/*
 * 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
 *
 *      https://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.Collections;
import java.util.Comparator;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.Objects;
import java.util.Set;

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;
import org.apache.commons.collections4.multiset.HashMultiSet;

/**
 * Provides utility methods and decorators for {@link Collection} instances.
 * <p>
 * Various utility methods might put the input objects into a Set/Map/MultiSet. 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.
 * </p>
 * <p>
 * NOTE: From 4.0, method parameters will take {@link Iterable} objects when possible.
 * </p>
 *
 * @since 1.0
 */
public class CollectionUtils {

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

        static boolean equals(final Collection<?> a, final Collection<?> b) {
            return new HashMultiSet<>(a).equals(new HashMultiSet<>(b));
        }

        /** Contains the cardinality for each object in collection A. */
        final MultiSet<O> cardinalityA;

        /** Contains the cardinality for each object in collection B. */
        final MultiSet<O> cardinalityB;

        /**
         * Creates a new CardinalityHelper for two collections.
         *
         * @param a The first collection.
         * @param b The second collection.
         */
        CardinalityHelper(final Iterable<? extends O> a, final Iterable<? extends O> b) {
            cardinalityA = new HashMultiSet<>(a);
            cardinalityB = new HashMultiSet<>(b);
        }

        /**
         * Gets the frequency of this object in collection A.
         *
         * @param key The key whose associated frequency is to be returned.
         * @return The frequency of the object in collection A.
         */
        public int freqA(final Object key) {
            return getFreq(key, cardinalityA);
        }

        /**
         * Gets the frequency of this object in collection B.
         *
         * @param key The key whose associated frequency is to be returned.
         * @return The frequency of the object in collection B.
         */
        public int freqB(final Object key) {
            return getFreq(key, cardinalityB);
        }

        private int getFreq(final Object key, final MultiSet<?> freqMap) {
            return freqMap.getCount(key);
        }

        /**
         * Gets 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));
        }

        /**
         * Gets 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));
        }
    }

    /**
     * Wraps another object and uses the provided Equator to implement {@link #equals(Object)} and {@link #hashCode()}.
     * <p>
     * This class can be used to store objects into a Map.
     * </p>
     *
     * @param <O> the element type.
     * @since 4.0
     */
    private static final class EquatorWrapper<O> {

        private final Equator<? super O> equator;

        private final O object;

        EquatorWrapper(final Equator<? super O> equator, final O object) {
            this.equator = equator;
            this.object = object;
        }

        @Override
        public boolean equals(final Object obj) {
            if (!(obj instanceof EquatorWrapper)) {
                return false;
            }
            @SuppressWarnings("unchecked")
            final EquatorWrapper<O> otherObj = (EquatorWrapper<O>) obj;
            return equator.equate(object, otherObj.getObject());
        }

        public O getObject() {
            return object;
        }

        @Override
        public int hashCode() {
            return equator.hash(object);
        }
    }

    /**
     * Helper class for set-related operations, for example union, subtract, intersection.
     *
     * @param <O> the element type.
     */
    private static final class SetOperationCardinalityHelper<O> extends CardinalityHelper<O> implements Iterable<O> {

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

        /** Output collection. */
        private final List<O> newList;

        /**
         * Create a new set operation helper from the two collections.
         *
         * @param a The first collection.
         * @param b The second collection.
         */
        SetOperationCardinalityHelper(final Iterable<? extends O> a, final Iterable<? extends O> 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());
        }

        @Override
        public Iterator<O> iterator() {
            return elements.iterator();
        }

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

        /**
         * 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);
            }
        }
    }

    /**
     * The index value when an element is not found in a collection or array: {@code -1}.
     *
     * @since 4.5.0-M1
     */
    public static final int INDEX_NOT_FOUND = -1;

    /**
     * Default prefix used while converting an Iterator to its String representation.
     *
     * @since 4.5.0-M1
     */
    public static final String DEFAULT_TOSTRING_PREFIX = "[";

    /**
     * Default suffix used while converting an Iterator to its String representation.
     *
     * @since 4.5.0-M1
     */
    public static final String DEFAULT_TOSTRING_SUFFIX = "]";

    /**
     * A String for Colon  (":").
     *
     * @since 4.5.0-M1
     */
    public static final String COLON = ":";

    /**
     * A String for Comma (",").
     *
     * @since 4.5.0-M1
     */
    public static final String COMMA = ",";

    /**
     * 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 = Collections.emptyList();

    /**
     * Adds all elements in the array to the given collection.
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection to add to, must not be null.
     * @param elements   The array of elements to add, must not be null.
     * @return {@code true} if the collection was changed, {@code false} otherwise.
     * @throws NullPointerException if the collection or elements is null.
     */
    public static <C> boolean addAll(final Collection<C> collection, final C... elements) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(elements, "elements");
        boolean changed = false;
        for (final C element : elements) {
            changed |= collection.add(element);
        }
        return changed;
    }

    /**
     * Adds all elements in the enumeration to the given collection.
     *
     * @param <C>         the type of object the {@link Collection} contains.
     * @param collection  The collection to add to, must not be null.
     * @param enumeration The enumeration of elements to add, must not be null.
     * @return {@code true} if the collections was changed, {@code false} otherwise.
     * @throws NullPointerException if the collection or enumeration is null.
     */
    public static <C> boolean addAll(final Collection<C> collection, final Enumeration<? extends C> enumeration) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(enumeration, "enumeration");
        boolean changed = false;
        while (enumeration.hasMoreElements()) {
            changed |= collection.add(enumeration.nextElement());
        }
        return changed;
    }

    /**
     * Adds all elements in the {@link Iterable} to the given collection. If the {@link Iterable} is a {@link Collection} then it is cast and will be added
     * using {@link Collection#addAll(Collection)} instead of iterating.
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection to add to, must not be null.
     * @param iterable   The iterable of elements to add, must not be null.
     * @return A boolean indicating whether the collection has changed or not.
     * @throws NullPointerException if the collection or iterable is null.
     */
    public static <C> boolean addAll(final Collection<C> collection, final Iterable<? extends C> iterable) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(iterable, "iterable");
        if (iterable instanceof Collection<?>) {
            return collection.addAll((Collection<? extends C>) iterable);
        }
        return addAll(collection, iterable.iterator());
    }

    /**
     * Adds all elements in the iteration to the given collection.
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection to add to, must not be null.
     * @param iterator   The iterator of elements to add, must not be null.
     * @return A boolean indicating whether the collection has changed or not.
     * @throws NullPointerException if the collection or iterator is null.
     */
    public static <C> boolean addAll(final Collection<C> collection, final Iterator<? extends C> iterator) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(iterator, "iterator");
        boolean changed = false;
        while (iterator.hasNext()) {
            changed |= collection.add(iterator.next());
        }
        return changed;
    }

    /**
     * Adds an element to the collection unless the element is null.
     *
     * @param <T>        the type of object the {@link Collection} contains.
     * @param collection The collection to add to, must not be null.
     * @param object     The object to add, if null it will not be added.
     * @return true if the collection changed.
     * @throws NullPointerException if the collection is null.
     * @since 3.2
     */
    public static <T> boolean addIgnoreNull(final Collection<T> collection, final T object) {
        Objects.requireNonNull(collection, "collection");
        return object != null && collection.add(object);
    }

    /**
     * Returns the number of occurrences of <em>obj</em> in <em>coll</em>.
     *
     * @param obj        The object to find the cardinality of.
     * @param collection The {@link Iterable} to search.
     * @param <O>        The type of object that the {@link Iterable} may contain.
     * @return The number of occurrences of obj in coll.
     * @throws NullPointerException if collection is null.
     * @deprecated Since 4.1, use {@link IterableUtils#frequency(Iterable, Object)} instead. Be aware that the order of parameters has changed.
     */
    @Deprecated
    public static <O> int cardinality(final O obj, final Iterable<? super O> collection) {
        return IterableUtils.frequency(Objects.requireNonNull(collection, "collection"), obj);
    }

    /**
     * Ensures an index is not negative.
     *
     * @param index The index to check.
     * @throws IndexOutOfBoundsException if the index is negative.
     */
    static void checkIndexBounds(final int index) {
        if (index < 0) {
            throw new IndexOutOfBoundsException("Index cannot be negative: " + index);
        }
    }

    /**
     * Merges two sorted Collections, a and b, into a single, sorted List such that the natural ordering of the elements is retained.
     * <p>
     * Uses the standard O(n) merge algorithm for combining two sorted lists.
     * </p>
     *
     * @param <O> the element type.
     * @param a   The first collection, must not be null.
     * @param b   The second collection, must not be null.
     * @return A new sorted List, containing the elements of Collection a and b.
     * @throws NullPointerException if either collection is null.
     * @since 4.0
     */
    public static <O extends Comparable<? super O>> List<O> collate(final Iterable<? extends O> a, final Iterable<? extends O> b) {
        return collate(a, b, ComparatorUtils.<O>naturalComparator(), true);
    }

