libjava/java/util/Observable.java

   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., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 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 }