src/java.desktop/share/classes/javax/swing/JPanel.java

/*
 * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package javax.swing;

import java.awt.FlowLayout;
import java.awt.LayoutManager;
import java.beans.BeanProperty;
import java.beans.JavaBean;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serial;

import javax.accessibility.Accessible;
import javax.accessibility.AccessibleContext;
import javax.accessibility.AccessibleRole;
import javax.swing.plaf.PanelUI;

/**
 * <code>JPanel</code> is a generic lightweight container.
 * For examples and task-oriented documentation for JPanel, see
 * <a
 href="https://docs.oracle.com/javase/tutorial/uiswing/components/panel.html">How to Use Panels</a>,
 * a section in <em>The Java Tutorial</em>.
 * <p>
 * <strong>Warning:</strong> Swing is not thread safe. For more
 * information see <a
 * href="package-summary.html#threading">Swing's Threading
 * Policy</a>.
 * <p>
 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases. The current serialization support is
 * appropriate for short term storage or RMI between applications running
 * the same version of Swing.  As of 1.4, support for long term storage
 * of all JavaBeans
 * has been added to the <code>java.beans</code> package.
 * Please see {@link java.beans.XMLEncoder}.
 *
 * @author Arnaud Weber
 * @author Steve Wilson
 * @since 1.2
 */
@JavaBean(defaultProperty = "UI", description = "A generic lightweight container.")
@SuppressWarnings("serial") // Same-version serialization only
public class JPanel extends JComponent implements Accessible
{
    /**
     * @see #getUIClassID
     * @see #readObject
     */
    private static final String uiClassID = "PanelUI";

    /**
     * Creates a new JPanel with the specified layout manager and buffering
     * strategy.
     *
     * @param layout  the LayoutManager to use
     * @param isDoubleBuffered  a boolean, true for double-buffering, which
     *        uses additional memory space to achieve fast, flicker-free
     *        updates
     */
    public JPanel(LayoutManager layout, boolean isDoubleBuffered) {
        setLayout(layout);
        setDoubleBuffered(isDoubleBuffered);
        setUIProperty("opaque", Boolean.TRUE);
        updateUI();
    }

    /**
     * Create a new buffered JPanel with the specified layout manager
     *
     * @param layout  the LayoutManager to use
     */
    public JPanel(LayoutManager layout) {
        this(layout, true);
    }

    /**
     * Creates a new <code>JPanel</code> with <code>FlowLayout</code>
     * and the specified buffering strategy.
     * If <code>isDoubleBuffered</code> is true, the <code>JPanel</code>
     * will use a double buffer.
     *
     * @param isDoubleBuffered  a boolean, true for double-buffering, which
     *        uses additional memory space to achieve fast, flicker-free
     *        updates
     */
    public JPanel(boolean isDoubleBuffered) {
        this(new FlowLayout(), isDoubleBuffered);
    }

    /**
     * Creates a new <code>JPanel</code> with a double buffer
     * and a flow layout.
     */
    public JPanel() {
        this(true);
    }

    /**
     * Resets the UI property with a value from the current look and feel.
     *
     * @see JComponent#updateUI
     */
    public void updateUI() {
        setUI((PanelUI)UIManager.getUI(this));
    }

    /**
     * Returns the look and feel (L&amp;F) object that renders this component.
     *
     * @return the PanelUI object that renders this component
     * @since 1.4
     */
    public PanelUI getUI() {
        return (PanelUI)ui;
    }


    /**
     * Sets the look and feel (L&amp;F) object that renders this component.
     *
     * @param ui  the PanelUI L&amp;F object
     * @see UIDefaults#getUI
     * @since 1.4
     */
    @BeanProperty(hidden = true, visualUpdate = true, description
            = "The UI object that implements the Component's LookAndFeel.")
    public void setUI(PanelUI ui) {
        super.setUI(ui);
    }

    /**
     * Returns a string that specifies the name of the L&amp;F class
     * that renders this component.
     *
     * @return the string "PanelUI"
     * @see JComponent#getUIClassID
     * @see UIDefaults#getUI
     */
    @BeanProperty(bound = false, expert = true, description
            = "A string that specifies the name of the L&F class.")
    public String getUIClassID() {
        return uiClassID;
    }


    /**
     * See readObject() and writeObject() in JComponent for more
     * information about serialization in Swing.
     */
    @Serial
    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();
        if (getUIClassID().equals(uiClassID)) {
            byte count = JComponent.getWriteObjCounter(this);
            JComponent.setWriteObjCounter(this, --count);
            if (count == 0 && ui != null) {
                ui.installUI(this);
            }
        }
    }


    /**
     * Returns a string representation of this JPanel. This method
     * is intended to be used only for debugging purposes, and the
     * content and format of the returned string may vary between
     * implementations. The returned string may be empty but may not
     * be <code>null</code>.
     *
     * @return  a string representation of this JPanel.
     */
    protected String paramString() {
        return super.paramString();
    }

/////////////////
// Accessibility support
////////////////

    /**
     * Gets the AccessibleContext associated with this JPanel.
     * For JPanels, the AccessibleContext takes the form of an
     * AccessibleJPanel.
     * A new AccessibleJPanel instance is created if necessary.
     *
     * @return an AccessibleJPanel that serves as the
     *         AccessibleContext of this JPanel
     */
    @BeanProperty(bound = false)
    public AccessibleContext getAccessibleContext() {
        if (accessibleContext == null) {
            accessibleContext = new AccessibleJPanel();
        }
        return accessibleContext;
    }

    /**
     * This class implements accessibility support for the
     * <code>JPanel</code> class.  It provides an implementation of the
     * Java Accessibility API appropriate to panel user-interface
     * elements.
     * <p>
     * <strong>Warning:</strong>
     * Serialized objects of this class will not be compatible with
     * future Swing releases. The current serialization support is
     * appropriate for short term storage or RMI between applications running
     * the same version of Swing.  As of 1.4, support for long term storage
     * of all JavaBeans
     * has been added to the <code>java.beans</code> package.
     * Please see {@link java.beans.XMLEncoder}.
     */
    @SuppressWarnings("serial") // Same-version serialization only
    protected class AccessibleJPanel extends AccessibleJComponent {

        /**
         * Constructs an {@code AccessibleJPanel}.
         */
        protected AccessibleJPanel() {}

        /**
         * Get the role of this object.
         *
         * @return an instance of AccessibleRole describing the role of the
         * object
         */
        public AccessibleRole getAccessibleRole() {
            return AccessibleRole.PANEL;
        }
    }
}