This
class is the superclass for classes that want to provide
notifications of state changes to interested
Observer objects. Register an
Observer to be notified by passing it to the
addObserver( ) method of an
Observable, and de-register it by passing it to
the deleteObserver( ) method. You can delete all
observers registered for an Observable with
deleteObservers( ), and can find out how many
observers have been added with countObservers( ).
Note that there is not a method to enumerate the particular
Observer objects that have been added.
An Observable subclass should call the protected
method setChanged( ) when its state has changed in
some way. This sets a "state
changed" flag. After an operation or series of
operations that may have caused the state to change, the
Observable subclass should call
notifyObservers( ), optionally passing an
arbitrary Object argument. If the state changed
flag is set, this notifyObservers( ) calls the
update( ) method of each registered
Observer (in some arbitrary order), passing the
Observable object, and the optional argument, if
any. Once the update( ) method of each
Observable has been called,
notifyObservers( ) calls clearChanged(
) to clear the state changed flag. If
notifyObservers( ) is called when the state
changed flag is not set, it does not do anything. You can use
hasChanged( ) to query the current state of the
changed flag.
The Observable class and
Observer interface are not commonly used. Most
applications prefer the event-based notification model defined by the
JavaBeans component framework and by the
EventObject class and
EventListener interface of this package.
public class
Observable {
// Public Constructors
public
Observable ( );
// Public Instance Methods
public void
addObserver (Observer
o ); synchronized
public int
countObservers ( ); synchronized
public void
deleteObserver (Observer
o ); synchronized
public void
deleteObservers ( ); synchronized
public boolean
hasChanged ( ); synchronized
public void
notifyObservers ( );
public void
notifyObservers (Object
arg );
// Protected Instance Methods
protected void
clearChanged ( ); synchronized
protected void
setChanged ( ); synchronized
}