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