    /**
     * Merges two sorted Collections, a and b, into a single, sorted List such that the natural ordering of the elements is retained.
     * <p>
     * Uses the standard O(n) merge algorithm for combining two sorted lists.
     * </p>
     *
     * @param <O>               the element type.
     * @param a                 The first collection, must not be null.
     * @param b                 The second collection, must not be null.
     * @param includeDuplicates if {@code true} duplicate elements will be retained, otherwise they will be removed in the output collection.
     * @return A new sorted List, containing the elements of Collection a and b.
     * @throws NullPointerException if either collection is null.
     * @since 4.0
     */
    public static <O extends Comparable<? super O>> List<O> collate(final Iterable<? extends O> a, final Iterable<? extends O> b,
            final boolean includeDuplicates) {
        return collate(a, b, ComparatorUtils.<O>naturalComparator(), includeDuplicates);
    }

    /**
     * Merges two sorted Collections, a and b, into a single, sorted List such that the ordering of the elements according to Comparator c is retained.
     * <p>
     * Uses the standard O(n) merge algorithm for combining two sorted lists.
     * </p>
     *
     * @param <O> the element type.
     * @param a   The first collection, must not be null.
     * @param b   The second collection, must not be null.
     * @param c   The comparator to use for the merge.
     * @return A new sorted List, containing the elements of Collection a and b.
     * @throws NullPointerException if either collection or the comparator is null.
     * @since 4.0
     */
    public static <O> List<O> collate(final Iterable<? extends O> a, final Iterable<? extends O> b, final Comparator<? super O> c) {
        return collate(a, b, c, true);
    }

    /**
     * Merges two sorted Collections, a and b, into a single, sorted List such that the ordering of the elements according to Comparator c is retained.
     * <p>
     * Uses the standard O(n) merge algorithm for combining two sorted lists.
     * </p>
     *
     * @param <O>               the element type.
     * @param iterableA         The first collection, must not be null.
     * @param iterableB         The second collection, must not be null.
     * @param comparator        The comparator to use for the merge.
     * @param includeDuplicates if {@code true} duplicate elements will be retained, otherwise they will be removed in the output collection.
     * @return A new sorted List, containing the elements of Collection a and b.
     * @throws NullPointerException if either collection or the comparator is null.
     * @since 4.0
     */
    public static <O> List<O> collate(final Iterable<? extends O> iterableA, final Iterable<? extends O> iterableB, final Comparator<? super O> comparator,
            final boolean includeDuplicates) {
        Objects.requireNonNull(iterableA, "iterableA");
        Objects.requireNonNull(iterableB, "iterableB");
        Objects.requireNonNull(comparator, "comparator");
        // if both Iterables are a Collection, we can estimate the size
        final int totalSize = iterableA instanceof Collection<?> && iterableB instanceof Collection<?>
                ? Math.max(1, ((Collection<?>) iterableA).size() + ((Collection<?>) iterableB).size())
                : 10;
        final Iterator<O> iterator = new CollatingIterator<>(comparator, iterableA.iterator(), iterableB.iterator());
        if (includeDuplicates) {
            return IteratorUtils.toList(iterator, totalSize);
        }
        final ArrayList<O> mergedList = new ArrayList<>(totalSize);
        O lastItem = null;
        boolean first = true;
        while (iterator.hasNext()) {
            final O item = iterator.next();
            if (first || !Objects.equals(lastItem, item)) {
                mergedList.add(item);
            }
            lastItem = item;
            first = false;
        }
        mergedList.trimToSize();
        return mergedList;
    }

    /**
     * Transforms all elements from input collection with the given transformer and adds them to the output collection.
     * <p>
     * If the input collection or transformer is null, there is no change to the output collection.
     * </p>
     *
     * @param <I>              the type of object in the input collection.
     * @param <O>              the type of object in the output collection.
     * @param <R>              the type of the output collection.
     * @param inputCollection  The collection to get the input from, may be null.
     * @param transformer      The transformer to use, may be null.
     * @param outputCollection The collection to output into, may not be null if inputCollection and transformer are not null.
     * @return The output collection with the transformed input added.
     * @throws NullPointerException if the outputCollection is null and both, inputCollection and transformer are not null.
     */
    public static <I, O, R extends Collection<? super O>> R collect(final Iterable<? extends I> inputCollection,
            final Transformer<? super I, ? extends O> transformer, final R outputCollection) {
        if (inputCollection != null) {
            return collect(inputCollection.iterator(), transformer, outputCollection);
        }
        return outputCollection;
    }

    /**
     * Returns a new Collection containing all elements of the input collection transformed by the given transformer.
     * <p>
     * If the input collection or transformer is null, the result is an empty list.
     * </p>
     *
     * @param <I>             the type of object in the input collection.
     * @param <O>             the type of object in the output collection.
     * @param inputCollection The collection to get the input from, may not be null.
     * @param transformer     The transformer to use, may be null.
     * @return The transformed result (new list).
     * @throws NullPointerException if the outputCollection is null and both, inputCollection and transformer are not null.
     */
    public static <I, O> Collection<O> collect(final Iterable<I> inputCollection, final Transformer<? super I, ? extends O> transformer) {
        int size = 0;
        if (inputCollection != null) {
            size = inputCollection instanceof Collection<?> ? ((Collection<?>) inputCollection).size() : 0;
        }
        final Collection<O> answer = size == 0 ? new ArrayList<>() : new ArrayList<>(size);
        return collect(inputCollection, transformer, answer);
    }

    /**
     * Transforms all elements from the input iterator with the given transformer and adds them to the output collection.
     * <p>
     * If the input iterator or transformer is null, there is no change to the output collection.
     * </p>
     *
     * @param <I>              the type of object in the input collection.
     * @param <O>              the type of object in the output collection.
     * @param <R>              the type of the output collection.
     * @param inputIterator    The iterator to get the input from, may be null.
     * @param transformer      The transformer to use, may be null.
     * @param outputCollection The collection to output into, may not be null if inputIterator and transformer are not null.
     * @return The outputCollection with the transformed input added.
     * @throws NullPointerException if the output collection is null and both, inputIterator and transformer are not null.
     */
    public static <I, O, R extends Collection<? super O>> R collect(final Iterator<? extends I> inputIterator,
            final Transformer<? super I, ? extends O> transformer, final R outputCollection) {
        if (inputIterator != null && transformer != null) {
            while (inputIterator.hasNext()) {
                final I item = inputIterator.next();
                final O value = transformer.apply(item);
                outputCollection.add(value);
            }
        }
        return outputCollection;
    }

    /**
     * Transforms all elements from the input iterator with the given transformer and adds them to the output collection.
     * <p>
     * If the input iterator or transformer is null, the result is an empty list.
     * </p>
     *
     * @param <I>           the type of object in the input collection.
     * @param <O>           the type of object in the output collection.
     * @param inputIterator The iterator to get the input from, may be null.
     * @param transformer   The transformer to use, may be null.
     * @return The transformed result (new list).
     */
    public static <I, O> Collection<O> collect(final Iterator<I> inputIterator, final Transformer<? super I, ? extends O> transformer) {
        return collect(inputIterator, transformer, new ArrayList<>());
    }

