/*
 * 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.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.Dictionary;
import java.util.Enumeration;
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 java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.IntFunction;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import org.apache.commons.collections4.functors.EqualPredicate;
import org.apache.commons.collections4.iterators.ArrayIterator;
import org.apache.commons.collections4.iterators.ArrayListIterator;
import org.apache.commons.collections4.iterators.BoundedIterator;
import org.apache.commons.collections4.iterators.CollatingIterator;
import org.apache.commons.collections4.iterators.EmptyIterator;
import org.apache.commons.collections4.iterators.EmptyListIterator;
import org.apache.commons.collections4.iterators.EmptyMapIterator;
import org.apache.commons.collections4.iterators.EmptyOrderedIterator;
import org.apache.commons.collections4.iterators.EmptyOrderedMapIterator;
import org.apache.commons.collections4.iterators.EnumerationIterator;
import org.apache.commons.collections4.iterators.FilterIterator;
import org.apache.commons.collections4.iterators.FilterListIterator;
import org.apache.commons.collections4.iterators.IteratorChain;
import org.apache.commons.collections4.iterators.IteratorEnumeration;
import org.apache.commons.collections4.iterators.IteratorIterable;
import org.apache.commons.collections4.iterators.LazyIteratorChain;
import org.apache.commons.collections4.iterators.ListIteratorWrapper;
import org.apache.commons.collections4.iterators.LoopingIterator;
import org.apache.commons.collections4.iterators.LoopingListIterator;
import org.apache.commons.collections4.iterators.NodeListIterator;
import org.apache.commons.collections4.iterators.ObjectArrayIterator;
import org.apache.commons.collections4.iterators.ObjectArrayListIterator;
import org.apache.commons.collections4.iterators.ObjectGraphIterator;
import org.apache.commons.collections4.iterators.PeekingIterator;
import org.apache.commons.collections4.iterators.PushbackIterator;
import org.apache.commons.collections4.iterators.SingletonIterator;
import org.apache.commons.collections4.iterators.SingletonListIterator;
import org.apache.commons.collections4.iterators.SkippingIterator;
import org.apache.commons.collections4.iterators.TransformIterator;
import org.apache.commons.collections4.iterators.UnmodifiableIterator;
import org.apache.commons.collections4.iterators.UnmodifiableListIterator;
import org.apache.commons.collections4.iterators.UnmodifiableMapIterator;
import org.apache.commons.collections4.iterators.ZippingIterator;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

/**
 * Provides static utility methods and decorators for {@link Iterator} instances. The implementations are provided in the iterators subpackage.
 *
 * @since 2.1
 */
public class IteratorUtils {
    // validation is done in this class in certain cases because the
    // public classes allow invalid states

    /**
     * An iterator over no elements.
     */
    @SuppressWarnings("rawtypes")
    public static final ResettableIterator EMPTY_ITERATOR = EmptyIterator.RESETTABLE_INSTANCE;

    /**
     * A list iterator over no elements.
     */
    @SuppressWarnings("rawtypes")
    public static final ResettableListIterator EMPTY_LIST_ITERATOR = EmptyListIterator.RESETTABLE_INSTANCE;

    /**
     * An ordered iterator over no elements.
     */
    @SuppressWarnings("rawtypes")
    public static final OrderedIterator EMPTY_ORDERED_ITERATOR = EmptyOrderedIterator.INSTANCE;

    /**
     * A map iterator over no elements.
     */
    @SuppressWarnings("rawtypes")
    public static final MapIterator EMPTY_MAP_ITERATOR = EmptyMapIterator.INSTANCE;

    /**
     * An ordered map iterator over no elements.
     */
    @SuppressWarnings("rawtypes")
    public static final OrderedMapIterator EMPTY_ORDERED_MAP_ITERATOR = EmptyOrderedMapIterator.INSTANCE;

    /**
     * Default delimiter used to delimit elements while converting an Iterator to its String representation.
     */
    private static final String DEFAULT_TOSTRING_DELIMITER = ", ";

    private static <E, C extends Collection<E>> C addAll(final Iterator<? extends E> iterator, final C list) {
        Objects.requireNonNull(iterator, "iterator");
        while (iterator.hasNext()) {
            list.add(iterator.next());
        }
        return list;
    }

    /**
     * Gets an iterator over an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @return An iterator over the array.
     * @throws NullPointerException if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final E... array) {
        return new ObjectArrayIterator<>(array);
    }

    /**
     * Gets an iterator over the end part of an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @return An iterator over part of the array.
     * @throws IndexOutOfBoundsException if start is less than zero or greater than the length of the array.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final E[] array, final int start) {
        return new ObjectArrayIterator<>(array, start);
    }

    /**
     * Gets an iterator over part of an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @param end   The index to finish iterating at.
     * @return An iterator over part of the array.
     * @throws IndexOutOfBoundsException if array bounds are invalid.
     * @throws IllegalArgumentException  if end is before start.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final E[] array, final int start, final int end) {
        return new ObjectArrayIterator<>(array, start, end);
    }

    /**
     * Gets an iterator over an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @return An iterator over the array.
     * @throws IllegalArgumentException if the array is not an array.
     * @throws NullPointerException     if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final Object array) {
        return new ArrayIterator<>(array);
    }

    /**
     * Gets an iterator over the end part of an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @return An iterator over part of the array.
     * @throws IllegalArgumentException  if the array is not an array.
     * @throws IndexOutOfBoundsException if start is less than zero or greater than the length of the array.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final Object array, final int start) {
        return new ArrayIterator<>(array, start);
    }

    /**
     * Gets an iterator over part of an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @param end   The index to finish iterating at.
     * @return An iterator over part of the array.
     * @throws IllegalArgumentException  if the array is not an array or end is before start.
     * @throws IndexOutOfBoundsException if array bounds are invalid.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableIterator<E> arrayIterator(final Object array, final int start, final int end) {
        return new ArrayIterator<>(array, start, end);
    }

    /**
     * Gets a list iterator over an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @return A list iterator over the array.
     * @throws NullPointerException if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final E... array) {
        return new ObjectArrayListIterator<>(array);
    }

    /**
     * Gets a list iterator over the end part of an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @return A list iterator over part of the array.
     * @throws IndexOutOfBoundsException if start is less than zero.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final E[] array, final int start) {
        return new ObjectArrayListIterator<>(array, start);
    }

