JPanel


1   /*
2    * @(#)JPanel.java  1.46 03/12/19
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   package javax.swing;
8   
9   import java.awt.*;
10  
11  import javax.swing.plaf.*;
12  import javax.accessibility.*;
13  
14  import java.io.Serializable  ;
15  import java.io.ObjectOutputStream  ;
16  import java.io.ObjectInputStream  ;
17  import java.io.IOException  ;
18  
19  
20  /**
21   * <code>JPanel</code> is a generic lightweight container.
22   * For examples and task-oriented documentation for JPanel, see
23   * <a
24   href="http://java.sun.com/docs/books/tutorial/uiswing/components/panel.html">How to Use Panels</a>,
25   * a section in <em>The Java Tutorial</em>.
26   * <p>
27   * <strong>Warning:</strong>
28   * Serialized objects of this class will not be compatible with
29   * future Swing releases. The current serialization support is
30   * appropriate for short term storage or RMI between applications running
31   * the same version of Swing.  As of 1.4, support for long term storage
32   * of all JavaBeans<sup><font size="-2">TM</font></sup>
33   * has been added to the <code>java.beans</code> package.
34   * Please see {@link java.beans.XMLEncoder}.
35   *
36   * @beaninfo
37   * description: A generic lightweight container.
38   * 
39   * @version 1.46 12/19/03
40   * @author Arnaud Weber
41   * @author Steve Wilson
42   */
43  public class JPanel extends JComponent   implements Accessible
44  {
45      /**
46       * @see #getUIClassID
47       * @see #readObject
48       */
49      private static final String   uiClassID = "PanelUI";
50  
51      /**
52       * Creates a new JPanel with the specified layout manager and buffering
53       * strategy.
54       *
55       * @param layout  the LayoutManager to use
56       * @param isDoubleBuffered  a boolean, true for double-buffering, which
57       *        uses additional memory space to achieve fast, flicker-free 
58       *        updates
59       */
60      public JPanel(LayoutManager layout, boolean isDoubleBuffered) {
61          setLayout(layout);
62          setDoubleBuffered(isDoubleBuffered);
63          setUIProperty("opaque", Boolean.TRUE);
64          updateUI();
65      }
66  
67      /**
68       * Create a new buffered JPanel with the specified layout manager
69       *
70       * @param layout  the LayoutManager to use
71       */
72      public JPanel(LayoutManager layout) {
73          this(layout, true);
74      }
75  
76      /**
77       * Creates a new <code>JPanel</code> with <code>FlowLayout</code>
78       * and the specified buffering strategy.
79       * If <code>isDoubleBuffered</code> is true, the <code>JPanel</code>
80       * will use a double buffer.
81       *
82       * @param isDoubleBuffered  a boolean, true for double-buffering, which
83       *        uses additional memory space to achieve fast, flicker-free 
84       *        updates
85       */
86      public JPanel(boolean isDoubleBuffered) {
87          this(new FlowLayout(), isDoubleBuffered);
88      }
89  
90      /**
91       * Creates a new <code>JPanel</code> with a double buffer
92       * and a flow layout.
93       */
94      public JPanel() {
95          this(true);
96      }
97  
98      /**
99       * Resets the UI property with a value from the current look and feel.
100      *
101      * @see JComponent#updateUI
102      */
103     public void updateUI() {
104         setUI((PanelUI)UIManager.getUI(this));
105     }
106 
107     /**
108      * Returns the look and feel (L&F) object that renders this component.
109      *
110      * @return the PanelUI object that renders this component
111      * @since 1.4
112      */
113     public PanelUI getUI() {
114         return (PanelUI)ui;
115     }
116 
117 
118     /**
119      * Sets the look and feel (L&F) object that renders this component.
120      *
121      * @param ui  the PanelUI L&F object
122      * @see UIDefaults#getUI
123      * @since 1.4
124      * @beaninfo
125      *        bound: true
126      *       hidden: true
127      *    attribute: visualUpdate true
128      *  description: The UI object that implements the Component's LookAndFeel. 
129      */
130     public void setUI(PanelUI ui) {
131         super.setUI(ui);
132     }
133 
134     /**
135      * Returns a string that specifies the name of the L&F class
136      * that renders this component.
137      *
138      * @return "PanelUI"
139      * @see JComponent#getUIClassID
140      * @see UIDefaults#getUI
141      * @beaninfo
142      *        expert: true
143      *   description: A string that specifies the name of the L&F class.
144      */
145     public String   getUIClassID() {
146         return uiClassID;
147     }
148 
149 
150     /** 
151      * See readObject() and writeObject() in JComponent for more 
152      * information about serialization in Swing.
153      */
154     private void writeObject(ObjectOutputStream   s) throws IOException   {
155         s.defaultWriteObject();
156         if (getUIClassID().equals(uiClassID)) {
157             byte count = JComponent.getWriteObjCounter(this);
158             JComponent.setWriteObjCounter(this, --count);
159             if (count == 0 && ui != null) {
160                 ui.installUI(this);
161             }
162         }
163     }
164 
165 
166     /**
167      * Returns a string representation of this JPanel. This method 
168      * is intended to be used only for debugging purposes, and the 
169      * content and format of the returned string may vary between      
170      * implementations. The returned string may be empty but may not 
171      * be <code>null</code>.
172      * 
173      * @return  a string representation of this JPanel.
174      */
175     protected String   paramString() {
176     return super.paramString();
177     }
178 
179 /////////////////
180 // Accessibility support
181 ////////////////
182 
183     /**
184      * Gets the AccessibleContext associated with this JPanel. 
185      * For JPanels, the AccessibleContext takes the form of an 
186      * AccessibleJPanel. 
187      * A new AccessibleJPanel instance is created if necessary.
188      *
189      * @return an AccessibleJPanel that serves as the 
190      *         AccessibleContext of this JPanel
191      */
192     public AccessibleContext getAccessibleContext() {
193         if (accessibleContext == null) {
194             accessibleContext = new AccessibleJPanel();
195         }
196         return accessibleContext;
197     }
198 
199     /**
200      * This class implements accessibility support for the 
201      * <code>JPanel</code> class.  It provides an implementation of the 
202      * Java Accessibility API appropriate to panel user-interface 
203      * elements.
204      * <p>
205      * <strong>Warning:</strong>
206      * Serialized objects of this class will not be compatible with
207      * future Swing releases. The current serialization support is
208      * appropriate for short term storage or RMI between applications running
209      * the same version of Swing.  As of 1.4, support for long term storage
210      * of all JavaBeans<sup><font size="-2">TM</font></sup>
211      * has been added to the <code>java.beans</code> package.
212      * Please see {@link java.beans.XMLEncoder}.
213      */
214     protected class AccessibleJPanel extends AccessibleJComponent {
215 
216         /**
217          * Get the role of this object.
218          *
219          * @return an instance of AccessibleRole describing the role of the 
220          * object
221          */
222         public AccessibleRole getAccessibleRole() {
223             return AccessibleRole.PANEL;
224         }
225     }
226 }
227 
228
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