1 /*
   2  * Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 
  28 /**
  29  * This class represents an observable object, or "data"
  30  * in the model-view paradigm. It can be subclassed to represent an
  31  * object that the application wants to have observed.
  32  * <p>
  33  * An observable object can have one or more observers. An observer
  34  * may be any object that implements interface <tt>Observer</tt>. After an
  35  * observable instance changes, an application calling the
  36  * <code>Observable</code>'s <code>notifyObservers</code> method
  37  * causes all of its observers to be notified of the change by a call
  38  * to their <code>update</code> method.
  39  * <p>
  40  * The order in which notifications will be delivered is unspecified.
  41  * The default implementation provided in the Observable class will
  42  * notify Observers in the order in which they registered interest, but
  43  * subclasses may change this order, use no guaranteed order, deliver
  44  * notifications on separate threads, or may guarantee that their
  45  * subclass follows this order, as they choose.
  46  * <p>
  47  * Note that this notification mechanism has nothing to do with threads
  48  * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
  49  * mechanism of class <tt>Object</tt>.
  50  * <p>
  51  * When an observable object is newly created, its set of observers is
  52  * empty. Two observers are considered the same if and only if the
  53  * <tt>equals</tt> method returns true for them.
  54  *
  55  * @author  Chris Warth
  56  * @see     java.util.Observable#notifyObservers()
  57  * @see     java.util.Observable#notifyObservers(java.lang.Object)
  58  * @see     java.util.Observer
  59  * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
  60  * @since   JDK1.0
  61  */
  62 public class Observable {
  63     private boolean changed = false;
  64     private Vector obs;
  65 
  66     /** Construct an Observable with zero Observers. */
  67 
  68     public Observable() {
  69         obs = new Vector();
  70     }
  71 
  72     /**
  73      * Adds an observer to the set of observers for this object, provided
  74      * that it is not the same as some observer already in the set.
  75      * The order in which notifications will be delivered to multiple
  76      * observers is not specified. See the class comment.
  77      *
  78      * @param   o   an observer to be added.
  79      * @throws NullPointerException   if the parameter o is null.
  80      */
  81     public synchronized void addObserver(Observer o) {
  82         if (o == null)
  83             throw new NullPointerException();
  84         if (!obs.contains(o)) {
  85             obs.addElement(o);
  86         }
  87     }
  88 
  89     /**
  90      * Deletes an observer from the set of observers of this object.
  91      * Passing <CODE>null</CODE> to this method will have no effect.
  92      * @param   o   the observer to be deleted.
  93      */
  94     public synchronized void deleteObserver(Observer o) {
  95         obs.removeElement(o);
  96     }
  97 
  98     /**
  99      * If this object has changed, as indicated by the
 100      * <code>hasChanged</code> method, then notify all of its observers
 101      * and then call the <code>clearChanged</code> method to
 102      * indicate that this object has no longer changed.
 103      * <p>
 104      * Each observer has its <code>update</code> method called with two
 105      * arguments: this observable object and <code>null</code>. In other
 106      * words, this method is equivalent to:
 107      * <blockquote><tt>
 108      * notifyObservers(null)</tt></blockquote>
 109      *
 110      * @see     java.util.Observable#clearChanged()
 111      * @see     java.util.Observable#hasChanged()
 112      * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
 113      */
 114     public void notifyObservers() {
 115         notifyObservers(null);
 116     }
 117 
 118     /**
 119      * If this object has changed, as indicated by the
 120      * <code>hasChanged</code> method, then notify all of its observers
 121      * and then call the <code>clearChanged</code> method to indicate
 122      * that this object has no longer changed.
 123      * <p>
 124      * Each observer has its <code>update</code> method called with two
 125      * arguments: this observable object and the <code>arg</code> argument.
 126      *
 127      * @param   arg   any object.
 128      * @see     java.util.Observable#clearChanged()
 129      * @see     java.util.Observable#hasChanged()
 130      * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
 131      */
 132     public void notifyObservers(Object arg) {
 133         /*
 134          * a temporary array buffer, used as a snapshot of the state of
 135          * current Observers.
 136          */
 137         Object[] arrLocal;
 138 
 139         synchronized (this) {
 140             /* We don't want the Observer doing callbacks into
 141              * arbitrary code while holding its own Monitor.
 142              * The code where we extract each Observable from
 143              * the Vector and store the state of the Observer
 144              * needs synchronization, but notifying observers
 145              * does not (should not).  The worst result of any
 146              * potential race-condition here is that:
 147              * 1) a newly-added Observer will miss a
 148              *   notification in progress
 149              * 2) a recently unregistered Observer will be
 150              *   wrongly notified when it doesn't care
 151              */
 152             if (!changed)
 153                 return;
 154             arrLocal = obs.toArray();
 155             clearChanged();
 156         }
 157 
 158         for (int i = arrLocal.length-1; i>=0; i--)
 159             ((Observer)arrLocal[i]).update(this, arg);
 160     }
 161 
 162     /**
 163      * Clears the observer list so that this object no longer has any observers.
 164      */
 165     public synchronized void deleteObservers() {
 166         obs.removeAllElements();
 167     }
 168 
 169     /**
 170      * Marks this <tt>Observable</tt> object as having been changed; the
 171      * <tt>hasChanged</tt> method will now return <tt>true</tt>.
 172      */
 173     protected synchronized void setChanged() {
 174         changed = true;
 175     }
 176 
 177     /**
 178      * Indicates that this object has no longer changed, or that it has
 179      * already notified all of its observers of its most recent change,
 180      * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
 181      * This method is called automatically by the
 182      * <code>notifyObservers</code> methods.
 183      *
 184      * @see     java.util.Observable#notifyObservers()
 185      * @see     java.util.Observable#notifyObservers(java.lang.Object)
 186      */
 187     protected synchronized void clearChanged() {
 188         changed = false;
 189     }
 190 
 191     /**
 192      * Tests if this object has changed.
 193      *
 194      * @return  <code>true</code> if and only if the <code>setChanged</code>
 195      *          method has been called more recently than the
 196      *          <code>clearChanged</code> method on this object;
 197      *          <code>false</code> otherwise.
 198      * @see     java.util.Observable#clearChanged()
 199      * @see     java.util.Observable#setChanged()
 200      */
 201     public synchronized boolean hasChanged() {
 202         return changed;
 203     }
 204 
 205     /**
 206      * Returns the number of observers of this <tt>Observable</tt> object.
 207      *
 208      * @return  the number of observers of this object.
 209      */
 210     public synchronized int countObservers() {
 211         return obs.size();
 212     }
 213 }