    /**
     * Gets a list iterator over part of an object array.
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @param end   The index to finish iterating at.
     * @return A list iterator over part of the array.
     * @throws IndexOutOfBoundsException if array bounds are invalid.
     * @throws IllegalArgumentException  if end is before start.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final E[] array, final int start, final int end) {
        return new ObjectArrayListIterator<>(array, start, end);
    }

    /**
     * Gets a list iterator over an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @return A list iterator over the array.
     * @throws IllegalArgumentException if the array is not an array.
     * @throws NullPointerException     if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final Object array) {
        return new ArrayListIterator<>(array);
    }

    /**
     * Gets a list iterator over the end part of an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @return A list iterator over part of the array.
     * @throws IllegalArgumentException  if the array is not an array.
     * @throws IndexOutOfBoundsException if start is less than zero.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final Object array, final int start) {
        return new ArrayListIterator<>(array, start);
    }

    /**
     * Gets a list iterator over part of an object or primitive array.
     * <p>
     * This method will handle primitive arrays as well as object arrays. The primitives will be wrapped in the appropriate wrapper class.
     * </p>
     *
     * @param <E>   The element type.
     * @param array The array over which to iterate.
     * @param start The index to start iterating at.
     * @param end   The index to finish iterating at.
     * @return A list iterator over part of the array.
     * @throws IllegalArgumentException  if the array is not an array or end is before start.
     * @throws IndexOutOfBoundsException if array bounds are invalid.
     * @throws NullPointerException      if array is null.
     */
    public static <E> ResettableListIterator<E> arrayListIterator(final Object array, final int start, final int end) {
        return new ArrayListIterator<>(array, start, end);
    }

    /**
     * Gets an enumeration that wraps an iterator.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, may not be null.
     * @return A new enumeration.
     * @throws NullPointerException if iterator is null.
     */
    public static <E> Enumeration<E> asEnumeration(final Iterator<? extends E> iterator) {
        return new IteratorEnumeration<>(Objects.requireNonNull(iterator, "iterator"));
    }

    /**
     * Gets an {@link Iterable} that wraps an iterator. The returned {@link Iterable} can be used for a single iteration.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, may not be null.
     * @return A new, single use {@link Iterable}.
     * @throws NullPointerException if iterator is null.
     */
    public static <E> Iterable<E> asIterable(final Iterator<? extends E> iterator) {
        Objects.requireNonNull(iterator, "iterator");
        return new IteratorIterable<>(iterator, false);
    }

    /**
     * Gets an iterator that provides an iterator view of the given enumeration.
     *
     * @param <E>         The element type.
     * @param enumeration The enumeration to use, may not be null.
     * @return A new iterator.
     * @throws NullPointerException if enumeration is null.
     */
    public static <E> Iterator<E> asIterator(final Enumeration<? extends E> enumeration) {
        return new EnumerationIterator<>(Objects.requireNonNull(enumeration, "enumeration"));
    }

    /**
     * Gets an iterator that provides an iterator view of the given enumeration that will remove elements from the specified collection.
     *
     * @param <E>              The element type.
     * @param enumeration      The enumeration to use, may not be null.
     * @param removeCollection The collection to remove elements from, may not be null.
     * @return A new iterator.
     * @throws NullPointerException if enumeration or removeCollection is null.
     */
    public static <E> Iterator<E> asIterator(final Enumeration<? extends E> enumeration, final Collection<? super E> removeCollection) {
        return new EnumerationIterator<>(Objects.requireNonNull(enumeration, "enumeration"), Objects.requireNonNull(removeCollection, "removeCollection"));
    }

    /**
     * Gets an iterable that wraps an iterator. The returned iterable can be used for multiple iterations.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, may not be null.
     * @return A new, multiple use iterable.
     * @throws NullPointerException if iterator is null.
     */
    public static <E> Iterable<E> asMultipleUseIterable(final Iterator<? extends E> iterator) {
        Objects.requireNonNull(iterator, "iterator");
        return new IteratorIterable<>(iterator, true);
    }

    /**
     * Decorates the specified iterator to return at most the given number of elements.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to decorate.
     * @param max      The maximum number of elements returned by this iterator.
     * @return A new bounded iterator.
     * @throws NullPointerException     if the iterator is null.
     * @throws IllegalArgumentException if max is negative.
     * @since 4.1
     */
    public static <E> BoundedIterator<E> boundedIterator(final Iterator<? extends E> iterator, final long max) {
        return boundedIterator(iterator, 0, max);
    }

    /**
     * Decorates the specified iterator to return at most the given number of elements, skipping all elements until the iterator reaches the position at
     * {@code offset}.
     * <p>
     * The iterator is immediately advanced until it reaches the position at {@code offset}, incurring O(n) time.
     * </p>
     *
     * @param <E>      The element type.
     * @param iterator The iterator to decorate.
     * @param offset   The index of the first element of the decorated iterator to return.
     * @param max      The maximum number of elements returned by this iterator.
     * @return A new bounded iterator.
     * @throws NullPointerException     if the iterator is null.
     * @throws IllegalArgumentException if either offset or max is negative.
     * @since 4.1
     */
    public static <E> BoundedIterator<E> boundedIterator(final Iterator<? extends E> iterator, final long offset, final long max) {
        return new BoundedIterator<>(iterator, offset, max);
    }

    /**
     * Gets an iterator that iterates through a collections of {@link Iterator}s one after another.
     *
     * @param <E>       The element type
     * @param iterators The iterators to use, not null or empty or contain nulls
     * @return A combination iterator over the iterators
     * @throws NullPointerException if iterators collection is null or contains a null
     * @throws ClassCastException   if the iterators collection contains the wrong object type
     */
    public static <E> Iterator<E> chainedIterator(final Collection<? extends Iterator<? extends E>> iterators) {
        return new IteratorChain<>(iterators);
    }

    /**
     * Gets an iterator that iterates through an array of {@link Iterator}s one after another.
     *
     * @param <E>       The element type
     * @param iterators The iterators to use, not null or empty or contain nulls
     * @return A combination iterator over the iterators
     * @throws NullPointerException if iterators array is null or contains a null
     */
    public static <E> Iterator<E> chainedIterator(final Iterator<? extends E>... iterators) {
        return new IteratorChain<>(iterators);
    }

