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	/*
	* Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	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();
	}
	}

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