Miscellaneous Utilities - The Java Programming Language - page 943

22.3. Observer / Observable
The Observer / Observable types provide a protocol for an arbitrary number
of Observer objects to watch for changes and events in any number of
Observable objects. An Observable object subclasses the Observable class,
which provides methods to maintain a list of Observer objects that want to
know about changes in the Observable object. All objects in the "interes-
ted" list must implement the Observer interface. When an Observable ob-
ject experiences a noteworthy change or an event that Observer objects
may care about, the Observable object invokes its notifyObservers method,
which invokes each Observer object's update method.
The Observer interface consists of a single method:
public void update(Observable obj, Object arg)
This method is invoked when the Observable object obj has a
change or an event to report. The arg parameter is a way to
pass an arbitrary object to describe the change or event to the
observer.
The Observer / Observable mechanism is designed to be general. Each Ob-
servable class is left to define the circumstances under which an Observer
object's update method will be invoked. The Observable object maintains a
"changed" flag which subclass methods use to indicate when something
of interest has occurred.
protected void setChanged()
Marks this object as having been changed hasChanged will now
return true but does not notify observers.
protected void clearChanged()
Indicates that this object is no longer changed or has notified
all observers of the last change hasChanged will now return false .