    /**
     * Gets an iterator that iterates through two {@link Iterator}s one after another.
     *
     * @param <E>       The element type.
     * @param iterator1 The first iterator to use, not null.
     * @param iterator2 The second iterator to use, not null.
     * @return A combination iterator over the iterators.
     * @throws NullPointerException if either iterator is null.
     */
    public static <E> Iterator<E> chainedIterator(final Iterator<? extends E> iterator1, final Iterator<? extends E> iterator2) {
        // keep a version with two iterators to avoid the following warning in client code (Java 5 & 6)
        // "A generic array of E is created for a varargs parameter"
        return new IteratorChain<>(iterator1, iterator2);
    }

    /**
     * Gets an iterator that iterates through an {@link Iterator} of Iterators one after another.
     *
     * @param <E>       the element type.
     * @param iterators The iterators to use, not null or empty or contain nulls.
     * @return A combination iterator over the iterators.
     * @throws NullPointerException if iterators collection is null or contains a null.
     * @throws ClassCastException   if the iterators collection contains the wrong object type.
     * @since 4.5.0-M3
     */
    public static <E> Iterator<E> chainedIterator(final Iterator<? extends Iterator<? extends E>> iterators) {
        return new LazyIteratorChain<E>() {

            @Override
            protected Iterator<? extends E> nextIterator(final int count) {
                return iterators.hasNext() ? iterators.next() : null;
            }
        };
    }

    /**
     * Gets an iterator that provides an ordered iteration over the elements contained in a collection of {@link Iterator}s.
     * <p>
     * Given two ordered {@link Iterator}s {@code A} and {@code B}, the {@link Iterator#next()} method will return the lesser of {@code A.next()} and
     * {@code B.next()} and so on.
     * </p>
     * <p>
     * The comparator is optional. If null is specified then natural order is used.
     * </p>
     *
     * @param <E>        The element type.
     * @param comparator The comparator to use, may be null for natural order.
     * @param iterators  The iterators to use, not null or empty or contain nulls.
     * @return A combination iterator over the iterators.
     * @throws NullPointerException if iterators collection is null or contains a null.
     * @throws ClassCastException   if the iterators collection contains the wrong object type.
     */
    public static <E> Iterator<E> collatedIterator(final Comparator<? super E> comparator, final Collection<Iterator<? extends E>> iterators) {
        @SuppressWarnings("unchecked")
        final Comparator<E> comp = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : (Comparator<E>) comparator;
        return new CollatingIterator<>(comp, iterators);
    }

    /**
     * Gets an iterator that provides an ordered iteration over the elements contained in an array of {@link Iterator}s.
     * <p>
     * Given two ordered {@link Iterator}s {@code A} and {@code B}, the {@link Iterator#next()} method will return the lesser of {@code A.next()} and
     * {@code B.next()} and so on.
     * </p>
     * <p>
     * The comparator is optional. If null is specified then natural order is used.
     * </p>
     *
     * @param <E>        The element type.
     * @param comparator The comparator to use, may be null for natural order.
     * @param iterators  The iterators to use, not null or empty or contain nulls.
     * @return A combination iterator over the iterators.
     * @throws NullPointerException if iterators array is null or contains a null value.
     */
    public static <E> Iterator<E> collatedIterator(final Comparator<? super E> comparator, final Iterator<? extends E>... iterators) {
        @SuppressWarnings("unchecked")
        final Comparator<E> comp = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : (Comparator<E>) comparator;
        return new CollatingIterator<>(comp, iterators);
    }

    /**
     * Gets an iterator that provides an ordered iteration over the elements contained in a collection of ordered {@link Iterator}s.
     * <p>
     * Given two ordered {@link Iterator}s {@code A} and {@code B}, the {@link Iterator#next()} method will return the lesser of {@code A.next()} and
     * {@code B.next()}.
     * </p>
     * <p>
     * The comparator is optional. If null is specified then natural order is used.
     * </p>
     *
     * @param <E>        The element type.
     * @param comparator The comparator to use, may be null for natural order.
     * @param iterator1  The first iterators to use, not null.
     * @param iterator2  The first iterators to use, not null.
     * @return A combination iterator over the iterators.
     * @throws NullPointerException if either iterator is null.
     */
    public static <E> Iterator<E> collatedIterator(final Comparator<? super E> comparator, final Iterator<? extends E> iterator1,
            final Iterator<? extends E> iterator2) {
        @SuppressWarnings("unchecked")
        final Comparator<E> comp = comparator == null ? ComparatorUtils.NATURAL_COMPARATOR : (Comparator<E>) comparator;
        return new CollatingIterator<>(comp, iterator1, iterator2);
    }

    /**
     * Checks if the object is contained in the given iterator.
     * <p>
     * A {@code null} or empty iterator returns false.
     * </p>
     *
     * @param <E>      The type of object the {@link Iterator} contains.
     * @param iterator The iterator to check, may be null.
     * @param object   The object to check.
     * @return true if the object is contained in the iterator, false otherwise.
     * @since 4.1
     */
    public static <E> boolean contains(final Iterator<E> iterator, final Object object) {
        return matchesAny(iterator, EqualPredicate.equalPredicate(object));
    }

    /**
     * Gets an empty iterator.
     * <p>
     * This iterator is a valid iterator object that will iterate over nothing.
     * </p>
     *
     * @param <E> The element type.
     * @return An iterator over nothing.
     */
    public static <E> ResettableIterator<E> emptyIterator() {
        return EmptyIterator.<E>resettableEmptyIterator();
    }

    /**
     * Gets an empty list iterator.
     * <p>
     * This iterator is a valid list iterator object that will iterate over nothing.
     * </p>
     *
     * @param <E> The element type.
     * @return A list iterator over nothing.
     */
    public static <E> ResettableListIterator<E> emptyListIterator() {
        return EmptyListIterator.<E>resettableEmptyListIterator();
    }

    /**
     * Gets an empty map iterator.
     * <p>
     * This iterator is a valid map iterator object that will iterate over nothing.
     * </p>
     *
     * @param <K> The key type.
     * @param <V> The value type.
     * @return A map iterator over nothing.
     */
    public static <K, V> MapIterator<K, V> emptyMapIterator() {
        return EmptyMapIterator.<K, V>emptyMapIterator();
    }

