java.util

Class Observable

java.lang.Object

java.util.Observable

public class Observable
extends Object

This class represents an object which is observable. Other objects may register their intent to be notified when this object changes; and when this object does change, it will trigger the update method of each observer. Note that the notifyObservers() method of this class is unrelated to the notify() of Object.

Author:

Warren Levy (warrenl@cygnus.com), Eric Blake (ebb9@email.byu.edu)

See Also:

Observer

Constructor Summary

Constructors

Constructor and Description
Observable() Constructs an Observable with zero Observers.

Method Summary

Modifier and Type	Method and Description
void	addObserver(Observer observer) Adds an Observer.
int	countObservers() Returns the number of observers for this object.
void	deleteObserver(Observer victim) Deletes an Observer of this Observable.
void	deleteObservers() Deletes all Observers of this Observable.
boolean	hasChanged() True if setChanged has been called more recently than clearChanged.
void	notifyObservers() If the Observable has actually changed then tell all Observers about it, then reset state to unchanged.
void	notifyObservers(Object obj) If the Observable has actually changed then tell all Observers about it, then reset state to unchanged.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Observable

public Observable()

Constructs an Observable with zero Observers.

Method Detail

addObserver

public void addObserver(Observer observer)

Adds an Observer. If the observer was already added this method does nothing.

Parameters:

observer - Observer to add

Throws:

NullPointerException - if observer is null

countObservers

public int countObservers()

Returns the number of observers for this object.

Returns:

number of Observers for this

deleteObserver

public void deleteObserver(Observer victim)

Deletes an Observer of this Observable.

Parameters:

victim - Observer to delete

deleteObservers

public void deleteObservers()

Deletes all Observers of this Observable.

hasChanged

public boolean hasChanged()

True if setChanged has been called more recently than clearChanged.

Returns:

whether or not this Observable has changed

notifyObservers

public void notifyObservers()

If the Observable has actually changed then tell all Observers about it, then reset state to unchanged.

See Also:

notifyObservers(Object), Observer.update(Observable, Object)

notifyObservers

public void notifyObservers(Object obj)

If the Observable has actually changed then tell all Observers about it, then reset state to unchanged. Note that though the order of notification is unspecified in subclasses, in Observable it is in the order of registration.

Parameters:

obj - argument to Observer's update method

See Also:

Observer.update(Observable, Object)