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 */
039public class LoopingListIterator<E> implements ResettableListIterator<E> {
040
041    /** The list to base the iterator on */
042    private final List<E> list;
043    /** The current list iterator */
044    private ListIterator<E> iterator;
045
046    /**
047     * Constructor that wraps a list.
048     * <p>
049     * There is no way to reset a ListIterator instance without
050     * recreating it from the original source, so the List must be
051     * passed in and a reference to it held.
052     *
053     * @param list the list to wrap
054     * @throws NullPointerException if the list it null
055     */
056    public LoopingListIterator(final List<E> list) {
057        if (list == null) {
058            throw new NullPointerException("The list must not be null");
059        }
060        this.list = list;
061        _reset();
062    }
063
064    /**
065     * Returns whether this iterator has any more elements.
066     * <p>
067     * Returns false only if the list originally had zero elements, or
068     * all elements have been {@link #remove removed}.
069     *
070     * @return <code>true</code> if there are more elements
071     */
072    @Override
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    @Override
086    public E next() {
087        if (list.isEmpty()) {
088            throw new NoSuchElementException(
089                "There are no elements for this iterator to loop on");
090        }
091        if (iterator.hasNext() == false) {
092            reset();
093        }
094        return iterator.next();
095    }
096
097    /**
098     * Returns the index of the element that would be returned by a
099     * subsequent call to {@link #next}.
100     * <p>
101     * As would be expected, if the iterator is at the physical end of
102     * the underlying list, 0 is returned, signifying the beginning of
103     * the list.
104     *
105     * @return the index of the element that would be returned if next() were called
106     * @throws NoSuchElementException if there are no elements in the list
107     */
108    @Override
109    public int nextIndex() {
110        if (list.isEmpty()) {
111            throw new NoSuchElementException(
112                "There are no elements for this iterator to loop on");
113        }
114        if (iterator.hasNext() == false) {
115            return 0;
116        }
117        return iterator.nextIndex();
118    }
119
120    /**
121     * Returns whether this iterator has any more previous elements.
122     * <p>
123     * Returns false only if the list originally had zero elements, or
124     * all elements have been {@link #remove removed}.
125     *
126     * @return <code>true</code> if there are more elements
127     */
128    @Override
129    public boolean hasPrevious() {
130        return !list.isEmpty();
131    }
132
133    /**
134     * Returns the previous object in the list.
135     * <p>
136     * If at the beginning of the list, return the last element. Note
137     * that in this case, traversal to find that element takes linear time.
138     *
139     * @return the object before the last element returned
140     * @throws NoSuchElementException if there are no elements in the list
141     */
142    @Override
143    public E previous() {
144        if (list.isEmpty()) {
145            throw new NoSuchElementException(
146                "There are no elements for this iterator to loop on");
147        }
148        if (iterator.hasPrevious() == false) {
149            E result = null;
150            while (iterator.hasNext()) {
151                result = iterator.next();
152            }
153            iterator.previous();
154            return result;
155        }
156        return iterator.previous();
157    }
158
159    /**
160     * Returns the index of the element that would be returned by a
161     * subsequent call to {@link #previous}.
162     * <p>
163     * As would be expected, if at the iterator is at the physical
164     * beginning of the underlying list, the list's size minus one is
165     * returned, signifying the end of the list.
166     *
167     * @return the index of the element that would be returned if previous() were called
168     * @throws NoSuchElementException if there are no elements in the list
169     */
170    @Override
171    public int previousIndex() {
172        if (list.isEmpty()) {
173            throw new NoSuchElementException(
174                "There are no elements for this iterator to loop on");
175        }
176        if (iterator.hasPrevious() == false) {
177            return list.size() - 1;
178        }
179        return iterator.previousIndex();
180    }
181
182    /**
183     * Removes the previously retrieved item from the underlying list.
184     * <p>
185     * This feature is only supported if the underlying list's
186     * {@link List#iterator iterator} method returns an implementation
187     * that supports it.
188     * <p>
189     * This method can only be called after at least one {@link #next}
190     * or {@link #previous} method call. After a removal, the remove
191     * method may not be called again until another {@link #next} or
192     * {@link #previous} has been performed. If the {@link #reset} is
193     * called, then remove may not be called until {@link #next} or
194     * {@link #previous} is called again.
195     *
196     * @throws UnsupportedOperationException if the remove method is
197     * not supported by the iterator implementation of the underlying
198     * list
199     */
200    @Override
201    public void remove() {
202        iterator.remove();
203    }
204
205    /**
206     * Inserts the specified element into the underlying list.
207     * <p>
208     * The element is inserted before the next element that would be
209     * returned by {@link #next}, if any, and after the next element
210     * that would be returned by {@link #previous}, if any.
211     * <p>
212     * This feature is only supported if the underlying list's
213     * {@link List#listIterator} method returns an implementation
214     * that supports it.
215     *
216     * @param obj  the element to insert
217     * @throws UnsupportedOperationException if the add method is not
218     *  supported by the iterator implementation of the underlying list
219     */
220    @Override
221    public void add(final E obj) {
222        iterator.add(obj);
223    }
224
225    /**
226     * Replaces the last element that was returned by {@link #next} or
227     * {@link #previous}.
228     * <p>
229     * This feature is only supported if the underlying list's
230     * {@link List#listIterator} method returns an implementation
231     * that supports it.
232     *
233     * @param obj  the element with which to replace the last element returned
234     * @throws UnsupportedOperationException if the set method is not
235     *  supported by the iterator implementation of the underlying list
236     */
237    @Override
238    public void set(final E obj) {
239        iterator.set(obj);
240    }
241
242    /**
243     * Resets the iterator back to the start of the list.
244     */
245    @Override
246    public void reset() {
247        _reset();
248    }
249
250    private void _reset() {
251        iterator = list.listIterator();
252    }
253
254    /**
255     * Gets the size of the list underlying the iterator.
256     *
257     * @return the current list size
258     */
259    public int size() {
260        return list.size();
261    }
262
263}