    /**
     * Gets an empty ordered iterator.
     * <p>
     * This iterator is a valid iterator object that will iterate over nothing.
     * </p>
     *
     * @param <E> The element type.
     * @return An ordered iterator over nothing.
     */
    public static <E> OrderedIterator<E> emptyOrderedIterator() {
        return EmptyOrderedIterator.<E>emptyOrderedIterator();
    }

    /**
     * Gets an empty ordered map iterator.
     * <p>
     * This iterator is a valid map iterator object that will iterate over nothing.
     * </p>
     *
     * @param <K> The key type.
     * @param <V> The value type.
     * @return A map iterator over nothing.
     */
    public static <K, V> OrderedMapIterator<K, V> emptyOrderedMapIterator() {
        return EmptyOrderedMapIterator.<K, V>emptyOrderedMapIterator();
    }

    /**
     * Gets an iterator that filters another iterator.
     * <p>
     * The returned iterator will only return objects that match the specified filtering predicate.
     * </p>
     *
     * @param <E>       The element type.
     * @param iterator  The iterator to use, not null.
     * @param predicate The predicate to use as a filter, not null.
     * @return A new filtered iterator.
     * @throws NullPointerException if either parameter is null.
     */
    public static <E> Iterator<E> filteredIterator(final Iterator<? extends E> iterator, final Predicate<? super E> predicate) {
        Objects.requireNonNull(iterator, "iterator");
        Objects.requireNonNull(predicate, "predicate");
        return new FilterIterator<>(iterator, predicate);
    }

    /**
     * Gets a list iterator that filters another list iterator.
     * <p>
     * The returned iterator will only return objects that match the specified filtering predicate.
     * </p>
     *
     * @param <E>          The element type.
     * @param listIterator The list iterator to use, not null.
     * @param predicate    The predicate to use as a filter, not null.
     * @return A new filtered iterator.
     * @throws NullPointerException if either parameter is null.
     */
    public static <E> ListIterator<E> filteredListIterator(final ListIterator<? extends E> listIterator, final Predicate<? super E> predicate) {
        Objects.requireNonNull(listIterator, "listIterator");
        Objects.requireNonNull(predicate, "predicate");
        return new FilterListIterator<>(listIterator, predicate);
    }

    /**
     * Finds the first element in the given iterator which matches the given predicate.
     * <p>
     * A {@code null} or empty iterator returns null.
     * </p>
     *
     * @param <E>       The element type.
     * @param iterator  The iterator to search, may be null.
     * @param predicate The predicate to use, must not be null.
     * @return The first element of the iterator which matches the predicate or null if none could be found.
     * @throws NullPointerException if predicate is null.
     * @since 4.1
     */
    public static <E> E find(final Iterator<E> iterator, final Predicate<? super E> predicate) {
        return find(iterator, predicate, null);
    }

    /**
     * Finds the first element in the given iterator which matches the given predicate.
     * <p>
     * A {@code null} or empty iterator returns {@code defaultValue}.
     * </p>
     *
     * @param <E>          the element type.
     * @param iterator     The iterator to search, may be null.
     * @param predicate    The predicate to use, must not be null.
     * @param defaultValue The default value, may be null.
     * @return The first element of the iterator which matches the predicate or null if none could be found.
     * @throws NullPointerException if predicate is null.
     */
    private static <E> E find(final Iterator<E> iterator, final Predicate<? super E> predicate, final E defaultValue) {
        Objects.requireNonNull(predicate, "predicate");
        if (iterator != null) {
            while (iterator.hasNext()) {
                final E element = iterator.next();
                if (predicate.test(element)) {
                    return element;
                }
            }
        }
        return defaultValue;
    }

    /**
     * Shortcut for {@code get(iterator, 0)}.
     * <p>
     * Returns the {@code first} value in {@link Iterator}, throwing {@code IndexOutOfBoundsException} if there is no such element.
     * </p>
     * <p>
     * The Iterator is advanced to {@code 0} (or to the end, if {@code 0} exceeds the number of entries) as a side effect of this method.
     * </p>
     *
     * @param <E>      The type of object in the {@link Iterator}.
     * @param iterator The iterator to get a value from.
     * @return The first object.
     * @throws IndexOutOfBoundsException if the request is invalid.
     * @throws NullPointerException      if iterator is null.
     * @since 4.2
     */
    public static <E> E first(final Iterator<E> iterator) {
        return get(iterator, 0);
    }

    /**
     * Applies the closure to each element of the provided iterator.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, may be null.
     * @param closure  The closure to apply to each element, may not be null.
     * @throws NullPointerException if closure is null.
     * @since 4.1
     */
    public static <E> void forEach(final Iterator<E> iterator, final Closure<? super E> closure) {
        Objects.requireNonNull(closure, "closure");
        if (iterator != null) {
            while (iterator.hasNext()) {
                closure.accept(iterator.next());
            }
        }
    }

    /**
     * Executes the given closure on each but the last element in the iterator.
     * <p>
     * If the input iterator is null no change is made.
     * </p>
     *
     * @param <E>      The type of object the {@link Iterator} contains.
     * @param iterator The iterator to get the input from, may be null.
     * @param closure  The closure to perform, may not be null.
     * @return The last element in the iterator, or null if iterator is null or empty.
     * @throws NullPointerException if closure is null.
     * @since 4.1
     */
    public static <E> E forEachButLast(final Iterator<E> iterator, final Closure<? super E> closure) {
        Objects.requireNonNull(closure, "closure");
        if (iterator != null) {
            while (iterator.hasNext()) {
                final E element = iterator.next();
                if (!iterator.hasNext()) {
                    return element;
                }
                closure.accept(element);
            }
        }
        return null;
    }

    /**
     * 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 <E>      the type of object in the {@link Iterator}.
     * @param iterator The iterator to get a value from.
     * @param index    The index to get, 0-based.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     * @throws NullPointerException      if iterator is null.
     * @since 4.1
     */
    public static <E> E get(final Iterator<E> iterator, final int index) {
        return get(iterator, index, ioob -> {
            throw new IndexOutOfBoundsException("Entry does not exist: " + ioob);
        });
    }

