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    import java.util.HashSet;
021    import java.util.Set;
022    
023    import org.apache.commons.collections.Predicate;
024    
025    /**
026     * Predicate implementation that returns true the first time an object is
027     * passed into the predicate.
028     * 
029     * @since Commons Collections 3.0
030     * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
031     *
032     * @author Stephen Colebourne
033     */
034    public final class UniquePredicate implements Predicate, Serializable {
035    
036        /** Serial version UID */
037        private static final long serialVersionUID = -3319417438027438040L;
038        
039        /** The set of previously seen objects */
040        private final Set iSet = new HashSet();
041        
042        /**
043         * Factory to create the predicate.
044         * 
045         * @return the predicate
046         * @throws IllegalArgumentException if the predicate is null
047         */
048        public static Predicate getInstance() {
049            return new UniquePredicate();
050        }
051    
052        /**
053         * Constructor that performs no validation.
054         * Use <code>getInstance</code> if you want that.
055         */
056        public UniquePredicate() {
057            super();
058        }
059    
060        /**
061         * Evaluates the predicate returning true if the input object hasn't been
062         * received yet.
063         * 
064         * @param object  the input object
065         * @return true if this is the first time the object is seen
066         */
067        public boolean evaluate(Object object) {
068            return iSet.add(object);
069        }
070    
071    }