    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.Objects;
    022
    023import org.apache.commons.collections4.ResettableListIterator;
    024
    025/**
    026 * Iterates backwards through a List, starting with the last element
    027 * and continuing to the first. This is useful for looping around
    028 * a list in reverse order without needing to actually reverse the list.
    029 * <p>
    030 * The first call to {@code next()} will return the last element
    031 * from the list, and so on. The {@code hasNext()} method works
    032 * in concert with the {@code next()} method as expected.
    033 * However, the {@code nextIndex()} method returns the correct
    034 * index in the list, thus it starts high and reduces as the iteration
    035 * continues. The previous methods work similarly.
    036 * </p>
    037 *
    038 * @param <E> The type of elements returned by this iterator.
    039 * @since 3.2
    040 */
    041public class ReverseListIterator<E> implements ResettableListIterator<E> {
    042
    043    /** The list being wrapped. */
    044    private final List<E> list;
    045
    046    /** The list iterator being wrapped. */
    047    private ListIterator<E> iterator;
    048
    049    /** Flag to indicate if updating is possible at the moment. */
    050    private boolean validForUpdate = true;
    051
    052    /**
    053     * Constructor that wraps a list.
    054     *
    055     * @param list  The list to create a reversed iterator for
    056     * @throws NullPointerException if the list is null
    057     */
    058    public ReverseListIterator(final List<E> list) {
    059        this.list = Objects.requireNonNull(list, "list");
    060        iterator = list.listIterator(list.size());
    061    }
    062
    063    /**
    064     * Adds a new element to the list between the next and previous elements.
    065     *
    066     * @param obj  The object to add
    067     * @throws UnsupportedOperationException if the list is unmodifiable
    068     * @throws IllegalStateException if the iterator is not in a valid state for set
    069     */
    070    @Override
    071    public void add(final E obj) {
    072        // the validForUpdate flag is needed as the necessary previous()
    073        // method call re-enables remove and add
    074        if (!validForUpdate) {
    075            throw new IllegalStateException("Cannot add to list until next() or previous() called");
    076        }
    077        validForUpdate = false;
    078        iterator.add(obj);
    079        iterator.previous();
    080    }
    081
    082    /**
    083     * Checks whether there is another element.
    084     *
    085     * @return true if there is another element
    086     */
    087    @Override
    088    public boolean hasNext() {
    089        return iterator.hasPrevious();
    090    }
    091
    092    /**
    093     * Checks whether there is a previous element.
    094     *
    095     * @return true if there is a previous element
    096     */
    097    @Override
    098    public boolean hasPrevious() {
    099        return iterator.hasNext();
    100    }
    101
    102    /**
    103     * Gets the next element.
    104     * The next element is the previous in the list.
    105     *
    106     * @return The next element in the iterator
    107     */
    108    @Override
    109    public E next() {
    110        final E obj = iterator.previous();
    111        validForUpdate = true;
    112        return obj;
    113    }
    114
    115    /**
    116     * Gets the index of the next element.
    117     *
    118     * @return The index of the next element in the iterator
    119     */
    120    @Override
    121    public int nextIndex() {
    122        return iterator.previousIndex();
    123    }
    124
    125    /**
    126     * Gets the previous element.
    127     * The next element is the previous in the list.
    128     *
    129     * @return The previous element in the iterator
    130     */
    131    @Override
    132    public E previous() {
    133        final E obj = iterator.next();
    134        validForUpdate = true;
    135        return obj;
    136    }
    137
    138    /**
    139     * Gets the index of the previous element.
    140     *
    141     * @return The index of the previous element in the iterator
    142     */
    143    @Override
    144    public int previousIndex() {
    145        return iterator.nextIndex();
    146    }
    147
    148    /**
    149     * Removes the last returned element.
    150     *
    151     * @throws UnsupportedOperationException if the list is unmodifiable
    152     * @throws IllegalStateException if there is no element to remove
    153     */
    154    @Override
    155    public void remove() {
    156        if (!validForUpdate) {
    157            throw new IllegalStateException("Cannot remove from list until next() or previous() called");
    158        }
    159        iterator.remove();
    160    }
    161
    162    /**
    163     * Resets the iterator back to the start (which is the
    164     * end of the list as this is a reversed iterator)
    165     */
    166    @Override
    167    public void reset() {
    168        iterator = list.listIterator(list.size());
    169    }
    170
    171    /**
    172     * Replaces the last returned element.
    173     *
    174     * @param obj  The object to set
    175     * @throws UnsupportedOperationException if the list is unmodifiable
    176     * @throws IllegalStateException if the iterator is not in a valid state for set
    177     */
    178    @Override
    179    public void set(final E obj) {
    180        if (!validForUpdate) {
    181            throw new IllegalStateException("Cannot set to list until next() or previous() called");
    182        }
    183        iterator.set(obj);
    184    }
    185
    186}