http://xml.apache.org/commons/components/external/: xml-commons provides an Apache-hosted set of DOM, SAX, and JAXP interfaces for use in other xml-based projects. Our hope is that we can standardize on both a common version and packaging scheme for these critical XML standards interfaces to make the lives of both our developers and users easier. The External Components portion of xml-commons contains interfaces that are defined by external standards organizations. For DOM, that's the W3C; for SAX it's David Megginson and sax.sourceforge.net; for JAXP it's Sun. 
/*
 * Copyright (c) 2009 World Wide Web Consortium,
 *
 * (Massachusetts Institute of Technology, European Research Consortium for
 * Informatics and Mathematics, Keio University). All Rights Reserved. This
 * work is distributed under the W3C(r) Software License [1] 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.
 *
 * [1] http://www.w3.org/Consortium/Legal/2002/copyright-software-20021231
 */

package org.w3c.dom;


The ElementTraversal interface is a set of read-only attributes
which allow an author to easily navigate between elements in a document.

In conforming implementations of Element Traversal, all objects that implement Element must also implement the ElementTraversal interface. Four of the methods, getFirstElementChild, getLastElementChild, getPreviousElementSibling, and getNextElementSibling, each return a live reference to another element with the defined relationship to the current element, if the related element exists. The fifth method, getChildElementCount, exposes the number of child elements of an element, for preprocessing before navigation. 
See also the
Element Traversal Specification.

/**
 * The <code>ElementTraversal</code> interface is a set of read-only attributes
 * which allow an author to easily navigate between elements in a document.
 * <p>In conforming implementations of Element Traversal, all objects that
 * implement {@link Element} must also implement the
 * <code>ElementTraversal</code> interface. Four of the methods,
 * {@link #getFirstElementChild}, {@link #getLastElementChild},
 * {@link #getPreviousElementSibling}, and {@link #getNextElementSibling},
 * each return a live reference to another element with the defined
 * relationship to the current element, if the related element exists. The
 * fifth method, {@link #getChildElementCount}, exposes the number of child
 * elements of an element, for preprocessing before navigation.
 * <p>See also the
 * <a href='http://www.w3.org/TR/ElementTraversal/'><cite>Element Traversal Specification</cite></a>.
 */
public interface ElementTraversal {

    
Returns the first child element node of this element. null
if this element has no child elements.

/**
     * Returns the first child element node of this element. <code>null</code>
     * if this element has no child elements.
     */
    Element getFirstElementChild();

    
Returns the last child element node of this element. null
if this element has no child elements.

/**
     * Returns the last child element node of this element. <code>null</code>
     * if this element has no child elements.
     */
    Element getLastElementChild();

    
Returns the previous sibling element node of this element.
null if this element has no element sibling nodes that
come before this one in the document tree.

/**
     * Returns the previous sibling element node of this element.
     * <code>null</code> if this element has no element sibling nodes that
     * come before this one in the document tree.
     */
    Element getPreviousElementSibling();

    
Returns the next sibling element node of this element.
null if this element has no element sibling nodes that
come after this one in the document tree.

/**
     * Returns the next sibling element node of this element.
     * <code>null</code> if this element has no element sibling nodes that
     * come after this one in the document tree.
     */
    Element getNextElementSibling();

    
Returns the current number of element nodes that are children of this
element. 0 if this element has no child nodes that are of
nodeType 1.

/**
     * Returns the current number of element nodes that are children of this
     * element. <code>0</code> if this element has no child nodes that are of
     * <code>nodeType</code> <code>1</code>.
     */
    int getChildElementCount();
}