Miscellaneous Utilities - The Java Programming Language - page 947

if (users != watching)
throw new IllegalArgumentException();
UserState state = (UserState) whichState;
if (watching.loggedIn(state)) // user logged in
addUser(state); // add to my list
else
removeUser(state); // remove from list
}
// ...
}
Each Eye object watches a particular Users object. When a user logs in
or out, Eye is notified because it invoked the Users object's addObserver
method with itself as the interested object. When update is invoked, it
checks the correctness of its parameters and then modifies its display
depending on whether the user in question has logged in or out.
The check for what happened with the UserState object is simple here.
You could avoid it by passing an object describing what happened and
to whom instead of passing the UserState object itself. Such a design
makes it easier to add new actions without breaking existing code.
The Observer / Observable mechanism is a looser, more flexible analogue to
the wait / notify mechanism for threads described in " wait, notifyAll, and
notify " on page 354 . The thread mechanism ensures that synchronized
access protects you from undesired concurrency. The observation mech-
anism enables any relationship to be built between two participants,
whatever the threading model. Both patterns have producers of inform-
ation ( Observable and the invoker of notify ) and consumers of that in-
formation ( Observer and the invoker of wait ), but each one fills a differ-
ent need. Use wait / notify when you design a thread-based mechanism,
and use Observer / Observable when you need something more general.
Exercise 22.4 : Provide an implementation of the Attributed interface
that uses Observer / Observable to notify observers of changes.