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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: JDK1.0
/**
* 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 <tt>Observer</tt>. After an
* observable instance changes, an application calling the
* <code>Observable</code>'s <code>notifyObservers</code> method
* causes all of its observers to be notified of the change by a call
* to their <code>update</code> 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 <tt>wait</tt> and <tt>notify</tt>
* mechanism of class <tt>Object</tt>.
* <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
* <tt>equals</tt> 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 JDK1.0
*/
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</CODE> 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: - clearChanged.clearChanged()
- hasChanged()
- Observer.update(Observable, Object)
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and <code>null</code>. In other
* words, this method is equivalent to:
* <blockquote><tt>
* notifyObservers(null)</tt></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</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and the <code>arg</code> 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 <tt>Observable</tt> object as having been changed; the
* <tt>hasChanged</tt> method will now return <tt>true</tt>.
*/
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: - notifyObservers.notifyObservers()
- notifyObservers(Object)
/**
* 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 <tt>hasChanged</tt> method will now return <tt>false</tt>.
* This method is called automatically by the
* <code>notifyObservers</code> 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</code> if and only if the <code>setChanged</code>
* method has been called more recently than the
* <code>clearChanged</code> method on this object;
* <code>false</code> 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 <tt>Observable</tt> object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
}