    /**
     * Returns {@code true} iff all elements of {@code coll2} are also contained in {@code coll1}. The cardinality of values in {@code coll2} is not taken into
     * account, which is the same behavior as {@link Collection#containsAll(Collection)}.
     * <p>
     * In other words, this method returns {@code true} iff the {@link #intersection} of <em>coll1</em> and <em>coll2</em> has the same cardinality as the set
     * of unique values from {@code coll2}. In case {@code coll2} is empty, {@code true} will be returned.
     * </p>
     * <p>
     * This method is intended as a replacement for {@link Collection#containsAll(Collection)} with a guaranteed runtime complexity of {@code O(n + m)}.
     * Depending on the type of {@link Collection} provided, this method will be much faster than calling {@link Collection#containsAll(Collection)} instead,
     * though this will come at the cost of an additional space complexity O(n).
     * </p>
     *
     * @param coll1 The first collection, must not be null.
     * @param coll2 The second collection, must not be null.
     * @return {@code true} iff the intersection of the collections has the same cardinality as the set of unique elements from the second collection.
     * @throws NullPointerException if coll1 or coll2 is null.
     * @since 4.0
     */
    public static boolean containsAll(final Collection<?> coll1, final Collection<?> coll2) {
        Objects.requireNonNull(coll1, "coll1");
        Objects.requireNonNull(coll2, "coll2");
        if (coll2.isEmpty()) {
            return true;
        }
        final Set<Object> elementsAlreadySeen = new HashSet<>();
        for (final Object nextElement : coll2) {
            if (elementsAlreadySeen.contains(nextElement)) {
                continue;
            }
            boolean foundCurrentElement = false;
            for (final Object p : coll1) {
                elementsAlreadySeen.add(p);
                if (Objects.equals(nextElement, p)) {
                    foundCurrentElement = true;
                    break;
                }
            }
            if (!foundCurrentElement) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns {@code true} iff at least one element is in both collections.
     * <p>
     * In other words, this method returns {@code true} iff the {@link #intersection} of <em>coll1</em> and <em>coll2</em> is not empty.
     * </p>
     *
     * @param coll1 The first collection, must not be null.
     * @param coll2 The second collection, must not be null.
     * @return {@code true} iff the intersection of the collections is non-empty.
     * @throws NullPointerException if coll1 or coll2 is null.
     * @since 2.1
     * @see #intersection
     */
    public static boolean containsAny(final Collection<?> coll1, final Collection<?> coll2) {
        Objects.requireNonNull(coll1, "coll1");
        Objects.requireNonNull(coll2, "coll2");
        if (coll1.size() < coll2.size()) {
            for (final Object aColl1 : coll1) {
                if (coll2.contains(aColl1)) {
                    return true;
                }
            }
        } else {
            for (final Object aColl2 : coll2) {
                if (coll1.contains(aColl2)) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Returns {@code true} iff at least one element is in both collections.
     * <p>
     * In other words, this method returns {@code true} iff the {@link #intersection} of <em>coll1</em> and <em>coll2</em> is not empty.
     * </p>
     *
     * @param <T>   The type of object to lookup in {@code coll1}.
     * @param coll1 The first collection, must not be {@code null}.
     * @param coll2 The second collection, must not be {@code null}.
     * @return {@code true} iff the intersection of the collections is non-empty.
     * @throws NullPointerException if coll1 or coll2 is {@code null}.
     * @since 4.2
     * @see #intersection
     */
    public static <T> boolean containsAny(final Collection<?> coll1, @SuppressWarnings("unchecked") final T... coll2) {
        Objects.requireNonNull(coll1, "coll1");
        Objects.requireNonNull(coll2, "coll2");
        if (coll1.size() < coll2.length) {
            for (final Object aColl1 : coll1) {
                if (ArrayUtils.contains(coll2, aColl1)) {
                    return true;
                }
            }
        } else {
            for (final Object aColl2 : coll2) {
                if (coll1.contains(aColl2)) {
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Counts the number of elements in the input collection that match the predicate.
     * <p>
     * A {@code null} collection or predicate matches no elements.
     * </p>
     *
     * @param <C>       the type of object the {@link Iterable} contains.
     * @param input     The {@link Iterable} to get the input from, may be null.
     * @param predicate The predicate to use, may be null.
     * @return The number of matches for the predicate in the collection.
     * @deprecated Since 4.1, use {@link IterableUtils#countMatches(Iterable, Predicate)} instead.
     */
    @Deprecated
    public static <C> int countMatches(final Iterable<C> input, final Predicate<? super C> predicate) {
        return predicate == null ? 0 : (int) IterableUtils.countMatches(input, predicate);
    }

    /**
     * Returns a {@link Collection} containing the exclusive disjunction (symmetric difference) of the given {@link Iterable}s.
     * <p>
     * The cardinality of each element <em>e</em> in the returned {@link Collection} will be equal to
     * <code>max(cardinality(<em>e</em>,<em>a</em>),cardinality(<em>e</em>,<em>b</em>)) - min(cardinality(<em>e</em>,<em>a</em>),
     * cardinality(<em>e</em>,<em>b</em>))</code>.
     * </p>
     * <p>
     * This is equivalent to {@code {@link #subtract subtract}({@link #union union(a, b)},{@link #intersection intersection(a, b)})} or
     * {@code {@link #union union}({@link #subtract subtract(a, b)},{@link #subtract subtract(b, a)})}.
     * </p>
     *
     * @param a   The first collection, must not be null.
     * @param b   The second collection, must not be null.
     * @param <O> The generic type that is able to represent the types contained in both input collections.
     * @return The symmetric difference of the two collections.
     * @throws NullPointerException if either collection is null.
     */
    public static <O> Collection<O> disjunction(final Iterable<? extends O> a, final Iterable<? extends O> b) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        final SetOperationCardinalityHelper<O> helper = new SetOperationCardinalityHelper<>(a, b);
        for (final O obj : helper) {
            helper.setCardinality(obj, helper.max(obj) - helper.min(obj));
        }
        return helper.list();
    }

    /**
     * Returns the immutable EMPTY_COLLECTION with generic type safety.
     *
     * @param <T> The element type.
     * @return immutable empty collection.
     * @see #EMPTY_COLLECTION
     * @since 4.0
     */
    @SuppressWarnings("unchecked") // OK, empty collection is compatible with any type
    public static <T> Collection<T> emptyCollection() {
        return EMPTY_COLLECTION;
    }

    /**
     * Returns an immutable empty collection if the argument is {@code null}, or the argument itself otherwise.
     *
     * @param <T>        The element type.
     * @param collection The collection, possibly {@code null}.
     * @return An empty collection if the argument is {@code null}.
     */
    public static <T> Collection<T> emptyIfNull(final Collection<T> collection) {
        return collection == null ? emptyCollection() : collection;
    }

    /**
     * Answers true if a predicate is true for at least one element of a collection.
     * <p>
     * A {@code null} collection or predicate returns false.
     * </p>
     *
     * @param <C>       the type of object the {@link Iterable} contains.
     * @param input     The {@link Iterable} to get the input from, may be null.
     * @param predicate The predicate to use, may be null.
     * @return true if at least one element of the collection matches the predicate.
     * @deprecated Since 4.1, use {@link IterableUtils#matchesAny(Iterable, Predicate)} instead.
     */
    @Deprecated
    public static <C> boolean exists(final Iterable<C> input, final Predicate<? super C> predicate) {
        return predicate != null && IterableUtils.matchesAny(input, predicate);
    }

    /**
     * Extract the lone element of the specified Collection.
     *.
     * @param <E>        collection type
     * @param collection to read.
     * @return sole member of collection.
     * @throws NullPointerException     if collection is null.
     * @throws IllegalArgumentException if collection is empty or contains more than one element.
     * @since 4.0
     */
    public static <E> E extractSingleton(final Collection<E> collection) {
        Objects.requireNonNull(collection, "collection");
        if (collection.size() != 1) {
            throw new IllegalArgumentException("Can extract singleton only when collection size == 1");
        }
        return collection.iterator().next();
    }

    /**
     * Filter the collection by applying a Predicate to each element. If the predicate returns false, remove the element.
     * <p>
     * If the input collection or predicate is null, there is no change made.
     * </p>
     *
     * @param <T>        the type of object the {@link Iterable} contains.
     * @param collection The collection to get the input from, may be null.
     * @param predicate  The predicate to use as a filter, may be null.
     * @return true if the collection is modified by this call, false otherwise.
     */
    public static <T> boolean filter(final Iterable<T> collection, final Predicate<? super T> predicate) {
        boolean result = false;
        if (collection != null && predicate != null) {
            for (final Iterator<T> it = collection.iterator(); it.hasNext();) {
                if (!predicate.test(it.next())) {
                    it.remove();
                    result = true;
                }
            }
        }
        return result;
    }

    /**
     * Filter the collection by applying a Predicate to each element. If the predicate returns true, remove the element.
     * <p>
     * This is equivalent to {@code filter(collection, PredicateUtils.notPredicate(predicate))} if predicate is != null.
     * </p>
     * <p>
     * If the input collection or predicate is null, there is no change made.
     * </p>
     *
     * @param <T>        the type of object the {@link Iterable} contains.
     * @param collection The collection to get the input from, may be null.
     * @param predicate  The predicate to use as a filter, may be null.
     * @return true if the collection is modified by this call, false otherwise.
     */
    public static <T> boolean filterInverse(final Iterable<T> collection, final Predicate<? super T> predicate) {
        return filter(collection, predicate == null ? null : PredicateUtils.notPredicate(predicate));
    }

    /**
     * Finds the first element in the given collection which matches the given predicate.
     * <p>
     * If the input collection or predicate is null, or no element of the collection matches the predicate, null is returned.
     * </p>
     *
     * @param <T>        the type of object the {@link Iterable} contains.
     * @param collection The collection to search, may be null.
     * @param predicate  The predicate to use, may be null.
     * @return The first element of the collection which matches the predicate or null if none could be found.
     * @deprecated Since 4.1, use {@link IterableUtils#find(Iterable, Predicate)} instead.
     */
    @Deprecated
    public static <T> T find(final Iterable<T> collection, final Predicate<? super T> predicate) {
        return predicate != null ? IterableUtils.find(collection, predicate) : null;
    }

