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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
// $Id: XPathVariableResolver.java 446598 2006-09-15 12:55:40Z jeremias $

package javax.xml.xpath;

import javax.xml.namespace.QName;

/**
 * XPathVariableResolver provides access to the set of user defined XPath variables.
 *
 * The XPathVariableResolver and the XPath evaluator must adhere to a contract that
 * cannot be directly enforced by the API.  Although variables may be mutable,
 * that is, an application may wish to evaluate the same XPath expression more
 * than once with different variable values, in the course of evaluating any
 * single XPath expression, a variable's value must be immutable.
 *
 * @author  Norman Walsh
 * @author  Jeff Suttor
 * @version $Revision: 446598 $, $Date: 2006-09-15 08:55:40 -0400 (Fri, 15 Sep 2006) $
 * @since 1.5
 */
public interface XPathVariableResolver {
  /** 
   * Find a variable in the set of available variables.
   * 
   * If variableName is null, then a NullPointerException is thrown.
   * 
   * @param variableName The QName of the variable name.
   * 
   * @return The variables value, or null if no variable named variableName
   *   exists.  The value returned must be of a type appropriate for the underlying object model.
   * 
   * @throws NullPointerException If variableName is null.
   */
  public Object resolveVariable(QName variableName);
}