1 /*
2 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.util;
27
28 /**
29 * This class represents an observable object, or "data"
30 * in the model-view paradigm. It can be subclassed to represent an
31 * object that the application wants to have observed.
32 * <p>
33 * An observable object can have one or more observers. An observer
34 * may be any object that implements interface <tt>Observer</tt>. After an
35 * observable instance changes, an application calling the
36 * <code>Observable</code>'s <code>notifyObservers</code> method
37 * causes all of its observers to be notified of the change by a call
38 * to their <code>update</code> method.
39 * <p>
40 * The order in which notifications will be delivered is unspecified.
41 * The default implementation provided in the Observable class will
42 * notify Observers in the order in which they registered interest, but
43 * subclasses may change this order, use no guaranteed order, deliver
44 * notifications on separate threads, or may guarantee that their
45 * subclass follows this order, as they choose.
46 * <p>
47 * Note that this notification mechanism has nothing to do with threads
48 * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
49 * mechanism of class <tt>Object</tt>.
50 * <p>
51 * When an observable object is newly created, its set of observers is
52 * empty. Two observers are considered the same if and only if the
53 * <tt>equals</tt> method returns true for them.
54 *
55 * @author Chris Warth
56 * @see java.util.Observable#notifyObservers()
57 * @see java.util.Observable#notifyObservers(java.lang.Object)
58 * @see java.util.Observer
59 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
60 * @since JDK1.0
61 */
62 public class Observable {
63 private boolean changed = false;
64 private Vector<Observer> obs;
65
66 /** Construct an Observable with zero Observers. */
67
68 public Observable() {
69 obs = new Vector<>();
70 }
71
72 /**
73 * Adds an observer to the set of observers for this object, provided
74 * that it is not the same as some observer already in the set.
75 * The order in which notifications will be delivered to multiple
76 * observers is not specified. See the class comment.
77 *
78 * @param o an observer to be added.
79 * @throws NullPointerException if the parameter o is null.
80 */
81 public synchronized void addObserver(Observer o) {
82 if (o == null)
83 throw new NullPointerException();
84 if (!obs.contains(o)) {
85 obs.addElement(o);
86 }
87 }
88
89 /**
90 * Deletes an observer from the set of observers of this object.
91 * Passing <CODE>null</CODE> to this method will have no effect.
92 * @param o the observer to be deleted.
93 */
94 public synchronized void deleteObserver(Observer o) {
95 obs.removeElement(o);
96 }
97
98 /**
99 * If this object has changed, as indicated by the
100 * <code>hasChanged</code> method, then notify all of its observers
101 * and then call the <code>clearChanged</code> method to
102 * indicate that this object has no longer changed.
103 * <p>
104 * Each observer has its <code>update</code> method called with two
105 * arguments: this observable object and <code>null</code>. In other
106 * words, this method is equivalent to:
107 * <blockquote><tt>
108 * notifyObservers(null)</tt></blockquote>
109 *
110 * @see java.util.Observable#clearChanged()
111 * @see java.util.Observable#hasChanged()
112 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
113 */
114 public void notifyObservers() {
115 notifyObservers(null);
116 }
117
118 /**
119 * If this object has changed, as indicated by the
120 * <code>hasChanged</code> method, then notify all of its observers
121 * and then call the <code>clearChanged</code> method to indicate
122 * that this object has no longer changed.
123 * <p>
124 * Each observer has its <code>update</code> method called with two
125 * arguments: this observable object and the <code>arg</code> argument.
126 *
127 * @param arg any object.
128 * @see java.util.Observable#clearChanged()
129 * @see java.util.Observable#hasChanged()
130 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
131 */
132 public void notifyObservers(Object arg) {
133 /*
134 * a temporary array buffer, used as a snapshot of the state of
135 * current Observers.
136 */
137 Object[] arrLocal;
138
139 synchronized (this) {
140 /* We don't want the Observer doing callbacks into
141 * arbitrary code while holding its own Monitor.
142 * The code where we extract each Observable from
143 * the Vector and store the state of the Observer
144 * needs synchronization, but notifying observers
145 * does not (should not). The worst result of any
146 * potential race-condition here is that:
147 * 1) a newly-added Observer will miss a
148 * notification in progress
149 * 2) a recently unregistered Observer will be
150 * wrongly notified when it doesn't care
151 */
152 if (!changed)
153 return;
154 arrLocal = obs.toArray();
155 clearChanged();
156 }
157
158 for (int i = arrLocal.length-1; i>=0; i--)
159 ((Observer)arrLocal[i]).update(this, arg);
160 }
161
162 /**
163 * Clears the observer list so that this object no longer has any observers.
164 */
165 public synchronized void deleteObservers() {
166 obs.removeAllElements();
167 }
168
169 /**
170 * Marks this <tt>Observable</tt> object as having been changed; the
171 * <tt>hasChanged</tt> method will now return <tt>true</tt>.
172 */
173 protected synchronized void setChanged() {
174 changed = true;
175 }
176
177 /**
178 * Indicates that this object has no longer changed, or that it has
179 * already notified all of its observers of its most recent change,
180 * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
181 * This method is called automatically by the
182 * <code>notifyObservers</code> methods.
183 *
184 * @see java.util.Observable#notifyObservers()
185 * @see java.util.Observable#notifyObservers(java.lang.Object)
186 */
187 protected synchronized void clearChanged() {
188 changed = false;
189 }
190
191 /**
192 * Tests if this object has changed.
193 *
194 * @return <code>true</code> if and only if the <code>setChanged</code>
195 * method has been called more recently than the
196 * <code>clearChanged</code> method on this object;
197 * <code>false</code> otherwise.
198 * @see java.util.Observable#clearChanged()
199 * @see java.util.Observable#setChanged()
200 */
201 public synchronized boolean hasChanged() {
202 return changed;
203 }
204
205 /**
206 * Returns the number of observers of this <tt>Observable</tt> object.
207 *
208 * @return the number of observers of this object.
209 */
210 public synchronized int countObservers() {
211 return obs.size();
212 }
213 }