    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.text.MessageFormat;
    020import java.util.ArrayList;
    021import java.util.Iterator;
    022import java.util.List;
    023import java.util.ListIterator;
    024import java.util.NoSuchElementException;
    025import java.util.Objects;
    026
    027import org.apache.commons.collections4.ResettableIterator;
    028import org.apache.commons.collections4.ResettableListIterator;
    029
    030/**
    031 * Converts an {@link Iterator} into a {@link ResettableListIterator}.
    032 * For plain {@code Iterator}s this is accomplished by caching the returned
    033 * elements.  This class can also be used to simply add
    034 * {@link ResettableIterator ResettableIterator}
    035 * functionality to a given {@link ListIterator}.
    036 * <p>
    037 * The {@code ListIterator} interface has additional useful methods
    038 * for navigation - {@code previous()} and the index methods.
    039 * This class allows a regular {@code Iterator} to behave as a
    040 * {@code ListIterator}. It achieves this by building a list internally
    041 * of as the underlying iterator is traversed.
    042 * </p>
    043 * <p>
    044 * The optional operations of {@code ListIterator} are not supported for plain {@code Iterator}s.
    045 * </p>
    046 * <p>
    047 * This class implements ResettableListIterator from Commons Collections 3.2.
    048 * </p>
    049 *
    050 * @param <E> The type of elements in this iterator.
    051 * @since 2.1
    052 */
    053public class ListIteratorWrapper<E> implements ResettableListIterator<E> {
    054
    055    /** Message used when set or add are called. */
    056    private static final String UNSUPPORTED_OPERATION_MESSAGE =
    057        "ListIteratorWrapper does not support optional operations of ListIterator.";
    058
    059    /** Message used when set or add are called. */
    060    private static final String CANNOT_REMOVE_MESSAGE = "Cannot remove element at index {0}.";
    061
    062    /** The underlying iterator being decorated. */
    063    private final Iterator<? extends E> iterator;
    064
    065    /** The list being used to cache the iterator. */
    066    private final List<E> list = new ArrayList<>();
    067
    068    /** The current index of this iterator. */
    069    private int currentIndex;
    070
    071    /** The current index of the wrapped iterator. */
    072    private int wrappedIteratorIndex;
    073
    074    /** Recall whether the wrapped iterator's "cursor" is in such a state as to allow remove() to be called */
    075    private boolean removeState;
    076
    077    /**
    078     * Constructs a new {@code ListIteratorWrapper} that will wrap
    079     * the given iterator.
    080     *
    081     * @param iterator  The iterator to wrap
    082     * @throws NullPointerException if the iterator is null
    083     */
    084    public ListIteratorWrapper(final Iterator<? extends E> iterator) {
    085        this.iterator = Objects.requireNonNull(iterator, "iterator");
    086    }
    087
    088    /**
    089     * Throws {@link UnsupportedOperationException}
    090     * unless the underlying {@code Iterator} is a {@code ListIterator}.
    091     *
    092     * @param obj  The object to add
    093     * @throws UnsupportedOperationException if the underlying iterator is not of
    094     * type {@link ListIterator}
    095     */
    096    @Override
    097    public void add(final E obj) throws UnsupportedOperationException {
    098        if (iterator instanceof ListIterator) {
    099            @SuppressWarnings("unchecked")
    100            final ListIterator<E> li = (ListIterator<E>) iterator;
    101            li.add(obj);
    102            return;
    103        }
    104        throw new UnsupportedOperationException(UNSUPPORTED_OPERATION_MESSAGE);
    105    }
    106
    107    /**
    108     * Returns true if there are more elements in the iterator.
    109     *
    110     * @return true if there are more elements
    111     */
    112    @Override
    113    public boolean hasNext() {
    114        if (currentIndex == wrappedIteratorIndex || iterator instanceof ListIterator) {
    115            return iterator.hasNext();
    116        }
    117        return true;
    118    }
    119
    120    /**
    121     * Returns true if there are previous elements in the iterator.
    122     *
    123     * @return true if there are previous elements
    124     */
    125    @Override
    126    public boolean hasPrevious() {
    127        if (iterator instanceof ListIterator) {
    128            final ListIterator<?> li = (ListIterator<?>) iterator;
    129            return li.hasPrevious();
    130        }
    131        return currentIndex > 0;
    132    }
    133
    134    /**
    135     * Returns the next element from the iterator.