    /**
     * Executes the given closure on each but the last element in the collection.
     * <p>
     * If the input collection or closure is null, there is no change made.
     * </p>
     *
     * @param <T>        the type of object the {@link Iterable} contains.
     * @param <C>        the closure type.
     * @param collection The collection to get the input from, may be null.
     * @param closure    The closure to perform, may be null.
     * @return The last element in the collection, or null if either collection or closure is null.
     * @since 4.0
     * @deprecated Since 4.1, use {@link IterableUtils#forEachButLast(Iterable, Closure)} instead.
     */
    @Deprecated
    public static <T, C extends Closure<? super T>> T forAllButLastDo(final Iterable<T> collection, final C closure) {
        return closure != null ? IterableUtils.forEachButLast(collection, closure) : null;
    }

    /**
     * Executes the given closure on each but the last element in the collection.
     * <p>
     * If the input collection or closure is null, there is no change made.
     * </p>
     *
     * @param <T>      the type of object the {@link Collection} contains.
     * @param <C>      the closure type.
     * @param iterator The iterator to get the input from, may be null.
     * @param closure  The closure to perform, may be null.
     * @return The last element in the collection, or null if either iterator or closure is null.
     * @since 4.0
     * @deprecated Since 4.1, use {@link IteratorUtils#forEachButLast(Iterator, Closure)} instead.
     */
    @Deprecated
    public static <T, C extends Closure<? super T>> T forAllButLastDo(final Iterator<T> iterator, final C closure) {
        return closure != null ? IteratorUtils.forEachButLast(iterator, closure) : null;
    }

    /**
     * Executes the given closure on each element in the collection.
     * <p>
     * If the input collection or closure is null, there is no change made.
     * </p>
     *
     * @param <T>        the type of object the {@link Iterable} contains.
     * @param <C>        the closure type.
     * @param collection The collection to get the input from, may be null.
     * @param closure    The closure to perform, may be null.
     * @return closure
     * @deprecated Since 4.1, use {@link IterableUtils#forEach(Iterable, Closure)} instead.
     */
    @Deprecated
    public static <T, C extends Closure<? super T>> C forAllDo(final Iterable<T> collection, final C closure) {
        if (closure != null) {
            IterableUtils.forEach(collection, closure);
        }
        return closure;
    }

    /**
     * Executes the given closure on each element in the collection.
     * <p>
     * If the input collection or closure is null, there is no change made.
     * </p>
     *
     * @param <T>      the type of object the {@link Iterator} contains.
     * @param <C>      the closure type.
     * @param iterator The iterator to get the input from, may be null.
     * @param closure  The closure to perform, may be null.
     * @return closure
     * @since 4.0
     * @deprecated Since 4.1, use {@link IteratorUtils#forEach(Iterator, Closure)} instead.
     */
    @Deprecated
    public static <T, C extends Closure<? super T>> C forAllDo(final Iterator<T> iterator, final C closure) {
        if (closure != null) {
            IteratorUtils.forEach(iterator, closure);
        }
        return closure;
    }

    /**
     * Gets the {@code index}-th value in the {@code iterable}'s {@link Iterator}, throwing {@code IndexOutOfBoundsException} if there is no such element.
     * <p>
     * If the {@link Iterable} is a {@link List}, then it will use {@link List#get(int)}.
     * </p>
     *
     * @param iterable The {@link Iterable} to get a value from.
     * @param index    The index to get.
     * @param <T>      The type of object in the {@link Iterable}.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     * @throws NullPointerException      if iterable is null.
     * @deprecated Since 4.1, use {@code IterableUtils.get(Iterable, int)} instead.
     */
    @Deprecated
    public static <T> T get(final Iterable<T> iterable, final int index) {
        Objects.requireNonNull(iterable, "iterable");
        return IterableUtils.get(iterable, index);
    }

    /**
     * Gets the {@code index}-th value in {@link Iterator}, throwing {@code IndexOutOfBoundsException} if there is no such element.
     * <p>
     * The Iterator is advanced to {@code index} (or to the end, if {@code index} exceeds the number of entries) as a side effect of this method.
     * </p>
     *
     * @param iterator The iterator to get a value from.
     * @param index    The index to get.
     * @param <T>      The type of object in the {@link Iterator}.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     * @throws IllegalArgumentException  if the object type is invalid.
     * @throws NullPointerException      if iterator is null.
     * @deprecated Since 4.1, use {@code IteratorUtils.get(Iterator, int)} instead.
     */
    @Deprecated
    public static <T> T get(final Iterator<T> iterator, final int index) {
        Objects.requireNonNull(iterator, "iterator");
        return IteratorUtils.get(iterator, index);
    }

    /**
     * Gets the {@code index}-th {@code Map.Entry} in the {@code map}'s {@code entrySet}, throwing {@code IndexOutOfBoundsException} if there is no such
     * element.
     *
     * @param <K>   the key type in the {@link Map}.
     * @param <V>   the value type in the {@link Map}.
     * @param map   The object to get a value from.
     * @param index The index to get.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     * @throws NullPointerException      if map is null.
     */
    public static <K, V> Map.Entry<K, V> get(final Map<K, V> map, final int index) {
        Objects.requireNonNull(map, "map");
        checkIndexBounds(index);
        return get(map.entrySet(), index);
    }

    /**
     * Gets the {@code index}-th value in {@code object}, throwing {@code IndexOutOfBoundsException} if there is no such element or
     * {@code IllegalArgumentException} if {@code object} is not an instance of one of the supported types.
     * <p>
     * The supported types, and associated semantics are:
     * </p>
     * <ul>
     * <li>Map -- the value returned is the {@code Map.Entry} in position {@code index} in the map's {@code entrySet} iterator, if there is such an entry.</li>
     * <li>List -- this method is equivalent to the list's get method.</li>
     * <li>Array -- the {@code index}-th array entry is returned, if there is such an entry; otherwise an {@code IndexOutOfBoundsException} is thrown.</li>
     * <li>Collection -- the value returned is the {@code index}-th object returned by the collection's default iterator, if there is such an element.</li>
     * <li>Iterator or Enumeration -- the value returned is the {@code index}-th object in the Iterator/Enumeration, if there is such an element. The
     * Iterator/Enumeration is advanced to {@code index} (or to the end, if {@code index} exceeds the number of entries) as a side effect of this method.</li>
     * </ul>
     *
     * @param object The object to get a value from.
     * @param index  The index to get.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     * @throws IllegalArgumentException  if the object type is invalid.
     */
    public static Object get(final Object object, final int index) {
        final int i = index;
        if (i < 0) {
            throw new IndexOutOfBoundsException("Index cannot be negative: " + i);
        }
        if (object instanceof Map<?, ?>) {
            final Map<?, ?> map = (Map<?, ?>) object;
            final Iterator<?> iterator = map.entrySet().iterator();
            return IteratorUtils.get(iterator, i);
        }
        if (object instanceof Object[]) {
            return ((Object[]) object)[i];
        }
        if (object instanceof Iterator<?>) {
            final Iterator<?> it = (Iterator<?>) object;
            return IteratorUtils.get(it, i);
        }
        if (object instanceof Iterable<?>) {
            final Iterable<?> iterable = (Iterable<?>) object;
            return IterableUtils.get(iterable, i);
        }
        if (object instanceof Enumeration<?>) {
            final Enumeration<?> it = (Enumeration<?>) object;
            return EnumerationUtils.get(it, i);
        }
        if (object == null) {
            throw new IllegalArgumentException("Unsupported object type: null");
        }
        try {
            return Array.get(object, i);
        } catch (final IllegalArgumentException ex) {
            throw new IllegalArgumentException("Unsupported object type: " + object.getClass().getName());
        }
    }

    /**
     * Gets a {@link Map} mapping each unique element in the given {@link Collection} to an {@link Integer} representing the number of occurrences of that
     * element in the {@link Collection}.
     * <p>
     * Only those elements present in the collection will appear as keys in the map.
     * </p>
     *
     * @param <O>  the type of object in the returned {@link Map}. This is a super type of &lt;I&gt;.
     * @param coll The collection to get the cardinality map for, must not be null.
     * @return The populated cardinality map.
     * @throws NullPointerException if coll is null.
     */
    public static <O> Map<O, Integer> getCardinalityMap(final Iterable<? extends O> coll) {
        Objects.requireNonNull(coll, "coll");
        final Map<O, Integer> count = new HashMap<>();
        for (final O obj : coll) {
            final Integer c = count.get(obj);
            if (c == null) {
                count.put(obj, Integer.valueOf(1));
            } else {
                count.put(obj, Integer.valueOf(c.intValue() + 1));
            }
        }
        return count;
    }

    /**
     * Returns the hash code of the input collection using the hash method of an equator.
     * <p>
     * Returns 0 if the input collection is {@code null}.
     * </p>
     *
     * @param <E>        the element type.
     * @param collection The input collection.
     * @param equator    The equator used for generate hashCode.
     * @return The hash code of the input collection using the hash method of an equator.
     * @throws NullPointerException if the equator is {@code null}.
     * @since 4.5.0-M1
     */
    public static <E> int hashCode(final Collection<? extends E> collection, final Equator<? super E> equator) {
        Objects.requireNonNull(equator, "equator");
        if (collection == null) {
            return 0;
        }
        int hashCode = 1;
        for (final E e : collection) {
            hashCode = 31 * hashCode + equator.hash(e);
        }
        return hashCode;
    }

