    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      https://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.iterators;
    018
    019import java.util.List;
    020import java.util.ListIterator;
    021import java.util.NoSuchElementException;
    022import java.util.Objects;
    023
    024import org.apache.commons.collections4.ResettableListIterator;
    025
    026/**
    027 * A ListIterator that restarts when it reaches the end or when it
    028 * reaches the beginning.
    029 * <p>
    030 * The iterator will loop continuously around the provided list,
    031 * unless there are no elements in the collection to begin with, or
    032 * all of the elements have been {@link #remove removed}.
    033 * </p>
    034 * <p>
    035 * Concurrent modifications are not directly supported, and for most
    036 * collection implementations will throw a
    037 * ConcurrentModificationException.
    038 * </p>
    039 *
    040 * @param <E> The type of elements returned by this iterator.
    041 * @since 3.2
    042 */
    043public class LoopingListIterator<E> implements ResettableListIterator<E> {
    044
    045    /** The list to base the iterator on */
    046    private final List<E> list;
    047
    048    /** The current list iterator */
    049    private ListIterator<E> iterator;
    050
    051    /**
    052     * Constructor that wraps a list.
    053     * <p>
    054     * There is no way to reset a ListIterator instance without
    055     * recreating it from the original source, so the List must be
    056     * passed in and a reference to it held.
    057     * </p>
    058     *
    059     * @param list The list to wrap
    060     * @throws NullPointerException if the list is null
    061     */
    062    public LoopingListIterator(final List<E> list) {
    063        this.list = Objects.requireNonNull(list, "collection");
    064        init();
    065    }
    066
    067    /**
    068     * Inserts the specified element into the underlying list.
    069     * <p>
    070     * The element is inserted before the next element that would be
    071     * returned by {@link #next}, if any, and after the next element
    072     * that would be returned by {@link #previous}, if any.
    073     * </p>
    074     * <p>
    075     * This feature is only supported if the underlying list's
    076     * {@link List#listIterator} method returns an implementation
    077     * that supports it.
    078     * </p>
    079     *
    080     * @param obj  The element to insert
    081     * @throws UnsupportedOperationException if the add method is not
    082     *  supported by the iterator implementation of the underlying list
    083     */
    084    @Override
    085    public void add(final E obj) {
    086        iterator.add(obj);
    087    }
    088
    089    /**
    090     * Returns whether this iterator has any more elements.
    091     * <p>
    092     * Returns false only if the list originally had zero elements, or
    093     * all elements have been {@link #remove removed}.
    094     * </p>
    095     *
    096     * @return {@code true} if there are more elements
    097     */
    098    @Override
    099    public boolean hasNext() {
    100        return !list.isEmpty();
    101    }
    102
    103    /**
    104     * Returns whether this iterator has any more previous elements.
    105     * <p>
    106     * Returns false only if the list originally had zero elements, or
    107     * all elements have been {@link #remove removed}.
    108     * </p>
    109     *
    110     * @return {@code true} if there are more elements
    111     */
    112    @Override
    113    public boolean hasPrevious() {
    114        return !list.isEmpty();
    115    }
    116
    117    private void init() {
    118        iterator = list.listIterator();
    119    }
    120
    121    /**
    122     * Returns the next object in the list.
    123     * <p>
    124     * If at the end of the list, returns the first element.
    125     * </p>
    126     *
    127     * @return The object after the last element returned
    128     * @throws NoSuchElementException if there are no elements in the list
    129     */
    130    @Override
    131    public E next() {
    132        if (list.isEmpty()) {
    133            throw new NoSuchElementException(
    134                "There are no elements for this iterator to loop on");
    135        }
    136        if (!iterator.hasNext()) {
    137            reset();
    138        }
    139        return iterator.next();
    140    }
    141
    142    /**
    143     * Returns the index of the element that would be returned by a
    144     * subsequent call to {@link #next}.
    145     * <p>
    146     * As would be expected, if the iterator is at the physical end of
    147     * the underlying list, 0 is returned, signifying the beginning of
    148     * the list.