    136     *
    137     * @return The next element from the iterator
    138     * @throws NoSuchElementException if there are no more elements
    139     */
    140    @Override
    141    public E next() throws NoSuchElementException {
    142        if (iterator instanceof ListIterator) {
    143            return iterator.next();
    144        }
    145
    146        if (currentIndex < wrappedIteratorIndex) {
    147            ++currentIndex;
    148            return list.get(currentIndex - 1);
    149        }
    150
    151        final E retval = iterator.next();
    152        list.add(retval);
    153        ++currentIndex;
    154        ++wrappedIteratorIndex;
    155        removeState = true;
    156        return retval;
    157    }
    158
    159    /**
    160     * Returns the index of the next element.
    161     *
    162     * @return The index of the next element
    163     */
    164    @Override
    165    public int nextIndex() {
    166        if (iterator instanceof ListIterator) {
    167            final ListIterator<?> li = (ListIterator<?>) iterator;
    168            return li.nextIndex();
    169        }
    170        return currentIndex;
    171    }
    172
    173    /**
    174     * Returns the previous element.
    175     *
    176     * @return The previous element
    177     * @throws NoSuchElementException  if there are no previous elements
    178     */
    179    @Override
    180    public E previous() throws NoSuchElementException {
    181        if (iterator instanceof ListIterator) {
    182            @SuppressWarnings("unchecked")
    183            final ListIterator<E> li = (ListIterator<E>) iterator;
    184            return li.previous();
    185        }
    186
    187        if (currentIndex == 0) {
    188            throw new NoSuchElementException();
    189        }
    190        removeState = wrappedIteratorIndex == currentIndex;
    191        return list.get(--currentIndex);
    192    }
    193
    194    /**
    195     * Returns the index of the previous element.
    196     *
    197     * @return  the index of the previous element
    198     */
    199    @Override
    200    public int previousIndex() {
    201        if (iterator instanceof ListIterator) {
    202            final ListIterator<?> li = (ListIterator<?>) iterator;
    203            return li.previousIndex();
    204        }
    205        return currentIndex - 1;
    206    }
    207
    208    /**
    209     * Removes the last element that was returned by {@link #next()} or {@link #previous()} from the underlying collection.
    210     * This call can only be made once per call to {@code next} or {@code previous} and only if {@link #add(Object)} was not called in between.
    211     *
    212     * @throws IllegalStateException if {@code next} or {@code previous} have not been called before, or if {@code remove} or {@code add} have been called after the last call to {@code next} or {@code previous}
    213     */
    214    @Override
    215    public void remove() throws IllegalStateException {
    216        if (iterator instanceof ListIterator) {
    217            iterator.remove();
    218            return;
    219        }
    220        int removeIndex = currentIndex;
    221        if (currentIndex == wrappedIteratorIndex) {
    222            --removeIndex;
    223        }
    224        if (!removeState || wrappedIteratorIndex - currentIndex > 1) {
    225            throw new IllegalStateException(MessageFormat.format(CANNOT_REMOVE_MESSAGE, Integer.valueOf(removeIndex)));
    226        }
    227        iterator.remove();
    228        list.remove(removeIndex);
    229        currentIndex = removeIndex;
    230        wrappedIteratorIndex--;
    231        removeState = false;
    232    }
    233
    234    /**
    235     * Resets this iterator back to the position at which the iterator
    236     * was created.
    237     *
    238     * @since 3.2
    239     */
    240    @Override
    241    public void reset()  {
    242        if (iterator instanceof ListIterator) {
    243            final ListIterator<?> li = (ListIterator<?>) iterator;
    244            while (li.previousIndex() >= 0) {
    245                li.previous();
    246            }
    247            return;
    248        }
    249        currentIndex = 0;
    250    }
    251
    252    /**
    253     * Throws {@link UnsupportedOperationException}
    254     * unless the underlying {@code Iterator} is a {@code ListIterator}.
    255     *
    256     * @param obj  The object to set
    257     * @throws UnsupportedOperationException if the underlying iterator is not of
    258     * type {@link ListIterator}
    259     */
    260    @Override
    261    public void set(final E obj) throws UnsupportedOperationException {
    262        if (iterator instanceof ListIterator) {
    263            @SuppressWarnings("unchecked")
    264            final ListIterator<E> li = (ListIterator<E>) iterator;
    265            li.set(obj);
    266            return;
    267        }
    268        throw new UnsupportedOperationException(UNSUPPORTED_OPERATION_MESSAGE);
    269    }
    270
    271}