1 /*
   2  * Copyright (c) 1997, 2015, 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 import java.beans.JavaBean;
  29 import java.beans.BeanProperty;
  30 
  31 import javax.swing.plaf.*;
  32 import javax.accessibility.*;
  33 
  34 import java.io.ObjectOutputStream;
  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  * @author Arnaud Weber
  60  * @author Steve Wilson
  61  * @since 1.2
  62  */
  63 @JavaBean(defaultProperty = "UI", description = "A generic lightweight container.")
  64 @SuppressWarnings("serial") // Same-version serialization only
  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      */
 147     @BeanProperty(hidden = true, visualUpdate = true, description
 148             = "The UI object that implements the Component's LookAndFeel.")
 149     public void setUI(PanelUI ui) {
 150         super.setUI(ui);
 151     }
 152 
 153     /**
 154      * Returns a string that specifies the name of the L&amp;F class
 155      * that renders this component.
 156      *
 157      * @return "PanelUI"
 158      * @see JComponent#getUIClassID
 159      * @see UIDefaults#getUI
 160      */
 161     @BeanProperty(bound = false, expert = true, description
 162             = "A string that specifies the name of the L&amp;F class.")
 163     public String getUIClassID() {
 164         return uiClassID;
 165     }
 166 
 167 
 168     /**
 169      * See readObject() and writeObject() in JComponent for more
 170      * information about serialization in Swing.
 171      */
 172     private void writeObject(ObjectOutputStream s) throws IOException {
 173         s.defaultWriteObject();
 174         if (getUIClassID().equals(uiClassID)) {
 175             byte count = JComponent.getWriteObjCounter(this);
 176             JComponent.setWriteObjCounter(this, --count);
 177             if (count == 0 && ui != null) {
 178                 ui.installUI(this);
 179             }
 180         }
 181     }
 182 
 183 
 184     /**
 185      * Returns a string representation of this JPanel. This method
 186      * is intended to be used only for debugging purposes, and the
 187      * content and format of the returned string may vary between
 188      * implementations. The returned string may be empty but may not
 189      * be <code>null</code>.
 190      *
 191      * @return  a string representation of this JPanel.
 192      */
 193     protected String paramString() {
 194         return super.paramString();
 195     }
 196 
 197 /////////////////
 198 // Accessibility support
 199 ////////////////
 200 
 201     /**
 202      * Gets the AccessibleContext associated with this JPanel.
 203      * For JPanels, the AccessibleContext takes the form of an
 204      * AccessibleJPanel.
 205      * A new AccessibleJPanel instance is created if necessary.
 206      *
 207      * @return an AccessibleJPanel that serves as the
 208      *         AccessibleContext of this JPanel
 209      */
 210     @BeanProperty(bound = false)
 211     public AccessibleContext getAccessibleContext() {
 212         if (accessibleContext == null) {
 213             accessibleContext = new AccessibleJPanel();
 214         }
 215         return accessibleContext;
 216     }
 217 
 218     /**
 219      * This class implements accessibility support for the
 220      * <code>JPanel</code> class.  It provides an implementation of the
 221      * Java Accessibility API appropriate to panel user-interface
 222      * elements.
 223      * <p>
 224      * <strong>Warning:</strong>
 225      * Serialized objects of this class will not be compatible with
 226      * future Swing releases. The current serialization support is
 227      * appropriate for short term storage or RMI between applications running
 228      * the same version of Swing.  As of 1.4, support for long term storage
 229      * of all JavaBeans&trade;
 230      * has been added to the <code>java.beans</code> package.
 231      * Please see {@link java.beans.XMLEncoder}.
 232      */
 233     @SuppressWarnings("serial") // Same-version serialization only
 234     protected class AccessibleJPanel extends AccessibleJComponent {
 235 
 236         /**
 237          * Get the role of this object.
 238          *
 239          * @return an instance of AccessibleRole describing the role of the
 240          * object
 241          */
 242         public AccessibleRole getAccessibleRole() {
 243             return AccessibleRole.PANEL;
 244         }
 245     }
 246 }