    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 *      http://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 *
    037 * @param <E> the type of elements returned by this iterator.
    038 * @since 3.2
    039 */
    040public class ReverseListIterator<E> implements ResettableListIterator<E> {
    041
    042    /** The list being wrapped. */
    043    private final List<E> list;
    044    /** The list iterator being wrapped. */
    045    private ListIterator<E> iterator;
    046    /** Flag to indicate if updating is possible at the moment. */
    047    private boolean validForUpdate = true;
    048
    049    /**
    050     * Constructor that wraps a list.
    051     *
    052     * @param list  the list to create a reversed iterator for
    053     * @throws NullPointerException if the list is null
    054     */
    055    public ReverseListIterator(final List<E> list) {
    056        this.list = Objects.requireNonNull(list, "list");
    057        iterator = list.listIterator(list.size());
    058    }
    059
    060    /**
    061     * Adds a new element to the list between the next and previous elements.
    062     *
    063     * @param obj  the object to add
    064     * @throws UnsupportedOperationException if the list is unmodifiable
    065     * @throws IllegalStateException if the iterator is not in a valid state for set
    066     */
    067    @Override
    068    public void add(final E obj) {
    069        // the validForUpdate flag is needed as the necessary previous()
    070        // method call re-enables remove and add
    071        if (!validForUpdate) {
    072            throw new IllegalStateException("Cannot add to list until next() or previous() called");
    073        }
    074        validForUpdate = false;
    075        iterator.add(obj);
    076        iterator.previous();
    077    }
    078
    079    /**
    080     * Checks whether there is another element.
    081     *
    082     * @return true if there is another element
    083     */
    084    @Override
    085    public boolean hasNext() {
    086        return iterator.hasPrevious();
    087    }
    088
    089    /**
    090     * Checks whether there is a previous element.
    091     *
    092     * @return true if there is a previous element
    093     */
    094    @Override
    095    public boolean hasPrevious() {
    096        return iterator.hasNext();
    097    }
    098
    099    /**
    100     * Gets the next element.
    101     * The next element is the previous in the list.
    102     *
    103     * @return the next element in the iterator
    104     */
    105    @Override
    106    public E next() {
    107        final E obj = iterator.previous();
    108        validForUpdate = true;
    109        return obj;
    110    }
    111
    112    /**
    113     * Gets the index of the next element.
    114     *
    115     * @return the index of the next element in the iterator
    116     */
    117    @Override
    118    public int nextIndex() {
    119        return iterator.previousIndex();
    120    }
    121
    122    /**
    123     * Gets the previous element.
    124     * The next element is the previous in the list.
    125     *
    126     * @return the previous element in the iterator
    127     */
    128    @Override
    129    public E previous() {
    130        final E obj = iterator.next();
    131        validForUpdate = true;
    132        return obj;
    133    }
    134
    135    /**
    136     * Gets the index of the previous element.
    137     *
    138     * @return the index of the previous element in the iterator
    139     */
    140    @Override
    141    public int previousIndex() {
    142        return iterator.nextIndex();
    143    }
    144
    145    /**
    146     * Removes the last returned element.
    147     *
    148     * @throws UnsupportedOperationException if the list is unmodifiable
    149     * @throws IllegalStateException if there is no element to remove
    150     */
    151    @Override
    152    public void remove() {
    153        if (!validForUpdate) {
    154            throw new IllegalStateException("Cannot remove from list until next() or previous() called");
    155        }
    156        iterator.remove();
    157    }
    158
    159    /**
    160     * Resets the iterator back to the start (which is the
    161     * end of the list as this is a reversed iterator)
    162     */
    163    @Override
    164    public void reset() {
    165        iterator = list.listIterator(list.size());
    166    }
    167
    168    /**
    169     * Replaces the last returned element.
    170     *
    171     * @param obj  the object to set
    172     * @throws UnsupportedOperationException if the list is unmodifiable
    173     * @throws IllegalStateException if the iterator is not in a valid state for set
    174     */
    175    @Override
    176    public void set(final E obj) {
    177        if (!validForUpdate) {
    178            throw new IllegalStateException("Cannot set to list until next() or previous() called");
    179        }
    180        iterator.set(obj);
    181    }
    182
    183}