    /**
     * Returns a {@link Collection} containing the intersection of the given {@link Iterable}s.
     * <p>
     * The cardinality of each element in the returned {@link Collection} will be equal to the minimum of the cardinality of that element in the two given
     * {@link Iterable}s.
     * </p>
     *
     * @param a   The first collection, must not be null.
     * @param b   The second collection, must not be null.
     * @param <O> The generic type that is able to represent the types contained in both input collections.
     * @return The intersection of the two collections.
     * @throws NullPointerException if either collection is null.
     * @see Collection#retainAll
     * @see #containsAny
     */
    public static <O> Collection<O> intersection(final Iterable<? extends O> a, final Iterable<? extends O> b) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        final SetOperationCardinalityHelper<O> helper = new SetOperationCardinalityHelper<>(a, b);
        for (final O obj : helper) {
            helper.setCardinality(obj, helper.min(obj));
        }
        return helper.list();
    }

    /**
     * Null-safe check if the specified collection is empty.
     * <p>
     * Null returns true.
     * </p>
     *
     * @param coll The collection to check, may be null.
     * @return true if empty or null
     * @since 3.2
     */
    public static boolean isEmpty(final Collection<?> coll) {
        return coll == null || coll.isEmpty();
    }

    /**
     * Returns {@code true} iff the given {@link Collection}s contain exactly the same elements with exactly the same cardinalities.
     * <p>
     * That is, iff the cardinality of <em>e</em> in <em>a</em> is equal to the cardinality of <em>e</em> in <em>b</em>, for each element <em>e</em> in
     * <em>a</em> or <em>b</em>.
     * </p>
     *
     * @param a The first collection, must not be null.
     * @param b The second collection, must not be null.
     * @return {@code true} iff the collections contain the same elements with the same cardinalities.
     * @throws NullPointerException if either collection is null.
     */
    public static boolean isEqualCollection(final Collection<?> a, final Collection<?> b) {
        return CardinalityHelper.equals(a, b);
    }

    /**
     * Returns {@code true} iff the given {@link Collection}s contain exactly the same elements with exactly the same cardinalities.
     * <p>
     * That is, iff the cardinality of <em>e</em> in <em>a</em> is equal to the cardinality of <em>e</em> in <em>b</em>, for each element <em>e</em> in
     * <em>a</em> or <em>b</em>.
     * </p>
     * <p>
     * <strong>Note:</strong> from version 4.1 onwards this method requires the input collections and equator to be of compatible type (using bounded
     * wildcards). Providing incompatible arguments (for example by casting to their rawtypes) will result in a {@code ClassCastException} thrown at runtime.
     * </p>
     *
     * @param <E>     the element type.
     * @param a       The first collection, must not be null.
     * @param b       The second collection, must not be null.
     * @param equator The Equator used for testing equality.
     * @return {@code true} iff the collections contain the same elements with the same cardinalities.
     * @throws NullPointerException if either collection or equator is null.
     * @since 4.0
     */
    public static <E> boolean isEqualCollection(final Collection<? extends E> a, final Collection<? extends E> b, final Equator<? super E> equator) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        Objects.requireNonNull(equator, "equator");
        if (a.size() != b.size()) {
            return false;
        }
        @SuppressWarnings({ "unchecked", "rawtypes" })
        final Transformer<E, ?> transformer = input -> new EquatorWrapper(equator, input);
        return isEqualCollection(collect(a, transformer), collect(b, transformer));
    }

    /**
     * Returns true if no more elements can be added to the Collection.
     * <p>
     * This method uses the {@link BoundedCollection} interface to determine the full status. If the collection does not implement this interface then false is
     * returned.
     * </p>
     * <p>
     * The collection does not have to implement this interface directly. If the collection has been decorated using the decorators subpackage then these will
     * be removed to access the BoundedCollection.
     * </p>
     *
     * @param collection The collection to check.
     * @return true if the BoundedCollection is full.
     * @throws NullPointerException if the collection is null.
     */
    public static boolean isFull(final Collection<? extends Object> collection) {
        Objects.requireNonNull(collection, "collection");
        if (collection instanceof BoundedCollection) {
            return ((BoundedCollection<?>) collection).isFull();
        }
        try {
            final BoundedCollection<?> bcoll = UnmodifiableBoundedCollection.unmodifiableBoundedCollection(collection);
            return bcoll.isFull();
        } catch (final IllegalArgumentException ex) {
            return false;
        }
    }

    /**
     * Null-safe check if the specified collection is not empty.
     * <p>
     * Null returns false.
     * </p>
     *
     * @param coll The collection to check, may be null.
     * @return true if non-null and non-empty.
     * @since 3.2
     */
    public static boolean isNotEmpty(final Collection<?> coll) {
        return !isEmpty(coll);
    }

    /**
     * Returns {@code true} iff <em>a</em> is a <em>proper</em> sub-collection of <em>b</em>, that is, iff the cardinality of <em>e</em> in <em>a</em> is less
     * than or equal to the cardinality of <em>e</em> in <em>b</em>, for each element <em>e</em> in <em>a</em>, and there is at least one element <em>f</em>
     * such that the cardinality of <em>f</em> in <em>b</em> is strictly greater than the cardinality of <em>f</em> in <em>a</em>.
     * <p>
     * The implementation assumes
     * </p>
     * <ul>
     * <li>{@code a.size()} and {@code b.size()} represent the total cardinality of <em>a</em> and <em>b</em>, resp.</li>
     * <li>{@code a.size() &lt; Integer.MAXVALUE}</li>
     * </ul>
     *
     * @param a The first (sub?) collection, must not be null.
     * @param b The second (super?) collection, must not be null.
     * @return {@code true} iff <em>a</em> is a <em>proper</em> sub-collection of <em>b</em>.
     * @throws NullPointerException if either collection is null.
     * @see #isSubCollection
     * @see Collection#containsAll
     */
    public static boolean isProperSubCollection(final Collection<?> a, final Collection<?> b) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        return a.size() < b.size() && isSubCollection(a, b);
    }

    /**
     * Returns {@code true} iff <em>a</em> is a sub-collection of <em>b</em>, that is, iff the cardinality of <em>e</em> in <em>a</em> is less than or equal to
     * the cardinality of <em>e</em> in <em>b</em>, for each element <em>e</em> in <em>a</em>.
     *
     * @param a The first (sub?) collection, must not be null.
     * @param b The second (super?) collection, must not be null.
     * @return {@code true} iff <em>a</em> is a sub-collection of <em>b</em>.
     * @throws NullPointerException if either collection is null.
     * @see #isProperSubCollection
     * @see Collection#containsAll
     */
    public static boolean isSubCollection(final Collection<?> a, final Collection<?> b) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        final CardinalityHelper<Object> helper = new CardinalityHelper<>(a, b);
        for (final Object obj : a) {
            if (helper.freqA(obj) > helper.freqB(obj)) {
                return false;
            }
        }
        return true;
    }

    /**
     * Answers true if a predicate is true for every element of a collection.
     * <p>
     * A {@code null} predicate returns false.
     * </p>
     * <p>
     * A {@code null} or empty collection returns true.
     * </p>
     *
     * @param <C>       the type of object the {@link Iterable} contains.
     * @param input     The {@link Iterable} to get the input from, may be null.
     * @param predicate The predicate to use, may be null.
     * @return true if every element of the collection matches the predicate or if the collection is empty, false otherwise.
     * @since 4.0
     * @deprecated Since 4.1, use {@link IterableUtils#matchesAll(Iterable, Predicate)} instead
     */
    @Deprecated
    public static <C> boolean matchesAll(final Iterable<C> input, final Predicate<? super C> predicate) {
        return predicate != null && IterableUtils.matchesAll(input, predicate);
    }

    /**
     * Gets the maximum number of elements that the Collection can contain.
     * <p>
     * This method uses the {@link BoundedCollection} interface to determine the maximum size. If the collection does not implement this interface then -1 is
     * returned.
     * </p>
     * <p>
     * The collection does not have to implement this interface directly. If the collection has been decorated using the decorators subpackage then these will
     * be removed to access the BoundedCollection.
     * </p>
     *
     * @param collection The collection to check.
     * @return The maximum size of the BoundedCollection, -1 if no maximum size.
     * @throws NullPointerException if the collection is null.
     */
    public static int maxSize(final Collection<? extends Object> collection) {
        Objects.requireNonNull(collection, "collection");
        if (collection instanceof BoundedCollection) {
            return ((BoundedCollection<?>) collection).maxSize();
        }
        try {
            final BoundedCollection<?> bcoll = UnmodifiableBoundedCollection.unmodifiableBoundedCollection(collection);
            return bcoll.maxSize();
        } catch (final IllegalArgumentException ex) {
            return -1;
        }
    }

