/*
 *  Copyright 2001-2004 The Apache Software Foundation
 *
 *  Licensed 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.
 */
package org.apache.commons.collections.functors;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

import org.apache.commons.collections.Predicate;

/**
 * Predicate implementation that returns true the first time an object is
 * passed into the predicate.
 * 
 * @since Commons Collections 3.0
 * @version $Revision: 1.4 $ $Date: 2004/05/16 11:16:01 $
 *
 * @author Stephen Colebourne
 */
public final class UniquePredicate implements Predicate, Serializable {

    /** Serial version UID */
    static final long serialVersionUID = -3319417438027438040L;
    
    /** The set of previously seen objects */
    private final Set iSet = new HashSet();
    
    /**
     * Factory to create the predicate.
     * 
     * @return the predicate
     * @throws IllegalArgumentException if the predicate is null
     */
    public static Predicate getInstance() {
        return new UniquePredicate();
    }

    /**
     * Constructor that performs no validation.
     * Use getInstance if you want that.
     */
    public UniquePredicate() {
        super();
    }

    /**
     * Evaluates the predicate returning true if the input object hasn't been
     * received yet.
     * 
     * @param object  the input object
     * @return true if this is the first time the object is seen
     */
    public boolean evaluate(Object object) {
        return iSet.add(object);
    }

}