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package java.util;
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.
Author: Chris Warth See Also: Since: 1.0 Deprecated: This class and the Observer interface have been deprecated. The event model supported by Observer and Observable is quite limited, the order of notifications delivered by Observable is unspecified, and state changes are not in one-for-one correspondence with notifications. For a richer event model, consider using the beans package. For reliable and ordered messaging among threads, consider using one of the concurrent data structures in the concurrent package. For reactive streams style programming, see the Flow API.
/**
* 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.
* <p>
* An observable object can have one or more observers. An observer
* may be any object that implements interface {@code Observer}. After an
* observable instance changes, an application calling the
* {@code Observable}'s {@code notifyObservers} method
* causes all of its observers to be notified of the change by a call
* to their {@code update} method.
* <p>
* 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.
* <p>
* Note that this notification mechanism has nothing to do with threads
* and is completely separate from the {@code wait} and {@code notify}
* mechanism of class {@code Object}.
* <p>
* When an observable object is newly created, its set of observers is
* empty. Two observers are considered the same if and only if the
* {@code equals} method returns true for them.
*
* @author Chris Warth
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
* @see java.util.Observer
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
* @since 1.0
*
* @deprecated
* This class and the {@link Observer} interface have been deprecated.
* The event model supported by {@code Observer} and {@code Observable}
* is quite limited, the order of notifications delivered by
* {@code Observable} is unspecified, and state changes are not in
* one-for-one correspondence with notifications.
* For a richer event model, consider using the
* {@link java.beans} package. For reliable and ordered
* messaging among threads, consider using one of the concurrent data
* structures in the {@link java.util.concurrent} package.
* For reactive streams style programming, see the
* {@link java.util.concurrent.Flow} API.
*/
@Deprecated(since="9")
public class Observable {
private boolean changed = false;
private Vector<Observer> obs;
Construct an Observable with zero Observers. /** Construct an Observable with zero Observers. */
public Observable() {
obs = new Vector<>();
}
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.
Params: - o – an observer to be added.
Throws: - NullPointerException – if the parameter o is null.
/**
* 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.
*
* @param o an observer to be added.
* @throws NullPointerException if the parameter o is null.
*/
public synchronized void addObserver(Observer o) {
if (o == null)
throw new NullPointerException();
if (!obs.contains(o)) {
obs.addElement(o);
}
}
Deletes an observer from the set of observers of this object. Passing null to this method will have no effect. Params: - o – the observer to be deleted.
/**
* Deletes an observer from the set of observers of this object.
* Passing {@code null} to this method will have no effect.
* @param o the observer to be deleted.
*/
public synchronized void deleteObserver(Observer o) {
obs.removeElement(o);
}
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:
/**
* If this object has changed, as indicated by the
* {@code hasChanged} method, then notify all of its observers
* and then call the {@code clearChanged} method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its {@code update} method called with two
* arguments: this observable object and {@code null}. In other
* words, this method is equivalent to:
* <blockquote>{@code
* notifyObservers(null)}</blockquote>
*
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers() {
notifyObservers(null);
}
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.
Params: - arg – any object.
See Also:
/**
* If this object has changed, as indicated by the
* {@code hasChanged} method, then notify all of its observers
* and then call the {@code clearChanged} method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its {@code update} method called with two
* arguments: this observable object and the {@code arg} argument.
*
* @param arg any object.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers(Object arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
Object[] arrLocal;
synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not). The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
* notification in progress
* 2) a recently unregistered Observer will be
* wrongly notified when it doesn't care
*/
if (!changed)
return;
arrLocal = obs.toArray();
clearChanged();
}
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
Clears the observer list so that this object no longer has any observers.
/**
* Clears the observer list so that this object no longer has any observers.
*/
public synchronized void deleteObservers() {
obs.removeAllElements();
}
Marks this Observable object as having been changed; the hasChanged method will now return true. /**
* Marks this {@code Observable} object as having been changed; the
* {@code hasChanged} method will now return {@code true}.
*/
protected synchronized void setChanged() {
changed = true;
}
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:
/**
* 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 {@code hasChanged} method will now return {@code false}.
* This method is called automatically by the
* {@code notifyObservers} methods.
*
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
*/
protected synchronized void clearChanged() {
changed = false;
}
Tests if this object has changed.
See Also: Returns: true if and only if the setChanged method has been called more recently than the clearChanged method on this object; false otherwise.
/**
* Tests if this object has changed.
*
* @return {@code true} if and only if the {@code setChanged}
* method has been called more recently than the
* {@code clearChanged} method on this object;
* {@code false} otherwise.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#setChanged()
*/
public synchronized boolean hasChanged() {
return changed;
}
Returns the number of observers of this Observable object. Returns: the number of observers of this object.
/**
* Returns the number of observers of this {@code Observable} object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
}