    /**
     * Returns a {@link Collection} of all the permutations of the input collection.
     * <p>
     * NOTE: the number of permutations of a given collection is equal to n!, where n is the size of the collection. Thus, the resulting collection will become
     * <strong>very</strong> large for collections &gt; 10 (for example 10! = 3628800, 15! = 1307674368000).
     * </p>
     * <p>
     * For larger collections it is advised to use a {@link PermutationIterator} to iterate over all permutations.
     * </p>
     *
     * @param <E>        the element type.
     * @param collection The collection to create permutations for, must not be null.
     * @return An unordered collection of all permutations of the input collection.
     * @throws NullPointerException if collection is null.
     * @see PermutationIterator
     * @since 4.0
     */
    public static <E> Collection<List<E>> permutations(final Collection<E> collection) {
        Objects.requireNonNull(collection, "collection");
        final PermutationIterator<E> it = new PermutationIterator<>(collection);
        final Collection<List<E>> result = new ArrayList<>();
        while (it.hasNext()) {
            result.add(it.next());
        }
        return result;
    }

    /**
     * Returns a predicated (validating) collection backed by the given collection.
     * <p>
     * Only objects that pass the test in the given predicate can be added to the collection. Trying to add an invalid object results in an
     * IllegalArgumentException. It is important not to use the original collection after invoking this method, as it is a backdoor for adding invalid objects.
     * </p>
     *
     * @param <C>        The type of objects in the Collection.
     * @param collection The collection to predicate, must not be null.
     * @param predicate  The predicate for the collection, must not be null.
     * @return A predicated collection backed by the given collection.
     * @throws NullPointerException if the collection or predicate is null.
     */
    public static <C> Collection<C> predicatedCollection(final Collection<C> collection, final Predicate<? super C> predicate) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(predicate, "predicate");
        return PredicatedCollection.predicatedCollection(collection, predicate);
    }

    /**
     * Removes the elements in {@code remove} from {@code collection}. That is, this method returns a collection containing all the elements in {@code c} that
     * are not in {@code remove}. The cardinality of an element {@code e} in the returned collection is the same as the cardinality of {@code e} in
     * {@code collection} unless {@code remove} contains {@code e}, in which case the cardinality is zero. This method is useful if you do not wish to modify
     * the collection {@code c} and thus cannot call {@code collection.removeAll(remove);}.
     * <p>
     * This implementation iterates over {@code collection}, checking each element in turn to see if it's contained in {@code remove}. If it's not contained,
     * it's added to the returned list. As a consequence, it is advised to use a collection type for {@code remove} that provides a fast (for example O(1))
     * implementation of {@link Collection#contains(Object)}.
     * </p>
     *
     * @param <E>        the type of object the {@link Collection} contains.
     * @param collection The collection from which items are removed (in the returned collection).
     * @param remove     The items to be removed from the returned {@code collection}.
     * @return A {@code Collection} containing all the elements of {@code collection} except any elements that also occur in {@code remove}.
     * @throws NullPointerException if either parameter is null.
     * @since 4.0 (method existed in 3.2 but was completely broken).
     */
    public static <E> Collection<E> removeAll(final Collection<E> collection, final Collection<?> remove) {
        return ListUtils.removeAll(collection, remove);
    }

    /**
     * Removes all elements in {@code remove} from {@code collection}. That is, this method returns a collection containing all the elements in
     * {@code collection} that are not in {@code remove}. The cardinality of an element {@code e} in the returned collection is the same as the cardinality of
     * {@code e} in {@code collection} unless {@code remove} contains {@code e}, in which case the cardinality is zero. This method is useful if you do not wish
     * to modify the collection {@code c} and thus cannot call {@code collection.removeAll(remove)}.
     * <p>
     * Moreover this method uses an {@link Equator} instead of {@link Object#equals(Object)} to determine the equality of the elements in {@code collection} and
     * {@code remove}. Hence this method is useful in cases where the equals behavior of an object needs to be modified without changing the object itself.
     * </p>
     *
     * @param <E>        The type of object the {@link Collection} contains.
     * @param collection The collection from which items are removed (in the returned collection).
     * @param remove     The items to be removed from the returned collection.
     * @param equator    The Equator used for testing equality.
     * @return A {@code Collection} containing all the elements of {@code collection} except any element that if equal according to the {@code equator}
     * @throws NullPointerException if any of the parameters is null.
     * @since 4.1
     */
    public static <E> Collection<E> removeAll(final Iterable<E> collection, final Iterable<? extends E> remove, final Equator<? super E> equator) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(remove, "remove");
        Objects.requireNonNull(equator, "equator");
        final Transformer<E, EquatorWrapper<E>> transformer = input -> new EquatorWrapper<>(equator, input);
        final Set<EquatorWrapper<E>> removeSet = collect(remove, transformer, new HashSet<>());
        final List<E> list = new ArrayList<>();
        for (final E element : collection) {
            if (!removeSet.contains(new EquatorWrapper<>(equator, element))) {
                list.add(element);
            }
        }
        return list;
    }

    /**
     * Removes the specified number of elements from the start index in the collection and returns them. This method modifies the input collections.
     *
     * @param <E>        the type of object the {@link Collection} contains.
     * @param input      The collection will be operated, can't be null.
     * @param startIndex The start index (inclusive) to remove element, can't be less than 0.
     * @param count      The specified number to remove, can't be less than 1.
     * @return collection of elements that removed from the input collection.
     * @throws NullPointerException      if input is null
     * @throws IndexOutOfBoundsException if startIndex is less than 0, count is less than 0, or the sum of startIndex and count is greater than the collection size.
     * @since 4.5.0-M1
     */
    public static <E> Collection<E> removeCount(final Collection<E> input, int startIndex, int count) {
        Objects.requireNonNull(input, "input");
        if (startIndex < 0) {
            throw new IndexOutOfBoundsException("The start index can't be less than 0.");
        }
        if (count < 0) {
            throw new IndexOutOfBoundsException("The count can't be less than 0.");
        }
        if (input.size() < startIndex + count) {
            throw new IndexOutOfBoundsException("The sum of start index and count can't be greater than the size of collection.");
        }
        final Collection<E> result = new ArrayList<>(count);
        final Iterator<E> iterator = input.iterator();
        while (count > 0) {
            if (startIndex > 0) {
                startIndex -= 1;
                iterator.next();
                continue;
            }
            count -= 1;
            result.add(iterator.next());
            iterator.remove();
        }
        return result;
    }

    /**
     * Removes elements whose index are between startIndex, inclusive and endIndex, exclusive in the collection and returns them. This method modifies the input
     * collections.
     *
     * @param <E>        the type of object the {@link Collection} contains.
     * @param input      The collection will be operated, must not be null.
     * @param startIndex The start index (inclusive) to remove element, must not be less than 0.
     * @param endIndex   The end index (exclusive) to remove, must not be less than startIndex.
     * @return collection of elements that removed from the input collection.
     * @throws NullPointerException      if input is null.
     * @throws IllegalArgumentException  if endIndex is less than startIndex.
     * @throws IndexOutOfBoundsException if endIndex is greater than the size of the collection, or startIndex is less than 0.
     * @since 4.5.0-M1
     */
    public static <E> Collection<E> removeRange(final Collection<E> input, final int startIndex, final int endIndex) {
        Objects.requireNonNull(input, "input");
        if (endIndex < startIndex) {
            throw new IllegalArgumentException("The end index can't be less than the start index.");
        }
        if (input.size() < endIndex) {
            throw new IndexOutOfBoundsException("The end index can't be greater than the size of collection.");
        }
        return removeCount(input, startIndex, endIndex - startIndex);
    }

    /**
     * Returns a collection containing all the elements in {@code collection} that are also in {@code retain}. The cardinality of an element {@code e} in the
     * returned collection is the same as the cardinality of {@code e} in {@code collection} unless {@code retain} does not contain {@code e}, in which case the
     * cardinality is zero. This method is useful if you do not wish to modify the collection {@code c} and thus cannot call {@code c.retainAll(retain);}.
     * <p>
     * This implementation iterates over {@code collection}, checking each element in turn to see if it's contained in {@code retain}. If it's contained, it's
     * added to the returned list. As a consequence, it is advised to use a collection type for {@code retain} that provides a fast (for example O(1))
     * implementation of {@link Collection#contains(Object)}.
     * </p>
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection whose contents are the target of the #retailAll operation.
     * @param retain     The collection containing the elements to be retained in the returned collection.
     * @return A {@code Collection} containing all the elements of {@code collection} that occur at least once in {@code retain}.
     * @throws NullPointerException if either parameter is null.
     * @since 3.2
     */
    public static <C> Collection<C> retainAll(final Collection<C> collection, final Collection<?> retain) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(retain, "retain");
        return ListUtils.retainAll(collection, retain);
    }

