OpenJDK 27 · java.util.Observable · APIdia, the JavaDoc alternative

This class represents an observable object, or "data" in the model-view paradigm. It can be subclassed to represent an object that the application wants to have observed.

An observable object can have one or more observers. An observer may be any object that implements interface Observer. After an observable instance changes, an application calling the Observable's notifyObservers method causes all of its observers to be notified of the change by a call to their update method.

The order in which notifications will be delivered is unspecified. The default implementation provided in the Observable class will notify Observers in the order in which they registered interest, but subclasses may change this order, use no guaranteed order, deliver notifications on separate threads, or may guarantee that their subclass follows this order, as they choose.

Note that this notification mechanism has nothing to do with threads and is completely separate from the wait and notify mechanism of class Object.

When an observable object is newly created, its set of observers is empty. Two observers are considered the same if and only if the equals method returns true for them.

Modifier and Type	Field and Description
private boolean	changed
private Vector<Observer>	obs

Constructor Summary

Access	Constructor and Description
public	Observable() Construct an Observable with zero Observers.

Method Summary

Modifier and Type	Method and Description
public synchronized void	addObserver(Observer an observer to be added. o) References Deprecated `Observer` is deprecated or references (maybe indirectly) at least one deprecated element. Adds an observer to the set of observers for this object, provided that it is not the same as some observer already in the set.
protected synchronized void	clearChanged() Indicates that this object has no longer changed, or that it has already notified all of its observers of its most recent change, so that the `hasChanged` method will now return `false`.
public synchronized int	Returns: the number of observers of this object. countObservers() Returns the number of observers of this `Observable` object.
public synchronized void	deleteObserver(Observer the observer to be deleted. o) References Deprecated `Observer` is deprecated or references (maybe indirectly) at least one deprecated element. Deletes an observer from the set of observers of this object.
public synchronized void	deleteObservers() Clears the observer list so that this object no longer has any observers.
public synchronized boolean	Returns: `true` if and only if the `setChanged` method has been called more recently than the `clearChanged` method on this object; `false` otherwise. hasChanged() Tests if this object has changed.
public void	notifyObservers() If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed.
public void	notifyObservers(Object any object. arg) If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed.
protected synchronized void	setChanged() Marks this `Observable` object as having been changed; the `hasChanged` method will now return `true`.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll toString wait wait wait

Field Detail

changed	back to summary
private boolean changed

obs	back to summary
private Vector<Observer> obs

Constructor Detail

Observable	back to summary
public Observable() Construct an Observable with zero Observers.

Method Detail

addObserver back to summary

addObserver	back to summary
public synchronized void addObserver(Observer o) References Deprecated `Observer` is deprecated or references (maybe indirectly) at least one deprecated element. See corresponding docs for further information. Adds an observer to the set of observers for this object, provided that it is not the same as some observer already in the set. The order in which notifications will be delivered to multiple observers is not specified. See the class comment. Parameters o:Observer an observer to be added. Exceptions NullPointerException: if the parameter o is null.

public synchronized void addObserver(Observer o)

References Deprecated

Observer is deprecated or references (maybe indirectly) at least one deprecated element.

See corresponding docs for further information.

Adds an observer to the set of observers for this object, provided that it is not the same as some observer already in the set. The order in which notifications will be delivered to multiple observers is not specified. See the class comment.

Parameters

o:Observer: an observer to be added.

Exceptions

NullPointerException:: if the parameter o is null.

clearChanged back to summary

clearChanged	back to summary
protected synchronized void clearChanged() Indicates that this object has no longer changed, or that it has already notified all of its observers of its most recent change, so that the `hasChanged` method will now return `false`. This method is called automatically by the `notifyObservers` methods. See Also `java.util.Observable#notifyObservers()`, `java.util.Observable#notifyObservers(java.lang.Object)`

protected synchronized void clearChanged()

Indicates that this object has no longer changed, or that it has already notified all of its observers of its most recent change, so that the hasChanged method will now return false. This method is called automatically by the notifyObservers methods.

See Also: java.util.Observable#notifyObservers(), java.util.Observable#notifyObservers(java.lang.Object)

countObservers back to summary

countObservers	back to summary
public synchronized int countObservers() Returns the number of observers of this `Observable` object. Returns:int the number of observers of this object.

public synchronized int countObservers()

Returns the number of observers of this Observable object.

Returns:int: the number of observers of this object.

deleteObserver back to summary

deleteObserver	back to summary
public synchronized void deleteObserver(Observer o) References Deprecated `Observer` is deprecated or references (maybe indirectly) at least one deprecated element. See corresponding docs for further information. Deletes an observer from the set of observers of this object. Passing `null` to this method will have no effect. Parameters o:Observer the observer to be deleted.

public synchronized void deleteObserver(Observer o)

References Deprecated

Observer is deprecated or references (maybe indirectly) at least one deprecated element.

See corresponding docs for further information.

Deletes an observer from the set of observers of this object. Passing null to this method will have no effect.

Parameters

o:Observer: the observer to be deleted.

deleteObservers	back to summary
public synchronized void deleteObservers() Clears the observer list so that this object no longer has any observers.

hasChanged back to summary

hasChanged	back to summary
public synchronized boolean hasChanged() Tests if this object has changed. Returns:boolean `true` if and only if the `setChanged` method has been called more recently than the `clearChanged` method on this object; `false` otherwise. See Also `java.util.Observable#clearChanged()`, `java.util.Observable#setChanged()`

public synchronized boolean hasChanged()

Tests if this object has changed.

Returns:boolean: true if and only if the setChanged method has been called more recently than the clearChanged method on this object; false otherwise.
See Also: java.util.Observable#clearChanged(), java.util.Observable#setChanged()

notifyObservers back to summary

notifyObservers	back to summary
public void notifyObservers() If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed. Each observer has its `update` method called with two arguments: this observable object and `null`. In other words, this method is equivalent to: `notifyObservers(null)` See Also `java.util.Observable#clearChanged()`, `java.util.Observable#hasChanged()`, `java.util.Observer#update(java.util.Observable, java.lang.Object)`

public void notifyObservers()

If this object has changed, as indicated by the hasChanged method, then notify all of its observers and then call the clearChanged method to indicate that this object has no longer changed.

Each observer has its update method called with two arguments: this observable object and null. In other words, this method is equivalent to:

notifyObservers(null)

See Also: java.util.Observable#clearChanged(), java.util.Observable#hasChanged(), java.util.Observer#update(java.util.Observable, java.lang.Object)

notifyObservers back to summary

notifyObservers	back to summary
public void notifyObservers(Object arg) If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed. Each observer has its `update` method called with two arguments: this observable object and the `arg` argument. Parameters arg:Object any object. See Also `java.util.Observable#clearChanged()`, `java.util.Observable#hasChanged()`, `java.util.Observer#update(java.util.Observable, java.lang.Object)`

public void notifyObservers(Object arg)

If this object has changed, as indicated by the hasChanged method, then notify all of its observers and then call the clearChanged method to indicate that this object has no longer changed.

Each observer has its update method called with two arguments: this observable object and the arg argument.

Parameters

arg:Object: any object.

setChanged	back to summary
protected synchronized void setChanged() Marks this `Observable` object as having been changed; the `hasChanged` method will now return `true`.

public Class Observable

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Method Summary

Field Detail

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Method Detail