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/*

 *    GeoTools - OpenSource mapping toolkit

 *    http://geotools.org

 *    (C) 2006, GeoTools Project Managment Committee (PMC)

 *    

 *    This library is free software; you can redistribute it and/or

 *    modify it under the terms of the GNU Lesser General Public

 *    License as published by the Free Software Foundation;

 *    version 2.1 of the License.

 *

 *    This library 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

 *    Lesser General Public License for more details.

 */

package org.gvsig.data.vectorial.filter;

import org.gvsig.data.vectorial.Feature;

import org.opengis.filter.expression.Expression;

import org.opengis.filter.expression.ExpressionVisitor;

//

/**

 * Abstract superclass of these Expression implementations.

 * <p>

 * Contains additional support for "Expression chaining". This allows

 * Expressions to be constructed as a chain of Java commands similar to the use

 * of the java collections api.

 * </p>

 * <p>

 * Note: Expression chaining is a simple developer convience, it has no effect

 * on the data model exposed by the GeoAPI interfaces.

 * </p>

 * <p>

 * Idea: We may also be able to teach this implementation to make use of

 * JXPath to extract "attribute values" from Java Beans, DOM, JDOM in addition

 * to the geotools & geoapi FeatureType models. It is a cunning plan - any

 * implementation will make use of this abstract base class.

 * </p>

 * 

 * @author Jody Garnett

 * @author jcarrasco

 * 

 */

public abstract class ExpressionAbstract implements Expression, ExpressionType {

          /** Defines the type of this expression. */

    protected short expressionType;

        /** Subclass should overide, default implementation returns null */

        public Object evaluate(Object object) {

                return null;

        }

        /**

         * Default implementation delegates handling of context

         * conversion to Value utility class.

         * <p>

         * Subclasses are expected to make use of the Value utility class

         * (as the easiest way to provide value morphing in confirmance with

         *  the Filter specification).

         *  

         */

        public abstract Object evaluate(Object object, Class context);

        /** Subclass should override, default implementation just returns extraData */

        public Object accept(ExpressionVisitor visitor, Object extraData) {

                return extraData;

        }

    /**

     * Helper method for subclasses to reduce null checks

     * @param expression

     * @param feature

     * @return value or null

     */

    protected Object eval( Expression expression, Feature feature ){

        if( expression == null || feature == null ) return null;

        return expression.evaluate( feature );

    }

        public short getType() {

                return expressionType;

        }

}