    /**
     * Returns a collection containing all the elements in {@code collection} that are also in {@code retain}. The cardinality of an element {@code e} in the
     * returned collection is the same as the cardinality of {@code e} in {@code collection} unless {@code retain} does not contain {@code e}, in which case the
     * cardinality is zero. This method is useful if you do not wish to modify the collection {@code c} and thus cannot call {@code c.retainAll(retain);}.
     * <p>
     * Moreover this method uses an {@link Equator} instead of {@link Object#equals(Object)} to determine the equality of the elements in {@code collection} and
     * {@code retain}. Hence this method is useful in cases where the equals behavior of an object needs to be modified without changing the object itself.
     * </p>
     *
     * @param <E>        The type of object the {@link Collection} contains.
     * @param collection The collection whose contents are the target of the {@code retainAll} operation.
     * @param retain     The collection containing the elements to be retained in the returned collection.
     * @param equator    The Equator used for testing equality.
     * @return A {@code Collection} containing all the elements of {@code collection} that occur at least once in {@code retain} according to the
     *         {@code equator}.
     * @throws NullPointerException if any of the parameters is null.
     * @since 4.1
     */
    public static <E> Collection<E> retainAll(final Iterable<E> collection, final Iterable<? extends E> retain, final Equator<? super E> equator) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(retain, "retain");
        Objects.requireNonNull(equator, "equator");
        final Transformer<E, EquatorWrapper<E>> transformer = input -> new EquatorWrapper<>(equator, input);
        final Set<EquatorWrapper<E>> retainSet = collect(retain, transformer, new HashSet<>());
        final List<E> list = new ArrayList<>();
        for (final E element : collection) {
            if (retainSet.contains(new EquatorWrapper<>(equator, element))) {
                list.add(element);
            }
        }
        return list;
    }

    /**
     * Reverses the order of the given array.
     *
     * @param array The array to reverse.
     * @throws NullPointerException if array is null.
     */
    public static void reverseArray(final Object[] array) {
        Objects.requireNonNull(array, "array");
        int i = 0;
        int j = array.length - 1;
        Object tmp;
        while (j > i) {
            tmp = array[j];
            array[j] = array[i];
            array[i] = tmp;
            j--;
            i++;
        }
    }

    /**
     * Selects all elements from input collection which match the given predicate into an output collection.
     * <p>
     * A {@code null} predicate matches no elements.
     * </p>
     *
     * @param <O>             the type of object the {@link Iterable} contains.
     * @param inputCollection The collection to get the input from, may not be null.
     * @param predicate       The predicate to use, may be null.
     * @return The elements matching the predicate (new list).
     */
    public static <O> Collection<O> select(final Iterable<? extends O> inputCollection, final Predicate<? super O> predicate) {
        int size = 0;
        if (inputCollection != null) {
            size = inputCollection instanceof Collection<?> ? ((Collection<?>) inputCollection).size() : 0;
        }
        final Collection<O> answer = size == 0 ? new ArrayList<>() : new ArrayList<>(size);
        return select(inputCollection, predicate, answer);
    }

    /**
     * Selects all elements from input collection which match the given predicate and adds them to outputCollection.
     * <p>
     * If the input collection or predicate is null, there is no change to the output collection.
     * </p>
     *
     * @param <O>              the type of object the {@link Iterable} contains.
     * @param <R>              the type of the output {@link Collection}.
     * @param inputCollection  The collection to get the input from, may be null.
     * @param predicate        The predicate to use, may be null.
     * @param outputCollection The collection to output into, may not be null if the inputCollection and predicate or not null.
     * @return The outputCollection
     */
    public static <O, R extends Collection<? super O>> R select(final Iterable<? extends O> inputCollection, final Predicate<? super O> predicate,
            final R outputCollection) {
        if (inputCollection != null && predicate != null) {
            for (final O item : inputCollection) {
                if (predicate.test(item)) {
                    outputCollection.add(item);
                }
            }
        }
        return outputCollection;
    }

    /**
     * Selects all elements from inputCollection into an output and rejected collection, based on the evaluation of the given predicate.
     * <p>
     * Elements matching the predicate are added to the {@code outputCollection}, all other elements are added to the {@code rejectedCollection}.
     * </p>
     * <p>
     * If the input predicate is {@code null}, no elements are added to {@code outputCollection} or {@code rejectedCollection}.
     * </p>
     * <p>
     * Note: calling the method is equivalent to the following code snippet:
     * </p>
     *
     * <pre>
     * select(inputCollection, predicate, outputCollection);
     * selectRejected(inputCollection, predicate, rejectedCollection);
     * </pre>
     *
     * @param <O>                the type of object the {@link Iterable} contains.
     * @param <R>                the type of the output {@link Collection}.
     * @param inputCollection    The collection to get the input from, may be null.
     * @param predicate          The predicate to use, may be null.
     * @param outputCollection   The collection to output selected elements into, may not be null if the inputCollection and predicate are not null.
     * @param rejectedCollection The collection to output rejected elements into, may not be null if the inputCollection or predicate are not null.
     * @return The outputCollection
     * @since 4.1
     */
    public static <O, R extends Collection<? super O>> R select(final Iterable<? extends O> inputCollection, final Predicate<? super O> predicate,
            final R outputCollection, final R rejectedCollection) {
        if (inputCollection != null && predicate != null) {
            for (final O element : inputCollection) {
                if (predicate.test(element)) {
                    outputCollection.add(element);
                } else {
                    rejectedCollection.add(element);
                }
            }
        }
        return outputCollection;
    }

    /**
     * Selects all elements from inputCollection which don't match the given predicate into an output collection.
     * <p>
     * If the input predicate is {@code null}, the result is an empty list.
     * </p>
     *
     * @param <O>             the type of object the {@link Iterable} contains.
     * @param inputCollection The collection to get the input from, may not be null.
     * @param predicate       The predicate to use, may be null.
     * @return The elements <strong>not</strong> matching the predicate (new list).
     */
    public static <O> Collection<O> selectRejected(final Iterable<? extends O> inputCollection, final Predicate<? super O> predicate) {
        int size = 0;
        if (inputCollection != null) {
            size = inputCollection instanceof Collection<?> ? ((Collection<?>) inputCollection).size() : 0;
        }
        final Collection<O> answer = size == 0 ? new ArrayList<>() : new ArrayList<>(size);
        return selectRejected(inputCollection, predicate, answer);
    }

    /**
     * Selects all elements from inputCollection which don't match the given predicate and adds them to outputCollection.
     * <p>
     * If the input predicate is {@code null}, no elements are added to {@code outputCollection}.
     * </p>
     *
     * @param <O>              the type of object the {@link Iterable} contains.
     * @param <R>              the type of the output {@link Collection}.
     * @param inputCollection  The collection to get the input from, may be null.
     * @param predicate        The predicate to use, may be null.
     * @param outputCollection The collection to output into, may not be null if the inputCollection and predicate or not null.
     * @return outputCollection
     */
    public static <O, R extends Collection<? super O>> R selectRejected(final Iterable<? extends O> inputCollection, final Predicate<? super O> predicate,
            final R outputCollection) {
        if (inputCollection != null && predicate != null) {
            for (final O item : inputCollection) {
                if (!predicate.test(item)) {
                    outputCollection.add(item);
                }
            }
        }
        return outputCollection;
    }

    /**
     * Gets the size of the collection/iterator specified.
     * <p>
     * This method can handles objects as follows
     * </p>
     * <ul>
     * <li>Collection - the collection size</li>
     * <li>Map - the map size</li>
     * <li>Array - the array size</li>
     * <li>Iterator - the number of elements remaining in the iterator</li>
     * <li>Enumeration - the number of elements remaining in the enumeration</li>
     * </ul>
     *
     * @param object The object to get the size of, may be null.
     * @return The size of the specified collection or 0 if the object was null.
     * @throws IllegalArgumentException thrown if object is not recognized.
     * @since 3.1
     */
    public static int size(final Object object) {
        if (object == null) {
            return 0;
        }
        int total = 0;
        if (object instanceof Map<?, ?>) {
            total = ((Map<?, ?>) object).size();
        } else if (object instanceof Collection<?>) {
            total = ((Collection<?>) object).size();
        } else if (object instanceof Iterable<?>) {
            total = IterableUtils.size((Iterable<?>) object);
        } else if (object instanceof Object[]) {
            total = ((Object[]) object).length;
        } else if (object instanceof Iterator<?>) {
            total = IteratorUtils.size((Iterator<?>) object);
        } else if (object instanceof Enumeration<?>) {
            final Enumeration<?> it = (Enumeration<?>) object;
            while (it.hasMoreElements()) {
                total++;
                it.nextElement();
            }
        } else {
            try {
                total = Array.getLength(object);
            } catch (final IllegalArgumentException ex) {
                throw new IllegalArgumentException("Unsupported object type: " + object.getClass().getName());
            }
        }
        return total;
    }

