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:     observers.add(observer);
  81:   }
  82: 
  83:   /**
  84:    * Reset this Observable's state to unchanged. This is called automatically
  85:    * by <code>notifyObservers</code> once all observers have been notified.
  86:    *
  87:    * @see #notifyObservers()
  88:    */
  89:   protected synchronized void clearChanged()
  90:   {
  91:     changed = false;
  92:   }
  93: 
  94:   /**
  95:    * Returns the number of observers for this object.
  96:    *
  97:    * @return number of Observers for this
  98:    */
  99:   public synchronized int countObservers()
 100:   {
 101:     return observers.size();
 102:   }
 103: 
 104:   /**
 105:    * Deletes an Observer of this Observable.
 106:    *
 107:    * @param victim Observer to delete
 108:    */
 109:   public synchronized void deleteObserver(Observer victim)
 110:   {
 111:     observers.remove(victim);
 112:   }
 113: 
 114:   /**
 115:    * Deletes all Observers of this Observable.
 116:    */
 117:   public synchronized void deleteObservers()
 118:   {
 119:     observers.clear();
 120:   }
 121: 
 122:   /**
 123:    * True if <code>setChanged</code> has been called more recently than
 124:    * <code>clearChanged</code>.
 125:    *
 126:    * @return whether or not this Observable has changed
 127:    */
 128:   public synchronized boolean hasChanged()
 129:   {
 130:     return changed;
 131:   }
 132: 
 133:   /**
 134:    * If the Observable has actually changed then tell all Observers about it,
 135:    * then reset state to unchanged.
 136:    *
 137:    * @see #notifyObservers(Object)
 138:    * @see Observer#update(Observable, Object)
 139:    */
 140:   public void notifyObservers()
 141:   {
 142:     notifyObservers(null);
 143:   }
 144: 
 145:   /**
 146:    * If the Observable has actually changed then tell all Observers about it,
 147:    * then reset state to unchanged. Note that though the order of
 148:    * notification is unspecified in subclasses, in Observable it is in the
 149:    * order of registration.
 150:    *
 151:    * @param obj argument to Observer's update method
 152:    * @see Observer#update(Observable, Object)
 153:    */
 154:   public void notifyObservers(Object obj)
 155:   {
 156:     if (! hasChanged())
 157:       return;
 158:     // Create clone inside monitor, as that is relatively fast and still
 159:     // important to keep threadsafe, but update observers outside of the
 160:     // lock since update() can call arbitrary code.
 161:     Set s;
 162:     synchronized (this)
 163:       {
 164:         s = (Set) observers.clone();
 165:       }
 166:     int i = s.size();
 167:     Iterator iter = s.iterator();
 168:     while (--i >= 0)
 169:       ((Observer) iter.next()).update(this, obj);
 170:     clearChanged();
 171:   }
 172: 
 173:   /**
 174:    * Marks this Observable as having changed.
 175:    */
 176:   protected synchronized void setChanged()
 177:   {
 178:     changed = true;
 179:   }
 180: }