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+/* Element.java -- Base class for object tree elements.
+   Copyright (C) 2005 Free Software Foundation, Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath 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 for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING.  If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library.  Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module.  An independent module is a module which is not derived from
+or based on this library.  If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so.  If you do not wish to do so, delete this
+exception statement from your version. */
+
+
+package gnu.java.beans.encoder.elements;
+
+import java.util.Iterator;
+import java.util.LinkedList;
+
+import gnu.java.beans.encoder.ObjectId;
+import gnu.java.beans.encoder.Writer;
+
+/** <code>Element</code> is the base class for the object tree elements.
+ * 
+ * <p>It provides the neccessary infrastructure every element subclass
+ * needs in order to interact with the {@link gnu.java.beans.encoder.Root}
+ * class.</p>
+ * 
+ * @author Robert Schuster (robertschuster@fsfe.org)
+ */
+public abstract class Element
+{
+  /**
+   * Stores the child elements.
+   */
+  private LinkedList children = new LinkedList();
+  
+  /**
+   * An optional ObjectId instance which is needed for certain subclasses
+   * only.
+   */
+  private ObjectId objectId;
+  
+  /** Sets an {@link gnu.java.beans.encoder.ObjectId} instance in this
+   * <code>Element</code>.
+   * 
+   * <p>This can only be done once.</p>
+   * 
+   * @param objectId An ObjectId instance.
+   */
+  public final void initId(ObjectId objectId)
+  {
+    assert (this.objectId == null);
+    assert (objectId != null);
+    
+    this.objectId = objectId;
+  }
+
+  /** Adds a child element to this <code>Element</code>.
+   * 
+   * @param elem The new child.
+   */
+  public final void addChild(Element elem)
+  {
+    children.add(elem);
+  }
+  
+  /** Removes the child element added last.
+   */
+  public final void removeLast()
+  {
+    children.removeLast();
+  }
+  
+  /** Provides access to the child elements via an iterator.
+   * 
+   * @return An iterator for the child elements.
+   */
+  public final Iterator iterator(){
+    return children.iterator();
+  }
+  
+  /** Clears all the stored child elements.
+   * 
+   */
+  public final void clear()
+  {
+   children.clear(); 
+  }
+  
+  /** Returns whether this element contains child elements.
+   * 
+   * <p>This method is useful to decide which formatting variant
+   * for the XML element can be chosen.</p>
+   * 
+   * @return Whether the element has child elements.
+   */
+  public final boolean isEmpty()
+  {
+    return children.isEmpty(); 
+  }
+  
+  /** Retrieves the element's {@link gnu.java.beans.encoder.ObjectId} instance
+   * if it has one.
+   * 
+   * @return The ObjectId instance or <code>null</code>.
+   */
+  public final ObjectId getId()
+  {
+    return objectId;
+  }
+  
+  /** Writes the opening XML tag.
+   * 
+   * @param writer The writer to be used for XML writing.
+   */
+  public abstract void writeStart(Writer writer);
+  
+  /** Writes the closing XML tag.
+   * 
+   * <p>By default this does <code>writer.writeEnd(children.isEmpty())</code>.
+   * Override if neccessary, for example when using the
+   * {@link gnu.java.beans.encoder.Writer#writeNoChildren}</code> method
+   * variants. 
+   * 
+   * @param writer The writer to be used for XML writing.
+   */
+  public void writeEnd(Writer writer)
+  {
+    writer.writeEnd(children.isEmpty());
+  }
+  
+}