    /**
     * 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 <E>             the type of object in the {@link Iterator}.
     * @param iterator        The iterator to get a value from.
     * @param index           The index to get, 0-based.
     * @param defaultSupplier supplies a default value at an index.
     * @return The object at the specified index.
     * @throws IndexOutOfBoundsException if the index is invalid.
     */
    static <E> E get(final Iterator<E> iterator, final int index, final IntFunction<E> defaultSupplier) {
        int i = index;
        CollectionUtils.checkIndexBounds(i);
        while (iterator.hasNext()) {
            i--;
            if (i == -1) {
                return iterator.next();
            }
            iterator.next();
        }
        return defaultSupplier.apply(i);
    }

    /**
     * Gets a suitable Iterator for the given object.
     * <p>
     * This method can handle objects as follows
     * </p>
     * <ul>
     * <li>null - empty iterator</li>
     * <li>Iterator - returned directly</li>
     * <li>Enumeration - wrapped</li>
     * <li>Collection - iterator from collection returned</li>
     * <li>Map - values iterator returned</li>
     * <li>Dictionary - values (elements) enumeration returned as iterator</li>
     * <li>array - iterator over array returned</li>
     * <li>object with iterator() public method accessed by reflection</li>
     * <li>object - singleton iterator</li>
     * <li>NodeList - iterator over the list</li>
     * <li>Node - iterator over the child nodes</li>
     * </ul>
     *
     * @param obj The object to convert to an iterator.
     * @return A suitable iterator, never null.
     */
    public static Iterator<?> getIterator(final Object obj) {
        if (obj == null) {
            return emptyIterator();
        }
        if (obj instanceof Iterator) {
            return (Iterator<?>) obj;
        }
        if (obj instanceof Iterable) {
            return ((Iterable<?>) obj).iterator();
        }
        if (obj instanceof Object[]) {
            return new ObjectArrayIterator<>((Object[]) obj);
        }
        if (obj instanceof Enumeration) {
            return new EnumerationIterator<>((Enumeration<?>) obj);
        }
        if (obj instanceof Map) {
            return ((Map<?, ?>) obj).values().iterator();
        }
        if (obj instanceof NodeList) {
            return new NodeListIterator((NodeList) obj);
        }
        if (obj instanceof Node) {
            return new NodeListIterator((Node) obj);
        }
        if (obj instanceof Dictionary) {
            return new EnumerationIterator<>(((Dictionary<?, ?>) obj).elements());
        }
        if (obj.getClass().isArray()) {
            return new ArrayIterator<>(obj);
        }
        try {
            final Method method = obj.getClass().getMethod("iterator", (Class[]) null);
            if (Iterator.class.isAssignableFrom(method.getReturnType())) {
                final Iterator<?> it = (Iterator<?>) method.invoke(obj, (Object[]) null);
                if (it != null) {
                    return it;
                }
            }
        } catch (final RuntimeException | ReflectiveOperationException ignore) { // NOPMD
            // ignore
        }
        return singletonIterator(obj);
    }

    /**
     * Returns the index of the first element in the specified iterator that matches the given predicate.
     * <p>
     * A {@code null} or empty iterator returns -1.
     * </p>
     *
     * @param <E>       The element type.
     * @param iterator  The iterator to search, may be null.
     * @param predicate The predicate to use, may not be null.
     * @return The index of the first element which matches the predicate or -1 if none matches.
     * @throws NullPointerException if predicate is null.
     * @since 4.1
     */
    public static <E> int indexOf(final Iterator<E> iterator, final Predicate<? super E> predicate) {
        Objects.requireNonNull(predicate, "predicate");
        if (iterator != null) {
            for (int index = 0; iterator.hasNext(); index++) {
                final E element = iterator.next();
                if (predicate.test(element)) {
                    return index;
                }
            }
        }
        return CollectionUtils.INDEX_NOT_FOUND;
    }

    /**
     * Checks if the given iterator is empty.
     * <p>
     * A {@code null} or empty iterator returns true.
     * </p>
     *
     * @param iterator The {@link Iterator} to use, may be null.
     * @return true if the iterator is exhausted or null, false otherwise.
     * @since 4.1
     */
    public static boolean isEmpty(final Iterator<?> iterator) {
        return iterator == null || !iterator.hasNext();
    }

    /**
     * Gets an iterator that loops continuously over the supplied collection.
     * <p>
     * The iterator will only stop looping if the remove method is called enough times to empty the collection, or if the collection is empty to start with.
     * </p>
     *
     * @param <E>        The element type.
     * @param collection The collection to iterate over, not null.
     * @return A new looping iterator.
     * @throws NullPointerException if the collection is null.
     */
    public static <E> ResettableIterator<E> loopingIterator(final Collection<? extends E> collection) {
        return new LoopingIterator<>(Objects.requireNonNull(collection, "collection"));
    }

    /**
     * Gets an iterator that loops continuously over the supplied list.
     * <p>
     * The iterator will only stop looping if the remove method is called enough times to empty the list, or if the list is empty to start with.
     * </p>
     *
     * @param <E>  The element type.
     * @param list The list to iterate over, not null.
     * @return A new looping iterator.
     * @throws NullPointerException if the list is null.
     * @since 3.2
     */
    public static <E> ResettableListIterator<E> loopingListIterator(final List<E> list) {
        return new LoopingListIterator<>(Objects.requireNonNull(list, "list"));
    }

    /**
     * Answers true if a predicate is true for every element of an iterator.
     * <p>
     * A {@code null} or empty iterator returns true.
     * </p>
     *
     * @param <E>       The type of object the {@link Iterator} contains.
     * @param iterator  The {@link Iterator} to use, may be null.
     * @param predicate The predicate to use, may not be null.
     * @return true if every element of the collection matches the predicate or if the collection is empty, false otherwise.
     * @throws NullPointerException if predicate is null.
     * @since 4.1
     */
    public static <E> boolean matchesAll(final Iterator<E> iterator, final Predicate<? super E> predicate) {
        Objects.requireNonNull(predicate, "predicate");
        if (iterator != null) {
            while (iterator.hasNext()) {
                final E element = iterator.next();
                if (!predicate.test(element)) {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * Answers true if a predicate is true for any element of the iterator.
     * <p>
     * A {@code null} or empty iterator returns false.
     * </p>
     *
     * @param <E>       The type of object the {@link Iterator} contains.
     * @param iterator  The {@link Iterator} to use, may be null.
     * @param predicate The predicate to use, may not be null.
     * @return true if any element of the collection matches the predicate, false otherwise.
     * @throws NullPointerException if predicate is null.
     * @since 4.1
     */
    public static <E> boolean matchesAny(final Iterator<E> iterator, final Predicate<? super E> predicate) {
        return indexOf(iterator, predicate) != -1;
    }

