001    /*
002     *  Licensed to the Apache Software Foundation (ASF) under one or more
003     *  contributor license agreements.  See the NOTICE file distributed with
004     *  this work for additional information regarding copyright ownership.
005     *  The ASF licenses this file to You under the Apache License, Version 2.0
006     *  (the "License"); you may not use this file except in compliance with
007     *  the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     *  Unless required by applicable law or agreed to in writing, software
012     *  distributed under the License is distributed on an "AS IS" BASIS,
013     *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     *  See the License for the specific language governing permissions and
015     *  limitations under the License.
016     */
017    package org.apache.commons.collections.functors;
018    
019    import java.io.Serializable;
020    
021    import org.apache.commons.collections.Transformer;
022    
023    /**
024     * Transformer implementation that returns the same constant each time.
025     * <p>
026     * No check is made that the object is immutable. In general, only immutable
027     * objects should use the constant factory. Mutable objects should
028     * use the prototype factory.
029     * 
030     * @since Commons Collections 3.0
031     * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
032     *
033     * @author Stephen Colebourne
034     */
035    public class ConstantTransformer implements Transformer, Serializable {
036    
037        /** Serial version UID */
038        private static final long serialVersionUID = 6374440726369055124L;
039        
040        /** Returns null each time */
041        public static final Transformer NULL_INSTANCE = new ConstantTransformer(null);
042    
043        /** The closures to call in turn */
044        private final Object iConstant;
045    
046        /**
047         * Transformer method that performs validation.
048         *
049         * @param constantToReturn  the constant object to return each time in the factory
050         * @return the <code>constant</code> factory.
051         */
052        public static Transformer getInstance(Object constantToReturn) {
053            if (constantToReturn == null) {
054                return NULL_INSTANCE;
055            }
056            return new ConstantTransformer(constantToReturn);
057        }
058        
059        /**
060         * Constructor that performs no validation.
061         * Use <code>getInstance</code> if you want that.
062         * 
063         * @param constantToReturn  the constant to return each time
064         */
065        public ConstantTransformer(Object constantToReturn) {
066            super();
067            iConstant = constantToReturn;
068        }
069    
070        /**
071         * Transforms the input by ignoring it and returning the stored constant instead.
072         * 
073         * @param input  the input object which is ignored
074         * @return the stored constant
075         */
076        public Object transform(Object input) {
077            return iConstant;
078        }
079    
080        /**
081         * Gets the constant.
082         * 
083         * @return the constant
084         * @since Commons Collections 3.1
085         */
086        public Object getConstant() {
087            return iConstant;
088        }
089    
090    }