Observable


1   /*
2    * @(#)Observable.java  1.38 04/01/12
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.util;
9   
10  /**
11   * This class represents an observable object, or "data"
12   * in the model-view paradigm. It can be subclassed to represent an 
13   * object that the application wants to have observed. 
14   * <p>
15   * An observable object can have one or more observers. An observer 
16   * may be any object that implements interface <tt>Observer</tt>. After an 
17   * observable instance changes, an application calling the 
18   * <code>Observable</code>'s <code>notifyObservers</code> method  
19   * causes all of its observers to be notified of the change by a call 
20   * to their <code>update</code> method. 
21   * <p>
22   * The order in which notifications will be delivered is unspecified.  
23   * The default implementation provided in the Observable class will
24   * notify Observers in the order in which they registered interest, but 
25   * subclasses may change this order, use no guaranteed order, deliver 
26   * notifications on separate threads, or may guarantee that their
27   * subclass follows this order, as they choose.
28   * <p>
29   * Note that this notification mechanism is has nothing to do with threads 
30   * and is completely separate from the <tt>wait</tt> and <tt>notify</tt> 
31   * mechanism of class <tt>Object</tt>.
32   * <p>
33   * When an observable object is newly created, its set of observers is 
34   * empty. Two observers are considered the same if and only if the 
35   * <tt>equals</tt> method returns true for them.
36   *
37   * @author  Chris Warth
38   * @version 1.38, 01/12/04
39   * @see     java.util.Observable#notifyObservers()
40   * @see     java.util.Observable#notifyObservers(java.lang.Object)
41   * @see     java.util.Observer
42   * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
43   * @since   JDK1.0
44   */
45  public class Observable {
46      private boolean changed = false;
47      private Vector   obs;
48     
49      /** Construct an Observable with zero Observers. */
50  
51      public Observable() {
52      obs = new Vector  ();
53      }
54  
55      /**
56       * Adds an observer to the set of observers for this object, provided 
57       * that it is not the same as some observer already in the set. 
58       * The order in which notifications will be delivered to multiple 
59       * observers is not specified. See the class comment.
60       *
61       * @param   o   an observer to be added.
62       * @throws NullPointerException   if the parameter o is null.
63       */
64      public synchronized void addObserver(Observer   o) {
65          if (o == null)
66              throw new NullPointerException  ();
67      if (!obs.contains(o)) {
68          obs.addElement(o);
69      }
70      }
71  
72      /**
73       * Deletes an observer from the set of observers of this object. 
74       * Passing <CODE>null</CODE> to this method will have no effect.
75       * @param   o   the observer to be deleted.
76       */
77      public synchronized void deleteObserver(Observer   o) {
78          obs.removeElement(o);
79      }
80  
81      /**
82       * If this object has changed, as indicated by the 
83       * <code>hasChanged</code> method, then notify all of its observers 
84       * and then call the <code>clearChanged</code> method to 
85       * indicate that this object has no longer changed. 
86       * <p>
87       * Each observer has its <code>update</code> method called with two
88       * arguments: this observable object and <code>null</code>. In other 
89       * words, this method is equivalent to:
90       * <blockquote><tt>
91       * notifyObservers(null)</tt></blockquote>
92       *
93       * @see     java.util.Observable#clearChanged()
94       * @see     java.util.Observable#hasChanged()
95       * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
96       */
97      public void notifyObservers() {
98      notifyObservers(null);
99      }
100 
101     /**
102      * If this object has changed, as indicated by the 
103      * <code>hasChanged</code> method, then notify all of its observers 
104      * and then call the <code>clearChanged</code> method to indicate 
105      * that this object has no longer changed. 
106      * <p>
107      * Each observer has its <code>update</code> method called with two
108      * arguments: this observable object and the <code>arg</code> argument.
109      *
110      * @param   arg   any object.
111      * @see     java.util.Observable#clearChanged()
112      * @see     java.util.Observable#hasChanged()
113      * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
114      */
115     public void notifyObservers(Object   arg) {
116     /*
117          * a temporary array buffer, used as a snapshot of the state of
118          * current Observers.
119          */
120         Object  [] arrLocal;
121 
122     synchronized (this) {
123         /* We don't want the Observer doing callbacks into
124          * arbitrary code while holding its own Monitor.
125          * The code where we extract each Observable from 
126          * the Vector and store the state of the Observer
127          * needs synchronization, but notifying observers
128          * does not (should not).  The worst result of any 
129          * potential race-condition here is that:
130          * 1) a newly-added Observer will miss a
131          *   notification in progress
132          * 2) a recently unregistered Observer will be
133          *   wrongly notified when it doesn't care
134          */
135         if (!changed)
136                 return;
137             arrLocal = obs.toArray();
138             clearChanged();
139         }
140 
141         for (int i = arrLocal.length-1; i>=0; i--)
142             ((Observer  )arrLocal[i]).update(this, arg);
143     }
144 
145     /**
146      * Clears the observer list so that this object no longer has any observers.
147      */
148     public synchronized void deleteObservers() {
149     obs.removeAllElements();
150     }
151 
152     /**
153      * Marks this <tt>Observable</tt> object as having been changed; the 
154      * <tt>hasChanged</tt> method will now return <tt>true</tt>.
155      */
156     protected synchronized void setChanged() {
157     changed = true;
158     }
159 
160     /**
161      * Indicates that this object has no longer changed, or that it has 
162      * already notified all of its observers of its most recent change, 
163      * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>. 
164      * This method is called automatically by the 
165      * <code>notifyObservers</code> methods. 
166      *
167      * @see     java.util.Observable#notifyObservers()
168      * @see     java.util.Observable#notifyObservers(java.lang.Object)
169      */
170     protected synchronized void clearChanged() {
171     changed = false;
172     }
173 
174     /**
175      * Tests if this object has changed. 
176      *
177      * @return  <code>true</code> if and only if the <code>setChanged</code> 
178      *          method has been called more recently than the 
179      *          <code>clearChanged</code> method on this object; 
180      *          <code>false</code> otherwise.
181      * @see     java.util.Observable#clearChanged()
182      * @see     java.util.Observable#setChanged()
183      */
184     public synchronized boolean hasChanged() {
185     return changed;
186     }
187 
188     /**
189      * Returns the number of observers of this <tt>Observable</tt> object.
190      *
191      * @return  the number of observers of this object.
192      */
193     public synchronized int countObservers() {
194     return obs.size();
195     }
196 }
197
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