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 */
017package org.apache.commons.collections4.functors;
018
019import java.io.Serializable;
020import java.util.HashSet;
021import java.util.Set;
022
023import org.apache.commons.collections4.Predicate;
024
025/**
026 * Predicate implementation that returns true the first time an object is
027 * passed into the predicate.
028 *
029 * @param <T> the type of the input to the predicate.
030 * @since 3.0
031 */
032public final class UniquePredicate<T>  extends AbstractPredicate<T> implements Serializable {
033
034    /** Serial version UID */
035    private static final long serialVersionUID = -3319417438027438040L;
036
037    /**
038     * Creates the predicate.
039     *
040     * @param <T> the type that the predicate queries
041     * @return the predicate
042     * @throws IllegalArgumentException if the predicate is null
043     */
044    public static <T> Predicate<T> uniquePredicate() {
045        return new UniquePredicate<>();
046    }
047
048    /** The set of previously seen objects */
049    private final Set<T> iSet = new HashSet<>();
050
051    /**
052     * Constructor that performs no validation.
053     * Use {@code uniquePredicate} if you want that.
054     */
055    public UniquePredicate() {
056    }
057
058    /**
059     * Evaluates the predicate returning true if the input object hasn't been
060     * received yet.
061     *
062     * @param object  the input object
063     * @return true if this is the first time the object is seen
064     */
065    @Override
066    public boolean test(final T object) {
067        return iSet.add(object);
068    }
069
070}