/*

 * Copyright 1994-2004 Sun Microsystems, Inc.  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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.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 is 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 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();

    }

}