Source for java.util.Observable

   1: /* Observable.java -- an object to be observed
   2:    Copyright (C) 1999, 2000, 2001, 2002, 2005  Free Software Foundation, Inc.
   3: 
   4: This file is part of GNU Classpath.
   5: 
   6: GNU Classpath is free software; you can redistribute it and/or modify
   7: it under the terms of the GNU General Public License as published by
   8: the Free Software Foundation; either version 2, or (at your option)
   9: any later version.
  10: 
  11: GNU Classpath is distributed in the hope that it will be useful, but
  12: WITHOUT ANY WARRANTY; without even the implied warranty of
  13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14: General Public License for more details.
  15: 
  16: You should have received a copy of the GNU General Public License
  17: along with GNU Classpath; see the file COPYING.  If not, write to the
  18: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 USA.
  20: 
  21: Linking this library statically or dynamically with other modules is
  22: making a combined work based on this library.  Thus, the terms and
  23: conditions of the GNU General Public License cover the whole
  24: combination.
  25: 
  26: As a special exception, the copyright holders of this library give you
  27: permission to link this library with independent modules to produce an
  28: executable, regardless of the license terms of these independent
  29: modules, and to copy and distribute the resulting executable under
  30: terms of your choice, provided that you also meet, for each linked
  31: independent module, the terms and conditions of the license of that
  32: module.  An independent module is a module which is not derived from
  33: or based on this library.  If you modify this library, you may extend
  34: this exception to your version of the library, but you are not
  35: obligated to do so.  If you do not wish to do so, delete this
  36: exception statement from your version. */
  37: 
  38: 
  39: package java.util;
  40: 
  41: /**
  42:  * This class represents an object which is observable.  Other objects may
  43:  * register their intent to be notified when this object changes; and when
  44:  * this object does change, it will trigger the <code>update</code> method
  45:  * of each observer.
  46:  *
  47:  * Note that the <code>notifyObservers()</code> method of this class is
  48:  * unrelated to the <code>notify()</code> of Object.
  49:  *
  50:  * @author Warren Levy (warrenl@cygnus.com)
  51:  * @author Eric Blake (ebb9@email.byu.edu)
  52:  * @see Observer
  53:  * @status updated to 1.4
  54:  */
  55: public class Observable
  56: {
  57:   /** Tracks whether this object has changed. */
  58:   private boolean changed;
  59: 
  60:   /* List of the Observers registered as interested in this Observable. */
  61:   private LinkedHashSet observers;
  62: 
  63:   /**
  64:    * Constructs an Observable with zero Observers.
  65:    */
  66:   public Observable()
  67:   {
  68:     observers = new LinkedHashSet();
  69:   }
  70: 
  71:   /**
  72:    * Adds an Observer. If the observer was already added this method does
  73:    * nothing.
  74:    *
  75:    * @param observer Observer to add
  76:    * @throws NullPointerException if observer is null
  77:    */
  78:   public synchronized void addObserver(Observer observer)
  79:   {
  80:     if (observer == null)
  81:       throw new NullPointerException("can't add null observer");
  82:     observers.add(observer);
  83:   }
  84: 
  85:   /**
  86:    * Reset this Observable's state to unchanged. This is called automatically
  87:    * by <code>notifyObservers</code> once all observers have been notified.
  88:    *
  89:    * @see #notifyObservers()
  90:    */
  91:   protected synchronized void clearChanged()
  92:   {
  93:     changed = false;
  94:   }
  95: 
  96:   /**
  97:    * Returns the number of observers for this object.
  98:    *
  99:    * @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: }