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;
021
022import org.apache.commons.collections4.ResettableListIterator;
023
024/**
025 * Iterates backwards through a List, starting with the last element
026 * and continuing to the first. This is useful for looping around
027 * a list in reverse order without needing to actually reverse the list.
028 * <p>
029 * The first call to <code>next()</code> will return the last element
030 * from the list, and so on. The <code>hasNext()</code> method works
031 * in concert with the <code>next()</code> method as expected.
032 * However, the <code>nextIndex()</code> method returns the correct
033 * index in the list, thus it starts high and reduces as the iteration
034 * continues. The previous methods work similarly.
035 *
036 * @since 3.2
037 */
038public class ReverseListIterator<E> implements ResettableListIterator<E> {
039
040    /** The list being wrapped. */
041    private final List<E> list;
042    /** The list iterator being wrapped. */
043    private ListIterator<E> iterator;
044    /** Flag to indicate if updating is possible at the moment. */
045    private boolean validForUpdate = true;
046
047    /**
048     * Constructor that wraps a list.
049     *
050     * @param list  the list to create a reversed iterator for
051     * @throws NullPointerException if the list is null
052     */
053    public ReverseListIterator(final List<E> list) {
054        super();
055        if (list == null) {
056            throw new NullPointerException("List must not be null.");
057        }
058        this.list = list;
059        iterator = list.listIterator(list.size());
060    }
061
062    //-----------------------------------------------------------------------
063    /**
064     * Checks whether there is another element.
065     *
066     * @return true if there is another element
067     */
068    @Override
069    public boolean hasNext() {
070        return iterator.hasPrevious();
071    }
072
073    /**
074     * Gets the next element.
075     * The next element is the previous in the list.
076     *
077     * @return the next element in the iterator
078     */
079    @Override
080    public E next() {
081        final E obj = iterator.previous();
082        validForUpdate = true;
083        return obj;
084    }
085
086    /**
087     * Gets the index of the next element.
088     *
089     * @return the index of the next element in the iterator
090     */
091    @Override
092    public int nextIndex() {
093        return iterator.previousIndex();
094    }
095
096    /**
097     * Checks whether there is a previous element.
098     *
099     * @return true if there is a previous element
100     */
101    @Override
102    public boolean hasPrevious() {
103        return iterator.hasNext();
104    }
105
106    /**
107     * Gets the previous element.
108     * The next element is the previous in the list.
109     *
110     * @return the previous element in the iterator
111     */
112    @Override
113    public E previous() {
114        final E obj = iterator.next();
115        validForUpdate = true;
116        return obj;
117    }
118
119    /**
120     * Gets the index of the previous element.
121     *
122     * @return the index of the previous element in the iterator
123     */
124    @Override
125    public int previousIndex() {
126        return iterator.nextIndex();
127    }
128
129    /**
130     * Removes the last returned element.
131     *
132     * @throws UnsupportedOperationException if the list is unmodifiable
133     * @throws IllegalStateException if there is no element to remove
134     */
135    @Override
136    public void remove() {
137        if (validForUpdate == false) {
138            throw new IllegalStateException("Cannot remove from list until next() or previous() called");
139        }
140        iterator.remove();
141    }
142
143    /**
144     * Replaces the last returned element.
145     *
146     * @param obj  the object to set
147     * @throws UnsupportedOperationException if the list is unmodifiable
148     * @throws IllegalStateException if the iterator is not in a valid state for set
149     */
150    @Override
151    public void set(final E obj) {
152        if (validForUpdate == false) {
153            throw new IllegalStateException("Cannot set to list until next() or previous() called");
154        }
155        iterator.set(obj);
156    }
157
158    /**
159     * Adds a new element to the list between the next and previous elements.
160     *
161     * @param obj  the object to add
162     * @throws UnsupportedOperationException if the list is unmodifiable
163     * @throws IllegalStateException if the iterator is not in a valid state for set
164     */
165    @Override
166    public void add(final E obj) {
167        // the validForUpdate flag is needed as the necessary previous()
168        // method call re-enables remove and add
169        if (validForUpdate == false) {
170            throw new IllegalStateException("Cannot add to list until next() or previous() called");
171        }
172        validForUpdate = false;
173        iterator.add(obj);
174        iterator.previous();
175    }
176
177    /**
178     * Resets the iterator back to the start (which is the
179     * end of the list as this is a reversed iterator)
180     */
181    @Override
182    public void reset() {
183        iterator = list.listIterator(list.size());
184    }
185
186}