Java tutorial
/* * Copyright (c) 1997, 2015, 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.*; import java.beans.JavaBean; import java.beans.BeanProperty; import javax.swing.plaf.*; import javax.accessibility.*; import java.io.ObjectOutputStream; import java.io.IOException; /** * <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&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&F) object that renders this component. * * @param ui the PanelUI L&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&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. */ 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 { /** * Get the role of this object. * * @return an instance of AccessibleRole describing the role of the * object */ public AccessibleRole getAccessibleRole() { return AccessibleRole.PANEL; } } }