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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.

There is a newer version: 4.5.0-M2

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.collections4.functors;

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

import org.apache.commons.collections4.Predicate;

/**
 * Predicate implementation that returns true the first time an object is
 * passed into the predicate.
 *
 * @since 3.0
 * @version $Id: UniquePredicate.java 1476582 2013-04-27 14:13:54Z tn $
 */
public final class UniquePredicate implements Predicate, Serializable {

    /** Serial version UID */
    private static final long serialVersionUID = -3319417438027438040L;

    /** The set of previously seen objects */
    private final Set iSet = new HashSet();

    /**
     * Factory to create the predicate.
     *
     * @param  the type that the predicate queries
     * @return the predicate
     * @throws IllegalArgumentException if the predicate is null
     */
    public static  Predicate uniquePredicate() {
        return new UniquePredicate();
    }

    /**
     * Constructor that performs no validation.
     * Use uniquePredicate 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(final T object) {
        return iSet.add(object);
    }

}