    /**
     * Gets an {@link Iterator} that wraps the specified node's childNodes. The returned {@link Iterator} can be used for a single iteration.
     * <p>
     * Convenience method, allows easy iteration over NodeLists:
     * </p>
     *
     * <pre>
     *   Iterator&lt;Node&gt; iterator = IteratorUtils.nodeListIterator(node);
     *   for (Node childNode : IteratorUtils.asIterable(iterator)) {
     *     ...
     *   }
     * </pre>
     *
     * @param node The node to use, may not be null.
     * @return A new, single use {@link Iterator}.
     * @throws NullPointerException if node is null.
     * @since 4.0
     */
    public static NodeListIterator nodeListIterator(final Node node) {
        return new NodeListIterator(Objects.requireNonNull(node, "node"));
    }

    /**
     * Gets an {@link Iterator} that wraps the specified {@link NodeList}. The returned {@link Iterator} can be used for a single iteration.
     *
     * @param nodeList The node list to use, may not be null.
     * @return A new, single use {@link Iterator}.
     * @throws NullPointerException if nodeList is null.
     * @since 4.0
     */
    public static NodeListIterator nodeListIterator(final NodeList nodeList) {
        return new NodeListIterator(Objects.requireNonNull(nodeList, "nodeList"));
    }

    /**
     * Gets an iterator that operates over an object graph.
     * <p>
     * This iterator can extract multiple objects from a complex tree-like object graph. The iteration starts from a single root object. It uses a
     * {@code Transformer} to extract the iterators and elements. Its main benefit is that no intermediate {@code List} is created.
     * </p>
     * <p>
     * For example, consider an object graph:
     * </p>
     *
     * <pre>
     *                 |- Branch -- Leaf
     *                 |         \- Leaf
     *         |- Tree |         /- Leaf
     *         |       |- Branch -- Leaf
     *  Forest |                 \- Leaf
     *         |       |- Branch -- Leaf
     *         |       |         \- Leaf
     *         |- Tree |         /- Leaf
     *                 |- Branch -- Leaf
     *                 |- Branch -- Leaf
     * </pre>
     * <p>
     * The following {@code Transformer}, used in this class, will extract all the Leaf objects without creating a combined intermediate list:
     * </p>
     *
     * <pre>
     *
     * public Object transform(Object input) {
     *     if (input instanceof Forest) {
     *         return ((Forest) input).treeIterator();
     *     }
     *     if (input instanceof Tree) {
     *         return ((Tree) input).branchIterator();
     *     }
     *     if (input instanceof Branch) {
     *         return ((Branch) input).leafIterator();
     *     }
     *     if (input instanceof Leaf) {
     *         return input;
     *     }
     *     throw new ClassCastException();
     * }
     * </pre>
     * <p>
     * Internally, iteration starts from the root object. When next is called, the transformer is called to examine the object. The transformer will return
     * either an iterator or an object. If the object is an Iterator, the next element from that iterator is obtained and the process repeats. If the element is
     * an object it is returned.
     * </p>
     * <p>
     * Under many circumstances, linking Iterators together in this manner is more efficient (and convenient) than using nested for loops to extract a list.
     * </p>
     *
     * @param <E>         The element type.
     * @param root        The root object to start iterating from, null results in an empty iterator.
     * @param transformer The transformer to use, see above, null uses no effect transformer.
     * @return A new object graph iterator.
     * @since 3.1
     */
    public static <E> Iterator<E> objectGraphIterator(final E root, final Transformer<? super E, ? extends E> transformer) {
        return new ObjectGraphIterator<>(root, transformer);
    }

    /**
     * Gets an iterator that supports one-element lookahead.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to decorate, not null.
     * @return A peeking iterator.
     * @throws NullPointerException if the iterator is null.
     * @since 4.0
     */
    public static <E> Iterator<E> peekingIterator(final Iterator<? extends E> iterator) {
        return PeekingIterator.peekingIterator(iterator);
    }

    /**
     * Gets an iterator that supports pushback of elements.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to decorate, not null.
     * @return A pushback iterator.
     * @throws NullPointerException if the iterator is null.
     * @since 4.0
     */
    public static <E> Iterator<E> pushbackIterator(final Iterator<? extends E> iterator) {
        return PushbackIterator.pushbackIterator(iterator);
    }

    /**
     * Gets a singleton iterator.
     * <p>
     * This iterator is a valid iterator object that will iterate over the specified object.
     * </p>
     *
     * @param <E>    The element type.
     * @param object The single object over which to iterate.
     * @return A singleton iterator over the object.
     */
    public static <E> ResettableIterator<E> singletonIterator(final E object) {
        return new SingletonIterator<>(object);
    }

    /**
     * Gets a singleton list iterator.
     * <p>
     * This iterator is a valid list iterator object that will iterate over the specified object.
     * </p>
     *
     * @param <E>    The element type.
     * @param object The single object over which to iterate.
     * @return A singleton list iterator over the object.
     */
    public static <E> ListIterator<E> singletonListIterator(final E object) {
        return new SingletonListIterator<>(object);
    }

    /**
     * Returns the number of elements contained in the given iterator.
     * <p>
     * A {@code null} or empty iterator returns {@code 0}.
     * </p>
     *
     * @param iterator The iterator to check, may be null.
     * @return The number of elements contained in the iterator.
     * @since 4.1
     */
    public static int size(final Iterator<?> iterator) {
        int size = 0;
        if (iterator != null) {
            while (iterator.hasNext()) {
                iterator.next();
                size++;
            }
        }
        return size;
    }

