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    1   /*
    2    * Copyright (c) 1994, 2004, 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 is 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   }

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