    149     * </p>
    150     *
    151     * @return The index of the element that would be returned if next() were called
    152     * @throws NoSuchElementException if there are no elements in the list
    153     */
    154    @Override
    155    public int nextIndex() {
    156        if (list.isEmpty()) {
    157            throw new NoSuchElementException(
    158                "There are no elements for this iterator to loop on");
    159        }
    160        if (!iterator.hasNext()) {
    161            return 0;
    162        }
    163        return iterator.nextIndex();
    164    }
    165
    166    /**
    167     * Returns the previous object in the list.
    168     * <p>
    169     * If at the beginning of the list, return the last element. Note
    170     * that in this case, traversal to find that element takes linear time.
    171     * </p>
    172     *
    173     * @return The object before the last element returned
    174     * @throws NoSuchElementException if there are no elements in the list
    175     */
    176    @Override
    177    public E previous() {
    178        if (list.isEmpty()) {
    179            throw new NoSuchElementException(
    180                "There are no elements for this iterator to loop on");
    181        }
    182        if (!iterator.hasPrevious()) {
    183            E result = null;
    184            while (iterator.hasNext()) {
    185                result = iterator.next();
    186            }
    187            iterator.previous();
    188            return result;
    189        }
    190        return iterator.previous();
    191    }
    192
    193    /**
    194     * Returns the index of the element that would be returned by a
    195     * subsequent call to {@link #previous}.
    196     * <p>
    197     * As would be expected, if at the iterator is at the physical
    198     * beginning of the underlying list, the list's size minus one is
    199     * returned, signifying the end of the list.
    200     * </p>
    201     *
    202     * @return The index of the element that would be returned if previous() were called
    203     * @throws NoSuchElementException if there are no elements in the list
    204     */
    205    @Override
    206    public int previousIndex() {
    207        if (list.isEmpty()) {
    208            throw new NoSuchElementException(
    209                "There are no elements for this iterator to loop on");
    210        }
    211        if (!iterator.hasPrevious()) {
    212            return list.size() - 1;
    213        }
    214        return iterator.previousIndex();
    215    }
    216
    217    /**
    218     * Removes the previously retrieved item from the underlying list.
    219     * <p>
    220     * This feature is only supported if the underlying list's
    221     * {@link List#iterator()} method returns an implementation
    222     * that supports it.
    223     * </p>
    224     * <p>
    225     * This method can only be called after at least one {@link #next}
    226     * or {@link #previous} method call. After a removal, the remove
    227     * method may not be called again until another {@link #next} or
    228     * {@link #previous} has been performed. If the {@link #reset} is
    229     * called, then remove may not be called until {@link #next} or
    230     * {@link #previous} is called again.
    231     * </p>
    232     *
    233     * @throws UnsupportedOperationException if the remove method is
    234     * not supported by the iterator implementation of the underlying
    235     * list
    236     */
    237    @Override
    238    public void remove() {
    239        iterator.remove();
    240    }
    241
    242    /**
    243     * Resets the iterator back to the start of the list.
    244     */
    245    @Override
    246    public void reset() {
    247        init();
    248    }
    249
    250    /**
    251     * Replaces the last element that was returned by {@link #next} or
    252     * {@link #previous}.
    253     * <p>
    254     * This feature is only supported if the underlying list's
    255     * {@link List#listIterator} method returns an implementation
    256     * that supports it.
    257     * </p>
    258     *
    259     * @param obj  The element with which to replace the last element returned
    260     * @throws UnsupportedOperationException if the set method is not
    261     *  supported by the iterator implementation of the underlying list
    262     */
    263    @Override
    264    public void set(final E obj) {
    265        iterator.set(obj);
    266    }
    267
    268    /**
    269     * Gets the size of the list underlying the iterator.
    270     *
    271     * @return The current list size
    272     */
    273    public int size() {
    274        return list.size();
    275    }
    276
    277}