    /**
     * Decorates the specified iterator to skip the first N elements.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to decorate.
     * @param offset   The first number of elements to skip.
     * @return A new skipping iterator.
     * @throws NullPointerException     if the iterator is null.
     * @throws IllegalArgumentException if offset is negative.
     * @since 4.1
     */
    public static <E> SkippingIterator<E> skippingIterator(final Iterator<E> iterator, final long offset) {
        return new SkippingIterator<>(iterator, offset);
    }

    /**
     * Creates a stream on the given Iterable.
     *
     * @param <E>      The type of elements in the Iterable.
     * @param iterable The Iterable to stream or null.
     * @return A new Stream or {@link Stream#empty()} if the Iterable is null.
     * @since 4.5.0-M3
     */
    public static <E> Stream<E> stream(final Iterable<E> iterable) {
        return iterable == null ? Stream.empty() : StreamSupport.stream(iterable.spliterator(), false);
    }

    /**
     * Creates a stream on the given Iterator.
     *
     * @param <E>      The type of elements in the Iterator.
     * @param iterator The Iterator to stream or null.
     * @return A new Stream or {@link Stream#empty()} if the Iterator is null.
     * @since 4.5.0-M3
     */
    public static <E> Stream<E> stream(final Iterator<E> iterator) {
        return iterator == null ? Stream.empty() : StreamSupport.stream(Spliterators.spliteratorUnknownSize(iterator, Spliterator.ORDERED), false);
    }

    /**
     * Gets an array based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, this is converted to an array.
     * </p>
     *
     * @param iterator The iterator to use, not null.
     * @return An array of the iterator contents.
     * @throws NullPointerException if iterator parameter is null.
     */
    public static Object[] toArray(final Iterator<?> iterator) {
        Objects.requireNonNull(iterator, "iterator");
        final List<?> list = toList(iterator, 100);
        return list.toArray();
    }

    /**
     * Gets an array based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, this is converted to an array.
     * </p>
     *
     * @param <E>        The element type.
     * @param iterator   The iterator to use, not null.
     * @param arrayClass The class of array to create.
     * @return An array of the iterator contents.
     * @throws NullPointerException if iterator parameter or arrayClass is null.
     * @throws ArrayStoreException  if the arrayClass is invalid.
     */
    public static <E> E[] toArray(final Iterator<? extends E> iterator, final Class<E> arrayClass) {
        Objects.requireNonNull(iterator, "iterator");
        Objects.requireNonNull(arrayClass, "arrayClass");
        final List<E> list = toList(iterator, 100);
        @SuppressWarnings("unchecked")
        final E[] array = (E[]) Array.newInstance(arrayClass, list.size());
        return list.toArray(array);
    }

    /**
     * Gets a list based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, the list is returned.
     * </p>
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, not null.
     * @return A list of the iterator contents.
     * @throws NullPointerException if iterator parameter is null.
     */
    public static <E> List<E> toList(final Iterator<? extends E> iterator) {
        return toList(iterator, 10);
    }

    /**
     * Gets a list based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, an ArrayList of its values is created. At the end, the list is returned.
     * </p>
     *
     * @param <E>           The element type.
     * @param iterator      The iterator to use, not null.
     * @param estimatedSize The initial size of the List.
     * @return A list of the iterator contents.
     * @throws NullPointerException     if iterator parameter is null.
     * @throws IllegalArgumentException if the size is less than 1.
     */
    public static <E> List<E> toList(final Iterator<? extends E> iterator, final int estimatedSize) {
        if (estimatedSize < 1) {
            throw new IllegalArgumentException("Estimated size must be greater than 0");
        }
        return addAll(iterator, new ArrayList<>(estimatedSize));
    }

    /**
     * Gets a list iterator based on a simple iterator.
     * <p>
     * As the wrapped Iterator is traversed, a LinkedList of its values is cached, permitting all required operations of ListIterator.
     * </p>
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, may not be null.
     * @return A new iterator.
     * @throws NullPointerException if iterator parameter is null.
     */
    public static <E> ListIterator<E> toListIterator(final Iterator<? extends E> iterator) {
        Objects.requireNonNull(iterator, "iterator");
        return new ListIteratorWrapper<>(iterator);
    }

    /**
     * Gets a set based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, a HashSet of its values is created. At the end, the set is returned.
     * </p>
     *
     * @param <E>      The element type.
     * @param iterator The iterator to use, not null.
     * @return A set of the iterator contents.
     * @throws NullPointerException if iterator parameter is null.
     * @since 4.5.0-M4
     */
    public static <E> Set<E> toSet(final Iterator<? extends E> iterator) {
        return toSet(iterator, 10);
    }

    /**
     * Gets a set based on an iterator.
     * <p>
     * As the wrapped Iterator is traversed, a HashSet of its values is created. At the end, the set is returned.
     * </p>
     *
     * @param <E>           The element type.
     * @param iterator      The iterator to use, not null.
     * @param estimatedSize The initial size of the HashSet.
     * @return A list of the iterator contents.
     * @throws NullPointerException     if iterator parameter is null.
     * @throws IllegalArgumentException if the size is less than 1.
     * @since 4.5.0-M4
     */
    public static <E> Set<E> toSet(final Iterator<? extends E> iterator, final int estimatedSize) {
        if (estimatedSize < 1) {
            throw new IllegalArgumentException("Estimated size must be greater than 0");
        }
        return addAll(iterator, new HashSet<>(estimatedSize));
    }

    /**
     * Returns a string representation of the elements of the specified iterator.
     * <p>
     * The string representation consists of a list of the iterator's elements, enclosed in square brackets ({@code "[]"}). Adjacent elements are separated by
     * the characters {@code ", "} (a comma followed by a space). Elements are converted to strings as by {@code String.valueOf(Object)}.
     * </p>
     *
     * @param <E>      The element type.
     * @param iterator The iterator to convert to a string, may be null.
     * @return A string representation of {@code iterator}.
     * @since 4.1
     */
    public static <E> String toString(final Iterator<E> iterator) {
        return toString(iterator, TransformerUtils.stringValueTransformer(), DEFAULT_TOSTRING_DELIMITER, CollectionUtils.DEFAULT_TOSTRING_PREFIX,
                CollectionUtils.DEFAULT_TOSTRING_SUFFIX);
    }

