java.util

Class Observable

java.lang.Object

java.util.Observable

public class Observable
extends Object

Observable is used to notify a group of Observer objects when a change occurs. On creation, the set of observers is empty. After a change occurred, the application can call the notifyObservers() method. This will cause the invocation of the update() method of all registered Observers. The order of invocation is not specified. This implementation will call the Observers in the order they registered. Subclasses are completely free in what order they call the update methods.

See Also:

Observer

Constructor Summary

Constructors

Constructor and Description
Observable() Constructs a new Observable object.

Method Summary

Modifier and Type	Method and Description
void	addObserver(Observer observer) Adds the specified observer to the list of observers.
protected void	clearChanged() Clears the changed flag for this Observable.
int	countObservers() Returns the number of observers registered to this Observable.
void	deleteObserver(Observer observer) Removes the specified observer from the list of observers.
void	deleteObservers() Removes all observers from the list of observers.
boolean	hasChanged() Returns the changed flag for this Observable.
void	notifyObservers() If hasChanged() returns true, calls the update() method for every observer in the list of observers using null as the argument.
void	notifyObservers(Object data) If hasChanged() returns true, calls the update() method for every Observer in the list of observers using the specified argument.
protected void	setChanged() Sets the changed flag for this Observable.

Methods inherited from class java.lang.Object

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

Constructor Detail

Observable

public Observable()

Constructs a new Observable object.

Method Detail

addObserver

public void addObserver(Observer observer)

Adds the specified observer to the list of observers. If it is already registered, it is not added a second time.

Parameters:

observer - the Observer to add.

clearChanged

protected void clearChanged()

Clears the changed flag for this Observable. After calling clearChanged(), hasChanged() will return false.

countObservers

public int countObservers()

Returns the number of observers registered to this Observable.

Returns:

the number of observers.

deleteObserver

public void deleteObserver(Observer observer)

Removes the specified observer from the list of observers. Passing null won't do anything.

Parameters:

observer - the observer to remove.

deleteObservers

public void deleteObservers()

Removes all observers from the list of observers.

hasChanged

public boolean hasChanged()

Returns the changed flag for this Observable.

Returns:

true when the changed flag for this Observable is set, false otherwise.

notifyObservers

public void notifyObservers()

If hasChanged() returns true, calls the update() method for every observer in the list of observers using null as the argument. Afterwards, calls clearChanged().

Equivalent to calling notifyObservers(null).

notifyObservers

public void notifyObservers(Object data)

If hasChanged() returns true, calls the update() method for every Observer in the list of observers using the specified argument. Afterwards calls clearChanged().

Parameters:

data - the argument passed to update().

setChanged

protected void setChanged()

Sets the changed flag for this Observable. After calling setChanged(), hasChanged() will return true.