    /**
     * Checks if the specified collection/array/iterator is empty.
     * <p>
     * This method can handles objects as follows
     * </p>
     * <ul>
     * <li>Collection - via collection isEmpty</li>
     * <li>Map - via map isEmpty</li>
     * <li>Array - using array size</li>
     * <li>Iterator - via hasNext</li>
     * <li>Enumeration - via hasMoreElements</li>
     * </ul>
     * <p>
     * Note: This method is named to avoid clashing with {@link #isEmpty(Collection)}.
     * </p>
     *
     * @param object The object to get the size of, may be null.
     * @return true if empty or null.
     * @throws IllegalArgumentException thrown if object is not recognized.
     * @since 3.2
     */
    public static boolean sizeIsEmpty(final Object object) {
        if (object == null) {
            return true;
        }
        if (object instanceof Collection<?>) {
            return ((Collection<?>) object).isEmpty();
        }
        if (object instanceof Iterable<?>) {
            return IterableUtils.isEmpty((Iterable<?>) object);
        }
        if (object instanceof Map<?, ?>) {
            return ((Map<?, ?>) object).isEmpty();
        }
        if (object instanceof Object[]) {
            return ((Object[]) object).length == 0;
        }
        if (object instanceof Iterator<?>) {
            return !((Iterator<?>) object).hasNext();
        }
        if (object instanceof Enumeration<?>) {
            return !((Enumeration<?>) object).hasMoreElements();
        }
        try {
            return Array.getLength(object) == 0;
        } catch (final IllegalArgumentException ex) {
            throw new IllegalArgumentException("Unsupported object type: " + object.getClass().getName());
        }
    }

    /**
     * Returns a new {@link Collection} containing {@code <em>a</em> - <em>b</em>}. The cardinality of each element <em>e</em> in the returned
     * {@link Collection} will be the cardinality of <em>e</em> in <em>a</em> minus the cardinality of <em>e</em> in <em>b</em>, or zero, whichever is greater.
     *
     * @param a   The collection to subtract from, must not be null.
     * @param b   The collection to subtract, must not be null.
     * @param <O> The generic type that is able to represent the types contained in both input collections.
     * @return A new collection with the results.
     * @throws NullPointerException if either collection is null.
     * @see Collection#removeAll
     */
    public static <O> Collection<O> subtract(final Iterable<? extends O> a, final Iterable<? extends O> b) {
        final Predicate<O> p = TruePredicate.truePredicate();
        return subtract(a, b, p);
    }

    /**
     * Returns a new {@link Collection} containing <em>a</em> minus a subset of <em>b</em>. Only the elements of <em>b</em> that satisfy the predicate
     * condition, <em>p</em> are subtracted from <em>a</em>.
     * <p>
     * The cardinality of each element <em>e</em> in the returned {@link Collection} that satisfies the predicate condition will be the cardinality of
     * <em>e</em> in <em>a</em> minus the cardinality of <em>e</em> in <em>b</em>, or zero, whichever is greater.
     * </p>
     * <p>
     * The cardinality of each element <em>e</em> in the returned {@link Collection} that does <strong>not</strong> satisfy the predicate condition will be
     * equal to the cardinality of <em>e</em> in <em>a</em>.
     * </p>
     *
     * @param a   The collection to subtract from, must not be null.
     * @param b   The collection to subtract, must not be null.
     * @param p   The condition used to determine which elements of <em>b</em> are subtracted.
     * @param <O> The generic type that is able to represent the types contained in both input collections.
     * @return A new collection with the results.
     * @throws NullPointerException if either collection or p is null.
     * @since 4.0
     * @see Collection#removeAll
     */
    public static <O> Collection<O> subtract(final Iterable<? extends O> a, final Iterable<? extends O> b, final Predicate<O> p) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        Objects.requireNonNull(p, "p");
        final ArrayList<O> list = new ArrayList<>();
        final HashMultiSet<O> multiSet = new HashMultiSet<>();
        for (final O element : b) {
            if (p.test(element)) {
                multiSet.add(element);
            }
        }
        for (final O element : a) {
            if (multiSet.remove(element, 1) == 0) {
                list.add(element);
            }
        }
        return list;
    }

    /**
     * Returns a synchronized collection backed by the given collection.
     * <p>
     * You must manually synchronize on the returned buffer's iterator to avoid non-deterministic behavior:
     * </p>
     *
     * <pre>
     * Collection c = CollectionUtils.synchronizedCollection(myCollection);
     * synchronized (c) {
     *     Iterator i = c.iterator();
     *     while (i.hasNext()) {
     *         process(i.next());
     *     }
     * }
     * </pre>
     * <p>
     * This method uses the implementation in the decorators subpackage.
     * </p>
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection to synchronize, must not be null.
     * @return A synchronized collection backed by the given collection.
     * @throws NullPointerException if the collection is null.
     * @deprecated Since 4.1, use {@link java.util.Collections#synchronizedCollection(Collection)} instead.
     */
    @Deprecated
    public static <C> Collection<C> synchronizedCollection(final Collection<C> collection) {
        Objects.requireNonNull(collection, "collection");
        return SynchronizedCollection.synchronizedCollection(collection);
    }

    /**
     * Transform the collection by applying a Transformer to each element.
     * <p>
     * If the input collection or transformer is null, there is no change made.
     * </p>
     * <p>
     * This routine is best for Lists, for which set() is used to do the transformations "in place." For other Collections, clear() and addAll() are used to
     * replace elements.
     * </p>
     * <p>
     * If the input collection controls its input, such as a Set, and the Transformer creates duplicates (or are otherwise invalid), the collection may reduce
     * in size due to calling this method.
     * </p>
     *
     * @param <C>         the type of object the {@link Collection} contains.
     * @param collection  The {@link Collection} to get the input from, may be null.
     * @param transformer The transformer to perform, may be null.
     */
    public static <C> void transform(final Collection<C> collection, final Transformer<? super C, ? extends C> transformer) {
        if (collection != null && transformer != null) {
            if (collection instanceof List<?>) {
                final List<C> list = (List<C>) collection;
                for (final ListIterator<C> it = list.listIterator(); it.hasNext();) {
                    it.set(transformer.apply(it.next()));
                }
            } else {
                final Collection<C> resultCollection = collect(collection, transformer);
                collection.clear();
                collection.addAll(resultCollection);
            }
        }
    }

    /**
     * Returns a transformed bag backed by the given collection.
     * <p>
     * Each object is passed through the transformer as it is added to the Collection. It is important not to use the original collection after invoking this
     * method, as it is a backdoor for adding untransformed objects.
     * </p>
     * <p>
     * Existing entries in the specified collection will not be transformed. If you want that behavior, see {@link TransformedCollection#transformedCollection}.
     * </p>
     *
     * @param <E>         The type of object the {@link Collection} contains.
     * @param collection  The collection to predicate, must not be null.
     * @param transformer The transformer for the collection, must not be null.
     * @return A transformed collection backed by the given collection.
     * @throws NullPointerException if the collection or transformer is null.
     */
    public static <E> Collection<E> transformingCollection(final Collection<E> collection, final Transformer<? super E, ? extends E> transformer) {
        Objects.requireNonNull(collection, "collection");
        Objects.requireNonNull(transformer, "transformer");
        return TransformedCollection.transformingCollection(collection, transformer);
    }

    /**
     * Returns a {@link Collection} containing the union of the given {@link Iterable}s.
     * <p>
     * The cardinality of each element in the returned {@link Collection} will be equal to the maximum of the cardinality of that element in the two given
     * {@link Iterable}s.
     * </p>
     *
     * @param a   The first collection, must not be null.
     * @param b   The second collection, must not be null.
     * @param <O> The generic type that is able to represent the types contained in both input collections.
     * @return The union of the two collections.
     * @throws NullPointerException if either collection is null.
     * @see Collection#addAll
     */
    public static <O> Collection<O> union(final Iterable<? extends O> a, final Iterable<? extends O> b) {
        Objects.requireNonNull(a, "a");
        Objects.requireNonNull(b, "b");
        final SetOperationCardinalityHelper<O> helper = new SetOperationCardinalityHelper<>(a, b);
        for (final O obj : helper) {
            helper.setCardinality(obj, helper.max(obj));
        }
        return helper.list();
    }

    /**
     * Returns an unmodifiable collection backed by the given collection.
     * <p>
     * This method uses the implementation in the decorators subpackage.
     * </p>
     *
     * @param <C>        the type of object the {@link Collection} contains.
     * @param collection The collection to make unmodifiable, must not be null.
     * @return An unmodifiable collection backed by the given collection.
     * @throws NullPointerException if the collection is null.
     * @deprecated Since 4.1, use {@link java.util.Collections#unmodifiableCollection(Collection)} instead.
     */
    @Deprecated
    public static <C> Collection<C> unmodifiableCollection(final Collection<? extends C> collection) {
        Objects.requireNonNull(collection, "collection");
        return UnmodifiableCollection.unmodifiableCollection(collection);
    }

    /**
     * Don't allow instances.
     */
    private CollectionUtils() {
        // empty
    }
}