    /**
     * Returns a string representation of the elements of the specified iterator.
     * <p>
     * The string representation consists of a list of the iterable's elements, enclosed in square brackets ({@code "[]"}). Adjacent elements are separated by
     * the characters {@code ", "} (a comma followed by a space). Elements are converted to strings as by using the provided {@code transformer}.
     * </p>
     *
     * @param <E>         The element type.
     * @param iterator    The iterator to convert to a string, may be null.
     * @param transformer The transformer used to get a string representation of an element.
     * @return A string representation of {@code iterator}.
     * @throws NullPointerException if {@code transformer} is null.
     * @since 4.1
     */
    public static <E> String toString(final Iterator<E> iterator, final Transformer<? super E, String> transformer) {
        return toString(iterator, transformer, DEFAULT_TOSTRING_DELIMITER, CollectionUtils.DEFAULT_TOSTRING_PREFIX, CollectionUtils.DEFAULT_TOSTRING_SUFFIX);
    }

    /**
     * Returns a string representation of the elements of the specified iterator.
     * <p>
     * The string representation consists of a list of the iterator's elements, enclosed by the provided {@code prefix} and {@code suffix}. Adjacent elements
     * are separated by the provided {@code delimiter}. Elements are converted to strings as by using the provided {@code transformer}.
     * </p>
     *
     * @param <E>         The element type.
     * @param iterator    The iterator to convert to a string, may be null.
     * @param transformer The transformer used to get a string representation of an element.
     * @param delimiter   The string to delimit elements.
     * @param prefix      The prefix, prepended to the string representation.
     * @param suffix      The suffix, appended to the string representation.
     * @return A string representation of {@code iterator}.
     * @throws NullPointerException if either transformer, delimiter, prefix or suffix is null.
     * @since 4.1
     */
    public static <E> String toString(final Iterator<E> iterator, final Transformer<? super E, String> transformer, final String delimiter, final String prefix,
            final String suffix) {
        Objects.requireNonNull(transformer, "transformer");
        Objects.requireNonNull(delimiter, "delimiter");
        Objects.requireNonNull(prefix, "prefix");
        Objects.requireNonNull(suffix, "suffix");
        final StringBuilder stringBuilder = new StringBuilder(prefix);
        if (iterator != null) {
            while (iterator.hasNext()) {
                final E element = iterator.next();
                stringBuilder.append(transformer.apply(element));
                stringBuilder.append(delimiter);
            }
            if (stringBuilder.length() > prefix.length()) {
                stringBuilder.setLength(stringBuilder.length() - delimiter.length());
            }
        }
        stringBuilder.append(suffix);
        return stringBuilder.toString();
    }

    /**
     * Gets an iterator that transforms the elements of another iterator.
     * <p>
     * The transformation occurs during the next() method and the underlying iterator is unaffected by the transformation.
     * </p>
     *
     * @param <I>         The input type.
     * @param <O>         The output type.
     * @param iterator    The iterator to use, not null.
     * @param transformer The transform to use, not null.
     * @return A new transforming iterator.
     * @throws NullPointerException if either parameter is null.
     */
    public static <I, O> Iterator<O> transformedIterator(final Iterator<? extends I> iterator, final Transformer<? super I, ? extends O> transformer) {
        Objects.requireNonNull(iterator, "iterator");
        Objects.requireNonNull(transformer, "transformer");
        return new TransformIterator<>(iterator, transformer);
    }

    /**
     * Gets an immutable version of an {@link Iterator}. The returned object will always throw an {@link UnsupportedOperationException} for the
     * {@link Iterator#remove} method.
     *
     * @param <E>      The element type.
     * @param iterator The iterator to make immutable.
     * @return An immutable version of the iterator.
     */
    public static <E> Iterator<E> unmodifiableIterator(final Iterator<E> iterator) {
        return UnmodifiableIterator.unmodifiableIterator(iterator);
    }

    /**
     * Gets an immutable version of a {@link ListIterator}. The returned object will always throw an {@link UnsupportedOperationException} for the
     * {@link Iterator#remove}, {@link ListIterator#add} and {@link ListIterator#set} methods.
     *
     * @param <E>          The element type.
     * @param listIterator The iterator to make immutable.
     * @return An immutable version of the iterator.
     */
    public static <E> ListIterator<E> unmodifiableListIterator(final ListIterator<E> listIterator) {
        return UnmodifiableListIterator.unmodifiableListIterator(listIterator);
    }

    /**
     * Gets an immutable version of a {@link MapIterator}. The returned object will always throw an {@link UnsupportedOperationException} for the
     * {@link Iterator#remove}, {@link MapIterator#setValue(Object)} methods.
     *
     * @param <K>         The key type.
     * @param <V>         The value type.
     * @param mapIterator The iterator to make immutable.
     * @return An immutable version of the iterator.
     */
    public static <K, V> MapIterator<K, V> unmodifiableMapIterator(final MapIterator<K, V> mapIterator) {
        return UnmodifiableMapIterator.unmodifiableMapIterator(mapIterator);
    }

    /**
     * Returns an iterator that interleaves elements from the decorated iterators.
     *
     * @param <E>       The element type.
     * @param iterators The array of iterators to interleave.
     * @return An iterator, interleaving the decorated iterators.
     * @throws NullPointerException if any iterator is null.
     * @since 4.1
     */
    public static <E> ZippingIterator<E> zippingIterator(final Iterator<? extends E>... iterators) {
        return new ZippingIterator<>(iterators);
    }

    /**
     * Returns an iterator that interleaves elements from the decorated iterators.
     *
     * @param <E> The element type.
     * @param a   The first iterator to interleave.
     * @param b   The second iterator to interleave.
     * @return An iterator, interleaving the decorated iterators.
     * @throws NullPointerException if any iterator is null.
     * @since 4.1
     */
    public static <E> ZippingIterator<E> zippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {
        return new ZippingIterator<>(a, b);
    }

    /**
     * Returns an iterator that interleaves elements from the decorated iterators.
     *
     * @param <E> The element type.
     * @param a   The first iterator to interleave.
     * @param b   The second iterator to interleave.
     * @param c   The third iterator to interleave.
     * @return An iterator, interleaving the decorated iterators.
     * @throws NullPointerException if any iterator is null.
     * @since 4.1
     */
    public static <E> ZippingIterator<E> zippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b, final Iterator<? extends E> c) {
        return new ZippingIterator<>(a, b, c);
    }

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