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 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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.
 * <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. */

 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.
 *
 * @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 <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
 * <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
 * <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.
 */
 public synchronized void deleteObservers() {
 obs.removeAllElements();
 }

 /**
 * 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 <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.
 *
 * @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 <tt>Observable</tt> object.
 *
 * @return the number of observers of this object.
 */
 public synchronized int countObservers() {
 return obs.size();
 }
}