1 /*
   2  * Copyright (c) 1997, 1998, 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 
  26 package javax.swing.plaf;
  27 
  28 
  29 /**
  30  * This interface is used to mark objects created by ComponentUI delegates.
  31  * The {@code ComponentUI.installUI()} and
  32  * {@code ComponentUI.uninstallUI()} methods can use this interface
  33  * to decide if a properties value has been overridden.  For example, the
  34  * JList cellRenderer property is initialized by BasicListUI.installUI(),
  35  * only if it's initial value is null:
  36  * <pre>
  37  * if (list.getCellRenderer() == null) {
  38  *     list.setCellRenderer((ListCellRenderer)(UIManager.get("List.cellRenderer")));
  39  * }
  40  * </pre>
  41  * At uninstallUI() time we reset the property to null if its value
  42  * is an instance of UIResource:
  43  * <pre>
  44  * if (list.getCellRenderer() instanceof UIResource) {
  45  *     list.setCellRenderer(null);
  46  * }
  47  *</pre>
  48  * This pattern applies to all properties except the java.awt.Component
  49  * properties font, foreground, and background.  If one of these
  50  * properties isn't initialized, or is explicitly set to null,
  51  * its container provides the value.  For this reason the
  52  * {@code "== null"} is unreliable when installUI() is called
  53  * to dynamically change a components look and feel.  So at installUI()
  54  * time we check to see if the current value is a UIResource:
  55  *<pre>
  56  * if (!(list.getFont() instanceof UIResource)) {
  57  *     list.setFont(UIManager.getFont("List.font"));
  58  * }
  59  * </pre>
  60  *
  61  * @see ComponentUI
  62  * @author Hans Muller
  63  *
  64  */
  65 
  66 public interface UIResource {
  67 }
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