001    /*
002     *    GeoAPI - Java interfaces for OGC/ISO standards
003     *    http://www.geoapi.org
004     *
005     *    Copyright (C) 2006-2012 Open Geospatial Consortium, Inc.
006     *    All Rights Reserved. http://www.opengeospatial.org/ogc/legal
007     *
008     *    Permission to use, copy, and modify this software and its documentation, with
009     *    or without modification, for any purpose and without fee or royalty is hereby
010     *    granted, provided that you include the following on ALL copies of the software
011     *    and documentation or portions thereof, including modifications, that you make:
012     *
013     *    1. The full text of this NOTICE in a location viewable to users of the
014     *       redistributed or derivative work.
015     *    2. Notice of any changes or modifications to the OGC files, including the
016     *       date changes were made.
017     *
018     *    THIS SOFTWARE AND DOCUMENTATION IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE
019     *    NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
020     *    TO, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT
021     *    THE USE OF THE SOFTWARE OR DOCUMENTATION WILL NOT INFRINGE ANY THIRD PARTY
022     *    PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS.
023     *
024     *    COPYRIGHT HOLDERS WILL NOT BE LIABLE FOR ANY DIRECT, INDIRECT, SPECIAL OR
025     *    CONSEQUENTIAL DAMAGES ARISING OUT OF ANY USE OF THE SOFTWARE OR DOCUMENTATION.
026     *
027     *    The name and trademarks of copyright holders may NOT be used in advertising or
028     *    publicity pertaining to the software without specific, written prior permission.
029     *    Title to copyright in this software and any associated documentation will at all
030     *    times remain with copyright holders.
031     */
032    package org.opengis.filter.expression;
033    
034    import java.io.ObjectStreamException;
035    import java.io.Serializable;
036    
037    
038    /**
039     * Placeholder class used to represent a NIL expression, evaluates to {@code null}.
040     * This placeholder class allows data structures to avoid the use of {@code null}.
041     * Please note that {@link Expression#NIL} is not considered a Literal with value
042     * {@code null} (since the literal may have its value changed).
043     *
044     * @author Jody Garnett (Refractions Research, Inc.)
045     * @author Martin Desruisseaux (Geomatys)
046     */
047    public final class NilExpression implements Expression, Serializable {
048        /**
049         * For cross-version compatibility.
050         */
051        private static final long serialVersionUID = 4999313240542653655L;
052    
053        /**
054         * Not extensible.
055         */
056        NilExpression() {
057        }
058    
059        /**
060         * Accepts a visitor.
061         */
062        public Object accept(ExpressionVisitor visitor, Object extraData) {
063            return visitor.visit(this, extraData);
064        }
065    
066        /**
067         * Returns {@code null}.
068         */
069        public Object evaluate(Object object) {
070            return null;
071        }
072    
073        /**
074         * Returns {@code null}.
075         */
076        public <T> T evaluate(Object object, Class<T> context) {
077            return null;
078        }
079    
080        /**
081         * Returns a string representation of this expression.
082         */
083        @Override
084        public String toString() {
085            return "Expression.NIL";
086        }
087    
088        /**
089         * Returns the canonical instance on deserialization.
090         */
091        private Object readResolve() throws ObjectStreamException {
092            return Expression.NIL;
093        }
094    }