is a boon. Event handlers are only tied to the event
type, so generalized mechanisms can deal with event delivery and processing
without regard for the specifics, unlike if different-typed objects were
passed around. Futhermore, events can carry semantics (attributes and
behavioral methods) that are tuned to the event aspect, which, of course,
domain objects should not do.


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