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1   /*
2    * Copyright (c) 1997, 2013, 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  package javax.swing;
26  
27  import java.awt.*;
28  
29  import javax.swing.plaf.*;
30  import javax.accessibility.*;
31  
32  import java.io.Serializable;
33  import java.io.ObjectOutputStream;
34  import java.io.ObjectInputStream;
35  import java.io.IOException;
36  
37  
38  /**
39   * <code>JPanel</code> is a generic lightweight container.
40   * For examples and task-oriented documentation for JPanel, see
41   * <a
42   href="http://docs.oracle.com/javase/tutorial/uiswing/components/panel.html">How to Use Panels</a>,
43   * a section in <em>The Java Tutorial</em>.
44   * <p>
45   * <strong>Warning:</strong> Swing is not thread safe. For more
46   * information see <a
47   * href="package-summary.html#threading">Swing's Threading
48   * Policy</a>.
49   * <p>
50   * <strong>Warning:</strong>
51   * Serialized objects of this class will not be compatible with
52   * future Swing releases. The current serialization support is
53   * appropriate for short term storage or RMI between applications running
54   * the same version of Swing.  As of 1.4, support for long term storage
55   * of all JavaBeans&trade;
56   * has been added to the <code>java.beans</code> package.
57   * Please see {@link java.beans.XMLEncoder}.
58   *
59   * @beaninfo
60   * description: A generic lightweight container.
61   *
62   * @author Arnaud Weber
63   * @author Steve Wilson
64   */
65  public class JPanel extends JComponent implements Accessible
66  {
67      /**
68       * @see #getUIClassID
69       * @see #readObject
70       */
71      private static final String uiClassID = "PanelUI";
72  
73      /**
74       * Creates a new JPanel with the specified layout manager and buffering
75       * strategy.
76       *
77       * @param layout  the LayoutManager to use
78       * @param isDoubleBuffered  a boolean, true for double-buffering, which
79       *        uses additional memory space to achieve fast, flicker-free
80       *        updates
81       */
82      public JPanel(LayoutManager layout, boolean isDoubleBuffered) {
83          setLayout(layout);
84          setDoubleBuffered(isDoubleBuffered);
85          setUIProperty("opaque", Boolean.TRUE);
86          updateUI();
87      }
88  
89      /**
90       * Create a new buffered JPanel with the specified layout manager
91       *
92       * @param layout  the LayoutManager to use
93       */
94      public JPanel(LayoutManager layout) {
95          this(layout, true);
96      }
97  
98      /**
99       * Creates a new <code>JPanel</code> with <code>FlowLayout</code>
100      * and the specified buffering strategy.
101      * If <code>isDoubleBuffered</code> is true, the <code>JPanel</code>
102      * will use a double buffer.
103      *
104      * @param isDoubleBuffered  a boolean, true for double-buffering, which
105      *        uses additional memory space to achieve fast, flicker-free
106      *        updates
107      */
108     public JPanel(boolean isDoubleBuffered) {
109         this(new FlowLayout(), isDoubleBuffered);
110     }
111 
112     /**
113      * Creates a new <code>JPanel</code> with a double buffer
114      * and a flow layout.
115      */
116     public JPanel() {
117         this(true);
118     }
119 
120     /**
121      * Resets the UI property with a value from the current look and feel.
122      *
123      * @see JComponent#updateUI
124      */
125     public void updateUI() {
126         setUI((PanelUI)UIManager.getUI(this));
127     }
128 
129     /**
130      * Returns the look and feel (L&amp;amp;F) object that renders this component.
131      *
132      * @return the PanelUI object that renders this component
133      * @since 1.4
134      */
135     public PanelUI getUI() {
136         return (PanelUI)ui;
137     }
138 
139 
140     /**
141      * Sets the look and feel (L&amp;F) object that renders this component.
142      *
143      * @param ui  the PanelUI L&amp;F object
144      * @see UIDefaults#getUI
145      * @since 1.4
146      * @beaninfo
147      *        bound: true
148      *       hidden: true
149      *    attribute: visualUpdate true
150      *  description: The UI object that implements the Component's LookAndFeel.
151      */
152     public void setUI(PanelUI ui) {
153         super.setUI(ui);
154     }
155 
156     /**
157      * Returns a string that specifies the name of the L&amp;F class
158      * that renders this component.
159      *
160      * @return "PanelUI"
161      * @see JComponent#getUIClassID
162      * @see UIDefaults#getUI
163      * @beaninfo
164      *        expert: true
165      *   description: A string that specifies the name of the L&amp;F class.
166      */
167     public String getUIClassID() {
168         return uiClassID;
169     }
170 
171 
172     /**
173      * See readObject() and writeObject() in JComponent for more
174      * information about serialization in Swing.
175      */
176     private void writeObject(ObjectOutputStream s) throws IOException {
177         s.defaultWriteObject();
178         if (getUIClassID().equals(uiClassID)) {
179             byte count = JComponent.getWriteObjCounter(this);
180             JComponent.setWriteObjCounter(this, --count);
181             if (count == 0 && ui != null) {
182                 ui.installUI(this);
183             }
184         }
185     }
186 
187 
188     /**
189      * Returns a string representation of this JPanel. This method
190      * is intended to be used only for debugging purposes, and the
191      * content and format of the returned string may vary between
192      * implementations. The returned string may be empty but may not
193      * be <code>null</code>.
194      *
195      * @return  a string representation of this JPanel.
196      */
197     protected String paramString() {
198         return super.paramString();
199     }
200 
201 /////////////////
202 // Accessibility support
203 ////////////////
204 
205     /**
206      * Gets the AccessibleContext associated with this JPanel.
207      * For JPanels, the AccessibleContext takes the form of an
208      * AccessibleJPanel.
209      * A new AccessibleJPanel instance is created if necessary.
210      *
211      * @return an AccessibleJPanel that serves as the
212      *         AccessibleContext of this JPanel
213      */
214     public AccessibleContext getAccessibleContext() {
215         if (accessibleContext == null) {
216             accessibleContext = new AccessibleJPanel();
217         }
218         return accessibleContext;
219     }
220 
221     /**
222      * This class implements accessibility support for the
223      * <code>JPanel</code> class.  It provides an implementation of the
224      * Java Accessibility API appropriate to panel user-interface
225      * elements.
226      * <p>
227      * <strong>Warning:</strong>
228      * Serialized objects of this class will not be compatible with
229      * future Swing releases. The current serialization support is
230      * appropriate for short term storage or RMI between applications running
231      * the same version of Swing.  As of 1.4, support for long term storage
232      * of all JavaBeans&trade;
233      * has been added to the <code>java.beans</code> package.
234      * Please see {@link java.beans.XMLEncoder}.
235      */
236     protected class AccessibleJPanel extends AccessibleJComponent {
237 
238         /**
239          * Get the role of this object.
240          *
241          * @return an instance of AccessibleRole describing the role of the
242          * object
243          */
244         public AccessibleRole getAccessibleRole() {
245             return AccessibleRole.PANEL;
246         }
247     }
248 }