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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;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.Set;

import org.apache.commons.collections4.bag.HashBag;
import org.apache.commons.collections4.collection.PredicatedCollection;
import org.apache.commons.collections4.collection.SynchronizedCollection;
import org.apache.commons.collections4.collection.TransformedCollection;
import org.apache.commons.collections4.collection.UnmodifiableBoundedCollection;
import org.apache.commons.collections4.collection.UnmodifiableCollection;
import org.apache.commons.collections4.functors.TruePredicate;
import org.apache.commons.collections4.iterators.CollatingIterator;
import org.apache.commons.collections4.iterators.PermutationIterator;

/**
 * Provides utility methods and decorators for {@link Collection} instances.
 * 
 * Various utility methods might put the input objects into a Set/Map/Bag. In case
 * the input objects override {@link Object#equals(Object)}, it is mandatory that
 * the general contract of the {@link Object#hashCode()} method is maintained.
 * 

 * NOTE: From 4.0, method parameters will take {@link Iterable} objects when possible.
 *
 * @since 1.0
 * @version $Id: CollectionUtils.java 1686855 2015-06-22 13:00:27Z tn $
 */
public class CollectionUtils {

    /**
     * Helper class to easily access cardinality properties of two collections.
     * @param   the element type
     */
    private static class CardinalityHelper {

        /** Contains the cardinality for each object in collection A. */
        final Map cardinalityA;

        /** Contains the cardinality for each object in collection B. */
        final Map cardinalityB;

        /**
         * Create a new CardinalityHelper for two collections.
         * @param a  the first collection
         * @param b  the second collection
         */
        public CardinalityHelper(final Iterable a, final Iterable b) {
            cardinalityA = CollectionUtils.getCardinalityMap(a);
            cardinalityB = CollectionUtils.getCardinalityMap(b);
        }

        /**
         * Returns the maximum frequency of an object.
         * @param obj  the object
         * @return the maximum frequency of the object
         */
        public final int max(final Object obj) {
            return Math.max(freqA(obj), freqB(obj));
        }

        /**
         * Returns the minimum frequency of an object.
         * @param obj  the object
         * @return the minimum frequency of the object
         */
        public final int min(final Object obj) {
            return Math.min(freqA(obj), freqB(obj));
        }

        /**
         * Returns the frequency of this object in collection A.
         * @param obj  the object
         * @return the frequency of the object in collection A
         */
        public int freqA(final Object obj) {
            return getFreq(obj, cardinalityA);
        }

        /**
         * Returns the frequency of this object in collection B.
         * @param obj  the object
         * @return the frequency of the object in collection B
         */
        public int freqB(final Object obj) {
            return getFreq(obj, cardinalityB);
        }

        private final int getFreq(final Object obj, final Map freqMap) {
            final Integer count = freqMap.get(obj);
            if (count != null) {
                return count.intValue();
            }
            return 0;
        }
    }

    /**
     * Helper class for set-related operations, e.g. union, subtract, intersection.
     * @param   the element type
     */
    private static class SetOperationCardinalityHelper extends CardinalityHelper implements Iterable {

        /** Contains the unique elements of the two collections. */
        private final Set elements;

        /** Output collection. */
        private final List newList;

        /**
         * Create a new set operation helper from the two collections.
         * @param a  the first collection
         * @param b  the second collection
         */
        public SetOperationCardinalityHelper(final Iterable a, final Iterable b) {
            super(a, b);
            elements = new HashSet();
            addAll(elements, a);
            addAll(elements, b);
            // the resulting list must contain at least each unique element, but may grow
            newList = new ArrayList(elements.size());
        }

        public Iterator iterator() {
            return elements.iterator();
        }

        /**
         * Add the object {@code count} times to the result collection.
         * @param obj  the object to add
         * @param count  the count
         */
        public void setCardinality(final O obj, final int count) {
            for (int i = 0; i < count; i++) {
                newList.add(obj);
            }
        }

        /**
         * Returns the resulting collection.
         * @return the result
         */
        public Collection list() {
            return newList;
        }

    }

    /**
     * An empty unmodifiable collection.
     * The JDK provides empty Set and List implementations which could be used for
     * this purpose. However they could be cast to Set or List which might be
     * undesirable. This implementation only implements Collection.
     */
    @SuppressWarnings("rawtypes") // we deliberately use the raw type here
    public static final Collection EMPTY_COLLECTION =
        UnmodifiableCollection.unmodifiableCollection(new ArrayList