16.54. Observers

Components that are to receive notifications of changes are called observers. To create an observer, extend the ChangeObserver abstract class and provide an implementation of the notify(Changes) method. Then, register the observer with an Observable using its register(ChangeObserver) method. The observer's notify(Changes) method will then be called with the changes that have been made to the Observable.

When an observer is no longer needed, it should be unregistered from all Observable instances with which it was registered. The ChangeObserver class automatically tracks which Observable instances it is registered with, and calling the observer's unregister() will unregister the observer from all of these Observables. Alternatively, an observer can be unregistered from a single Observable using the Observable's unregister(ChangeObserver) method.