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.NoSuchElementException;
022
023import org.apache.commons.collections4.ResettableListIterator;
024
025/**
026 * A ListIterator that restarts when it reaches the end or when it
027 * reaches the beginning.
028 * <p>
029 * The iterator will loop continuously around the provided list,
030 * unless there are no elements in the collection to begin with, or
031 * all of the elements have been {@link #remove removed}.
032 * <p>
033 * Concurrent modifications are not directly supported, and for most
034 * collection implementations will throw a
035 * ConcurrentModificationException.
036 *
037 * @since 3.2
038 * @version $Id: LoopingListIterator.html 972421 2015-11-14 20:00:04Z tn $
039 */
040public class LoopingListIterator<E> implements ResettableListIterator<E> {
041
042    /** The list to base the iterator on */
043    private final List<E> list;
044    /** The current list iterator */
045    private ListIterator<E> iterator;
046
047    /**
048     * Constructor that wraps a list.
049     * <p>
050     * There is no way to reset a ListIterator instance without
051     * recreating it from the original source, so the List must be
052     * passed in and a reference to it held.
053     *
054     * @param list the list to wrap
055     * @throws NullPointerException if the list it null
056     */
057    public LoopingListIterator(final List<E> list) {
058        if (list == null) {
059            throw new NullPointerException("The list must not be null");
060        }
061        this.list = list;
062        _reset();
063    }
064
065    /**
066     * Returns whether this iterator has any more elements.
067     * <p>
068     * Returns false only if the list originally had zero elements, or
069     * all elements have been {@link #remove removed}.
070     *
071     * @return <code>true</code> if there are more elements
072     */
073    public boolean hasNext() {
074        return !list.isEmpty();
075    }
076
077    /**
078     * Returns the next object in the list.
079     * <p>
080     * If at the end of the list, returns the first element.
081     *
082     * @return the object after the last element returned
083     * @throws NoSuchElementException if there are no elements in the list
084     */
085    public E next() {
086        if (list.isEmpty()) {
087            throw new NoSuchElementException(
088                "There are no elements for this iterator to loop on");
089        }
090        if (iterator.hasNext() == false) {
091            reset();
092        }
093        return iterator.next();
094    }
095
096    /**
097     * Returns the index of the element that would be returned by a
098     * subsequent call to {@link #next}.
099     * <p>
100     * As would be expected, if the iterator is at the physical end of
101     * the underlying list, 0 is returned, signifying the beginning of
102     * the list.
103     *
104     * @return the index of the element that would be returned if next() were called
105     * @throws NoSuchElementException if there are no elements in the list
106     */
107    public int nextIndex() {
108        if (list.isEmpty()) {
109            throw new NoSuchElementException(
110                "There are no elements for this iterator to loop on");
111        }
112        if (iterator.hasNext() == false) {
113            return 0;
114        }
115        return iterator.nextIndex();
116    }
117
118    /**
119     * Returns whether this iterator has any more previous elements.
120     * <p>
121     * Returns false only if the list originally had zero elements, or
122     * all elements have been {@link #remove removed}.
123     *
124     * @return <code>true</code> if there are more elements
125     */
126    public boolean hasPrevious() {
127        return !list.isEmpty();
128    }
129
130    /**
131     * Returns the previous object in the list.
132     * <p>
133     * If at the beginning of the list, return the last element. Note
134     * that in this case, traversal to find that element takes linear time.
135     *
136     * @return the object before the last element returned
137     * @throws NoSuchElementException if there are no elements in the list
138     */
139    public E previous() {
140        if (list.isEmpty()) {
141            throw new NoSuchElementException(
142                "There are no elements for this iterator to loop on");
143        }
144        if (iterator.hasPrevious() == false) {
145            E result = null;
146            while (iterator.hasNext()) {
147                result = iterator.next();
148            }
149            iterator.previous();
150            return result;
151        }
152        return iterator.previous();
153    }
154
155    /**
156     * Returns the index of the element that would be returned by a
157     * subsequent call to {@link #previous}.
158     * <p>
159     * As would be expected, if at the iterator is at the physical
160     * beginning of the underlying list, the list's size minus one is
161     * returned, signifying the end of the list.
162     *
163     * @return the index of the element that would be returned if previous() were called
164     * @throws NoSuchElementException if there are no elements in the list
165     */
166    public int previousIndex() {
167        if (list.isEmpty()) {
168            throw new NoSuchElementException(
169                "There are no elements for this iterator to loop on");
170        }
171        if (iterator.hasPrevious() == false) {
172            return list.size() - 1;
173        }
174        return iterator.previousIndex();
175    }
176
177    /**
178     * Removes the previously retrieved item from the underlying list.
179     * <p>
180     * This feature is only supported if the underlying list's
181     * {@link List#iterator iterator} method returns an implementation
182     * that supports it.
183     * <p>
184     * This method can only be called after at least one {@link #next}
185     * or {@link #previous} method call. After a removal, the remove
186     * method may not be called again until another {@link #next} or
187     * {@link #previous} has been performed. If the {@link #reset} is
188     * called, then remove may not be called until {@link #next} or
189     * {@link #previous} is called again.
190     *
191     * @throws UnsupportedOperationException if the remove method is
192     * not supported by the iterator implementation of the underlying
193     * list
194     */
195    public void remove() {
196        iterator.remove();
197    }
198
199    /**
200     * Inserts the specified element into the underlying list.
201     * <p>
202     * The element is inserted before the next element that would be
203     * returned by {@link #next}, if any, and after the next element
204     * that would be returned by {@link #previous}, if any.
205     * <p>
206     * This feature is only supported if the underlying list's
207     * {@link List#listIterator} method returns an implementation
208     * that supports it.
209     *
210     * @param obj  the element to insert
211     * @throws UnsupportedOperationException if the add method is not
212     *  supported by the iterator implementation of the underlying list
213     */
214    public void add(final E obj) {
215        iterator.add(obj);
216    }
217
218    /**
219     * Replaces the last element that was returned by {@link #next} or
220     * {@link #previous}.
221     * <p>
222     * This feature is only supported if the underlying list's
223     * {@link List#listIterator} method returns an implementation
224     * that supports it.
225     *
226     * @param obj  the element with which to replace the last element returned
227     * @throws UnsupportedOperationException if the set method is not
228     *  supported by the iterator implementation of the underlying list
229     */
230    public void set(final E obj) {
231        iterator.set(obj);
232    }
233
234    /**
235     * Resets the iterator back to the start of the list.
236     */
237    public void reset() {
238        _reset();
239    }
240
241    private void _reset() {
242        iterator = list.listIterator();
243    }
244
245    /**
246     * Gets the size of the list underlying the iterator.
247     *
248     * @return the current list size
249     */
250    public int size() {
251        return list.size();
252    }
253
254}