001    /* Observable.java -- an object to be observed
002       Copyright (C) 1999, 2000, 2001, 2002, 2005  Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010    
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    
039    package java.util;
040    
041    /**
042     * This class represents an object which is observable.  Other objects may
043     * register their intent to be notified when this object changes; and when
044     * this object does change, it will trigger the <code>update</code> method
045     * of each observer.
046     *
047     * Note that the <code>notifyObservers()</code> method of this class is
048     * unrelated to the <code>notify()</code> of Object.
049     *
050     * @author Warren Levy (warrenl@cygnus.com)
051     * @author Eric Blake (ebb9@email.byu.edu)
052     * @see Observer
053     * @status updated to 1.4
054     */
055    public class Observable
056    {
057      /** Tracks whether this object has changed. */
058      private boolean changed;
059    
060      /* List of the Observers registered as interested in this Observable. */
061      private LinkedHashSet observers;
062    
063      /**
064       * Constructs an Observable with zero Observers.
065       */
066      public Observable()
067      {
068        observers = new LinkedHashSet();
069      }
070    
071      /**
072       * Adds an Observer. If the observer was already added this method does
073       * nothing.
074       *
075       * @param observer Observer to add
076       * @throws NullPointerException if observer is null
077       */
078      public synchronized void addObserver(Observer observer)
079      {
080        if (observer == null)
081          throw new NullPointerException("can't add null observer");
082        observers.add(observer);
083      }
084    
085      /**
086       * Reset this Observable's state to unchanged. This is called automatically
087       * by <code>notifyObservers</code> once all observers have been notified.
088       *
089       * @see #notifyObservers()
090       */
091      protected synchronized void clearChanged()
092      {
093        changed = false;
094      }
095    
096      /**
097       * Returns the number of observers for this object.
098       *
099       * @return number of Observers for this
100       */
101      public synchronized int countObservers()
102      {
103        return observers.size();
104      }
105    
106      /**
107       * Deletes an Observer of this Observable.
108       *
109       * @param victim Observer to delete
110       */
111      public synchronized void deleteObserver(Observer victim)
112      {
113        observers.remove(victim);
114      }
115    
116      /**
117       * Deletes all Observers of this Observable.
118       */
119      public synchronized void deleteObservers()
120      {
121        observers.clear();
122      }
123    
124      /**
125       * True if <code>setChanged</code> has been called more recently than
126       * <code>clearChanged</code>.
127       *
128       * @return whether or not this Observable has changed
129       */
130      public synchronized boolean hasChanged()
131      {
132        return changed;
133      }
134    
135      /**
136       * If the Observable has actually changed then tell all Observers about it,
137       * then reset state to unchanged.
138       *
139       * @see #notifyObservers(Object)
140       * @see Observer#update(Observable, Object)
141       */
142      public void notifyObservers()
143      {
144        notifyObservers(null);
145      }
146    
147      /**
148       * If the Observable has actually changed then tell all Observers about it,
149       * then reset state to unchanged. Note that though the order of
150       * notification is unspecified in subclasses, in Observable it is in the
151       * order of registration.
152       *
153       * @param obj argument to Observer's update method
154       * @see Observer#update(Observable, Object)
155       */
156      public void notifyObservers(Object obj)
157      {
158        if (! hasChanged())
159          return;
160        // Create clone inside monitor, as that is relatively fast and still
161        // important to keep threadsafe, but update observers outside of the
162        // lock since update() can call arbitrary code.
163        Set s;
164        synchronized (this)
165          {
166            s = (Set) observers.clone();
167          }
168        int i = s.size();
169        Iterator iter = s.iterator();
170        while (--i >= 0)
171          ((Observer) iter.next()).update(this, obj);
172        clearChanged();
173      }
174    
175      /**
176       * Marks this Observable as having changed.
177       */
178      protected synchronized void setChanged()
179      {
180        changed = true;
181      }
182    }