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; 022import java.util.Objects; 023 024import org.apache.commons.collections4.ResettableListIterator; 025 026/** 027 * A ListIterator that restarts when it reaches the end or when it 028 * reaches the beginning. 029 * <p> 030 * The iterator will loop continuously around the provided list, 031 * unless there are no elements in the collection to begin with, or 032 * all of the elements have been {@link #remove removed}. 033 * <p> 034 * Concurrent modifications are not directly supported, and for most 035 * collection implementations will throw a 036 * ConcurrentModificationException. 037 * 038 * @param <E> the type of elements returned by this iterator. 039 * @since 3.2 040 */ 041public class LoopingListIterator<E> implements ResettableListIterator<E> { 042 043 /** The list to base the iterator on */ 044 private final List<E> list; 045 /** The current list iterator */ 046 private ListIterator<E> iterator; 047 048 /** 049 * Constructor that wraps a list. 050 * <p> 051 * There is no way to reset a ListIterator instance without 052 * recreating it from the original source, so the List must be 053 * passed in and a reference to it held. 054 * 055 * @param list the list to wrap 056 * @throws NullPointerException if the list is null 057 */ 058 public LoopingListIterator(final List<E> list) { 059 this.list = Objects.requireNonNull(list, "collection"); 060 _reset(); 061 } 062 063 private void _reset() { 064 iterator = list.listIterator(); 065 } 066 067 /** 068 * Inserts the specified element into the underlying list. 069 * <p> 070 * The element is inserted before the next element that would be 071 * returned by {@link #next}, if any, and after the next element 072 * that would be returned by {@link #previous}, if any. 073 * <p> 074 * This feature is only supported if the underlying list's 075 * {@link List#listIterator} method returns an implementation 076 * that supports it. 077 * 078 * @param obj the element to insert 079 * @throws UnsupportedOperationException if the add method is not 080 * supported by the iterator implementation of the underlying list 081 */ 082 @Override 083 public void add(final E obj) { 084 iterator.add(obj); 085 } 086 087 /** 088 * Returns whether this iterator has any more elements. 089 * <p> 090 * Returns false only if the list originally had zero elements, or 091 * all elements have been {@link #remove removed}. 092 * 093 * @return {@code true} if there are more elements 094 */ 095 @Override 096 public boolean hasNext() { 097 return !list.isEmpty(); 098 } 099 100 /** 101 * Returns whether this iterator has any more previous elements. 102 * <p> 103 * Returns false only if the list originally had zero elements, or 104 * all elements have been {@link #remove removed}. 105 * 106 * @return {@code true} if there are more elements 107 */ 108 @Override 109 public boolean hasPrevious() { 110 return !list.isEmpty(); 111 } 112 113 /** 114 * Returns the next object in the list. 115 * <p> 116 * If at the end of the list, returns the first element. 117 * 118 * @return the object after the last element returned 119 * @throws NoSuchElementException if there are no elements in the list 120 */ 121 @Override 122 public E next() { 123 if (list.isEmpty()) { 124 throw new NoSuchElementException( 125 "There are no elements for this iterator to loop on"); 126 } 127 if (!iterator.hasNext()) { 128 reset(); 129 } 130 return iterator.next(); 131 } 132 133 /** 134 * Returns the index of the element that would be returned by a 135 * subsequent call to {@link #next}. 136 * <p> 137 * As would be expected, if the iterator is at the physical end of 138 * the underlying list, 0 is returned, signifying the beginning of 139 * the list. 140 * 141 * @return the index of the element that would be returned if next() were called 142 * @throws NoSuchElementException if there are no elements in the list 143 */ 144 @Override 145 public int nextIndex() { 146 if (list.isEmpty()) { 147 throw new NoSuchElementException( 148 "There are no elements for this iterator to loop on"); 149 } 150 if (!iterator.hasNext()) { 151 return 0; 152 } 153 return iterator.nextIndex(); 154 } 155 156 /** 157 * Returns the previous object in the list. 158 * <p> 159 * If at the beginning of the list, return the last element. Note 160 * that in this case, traversal to find that element takes linear time. 161 * 162 * @return the object before the last element returned 163 * @throws NoSuchElementException if there are no elements in the list 164 */ 165 @Override 166 public E previous() { 167 if (list.isEmpty()) { 168 throw new NoSuchElementException( 169 "There are no elements for this iterator to loop on"); 170 } 171 if (!iterator.hasPrevious()) { 172 E result = null; 173 while (iterator.hasNext()) { 174 result = iterator.next(); 175 } 176 iterator.previous(); 177 return result; 178 } 179 return iterator.previous(); 180 } 181 182 /** 183 * Returns the index of the element that would be returned by a 184 * subsequent call to {@link #previous}. 185 * <p> 186 * As would be expected, if at the iterator is at the physical 187 * beginning of the underlying list, the list's size minus one is 188 * returned, signifying the end of the list. 189 * 190 * @return the index of the element that would be returned if previous() were called 191 * @throws NoSuchElementException if there are no elements in the list 192 */ 193 @Override 194 public int previousIndex() { 195 if (list.isEmpty()) { 196 throw new NoSuchElementException( 197 "There are no elements for this iterator to loop on"); 198 } 199 if (!iterator.hasPrevious()) { 200 return list.size() - 1; 201 } 202 return iterator.previousIndex(); 203 } 204 205 /** 206 * Removes the previously retrieved item from the underlying list. 207 * <p> 208 * This feature is only supported if the underlying list's 209 * {@link List#iterator iterator} method returns an implementation 210 * that supports it. 211 * <p> 212 * This method can only be called after at least one {@link #next} 213 * or {@link #previous} method call. After a removal, the remove 214 * method may not be called again until another {@link #next} or 215 * {@link #previous} has been performed. If the {@link #reset} is 216 * called, then remove may not be called until {@link #next} or 217 * {@link #previous} is called again. 218 * 219 * @throws UnsupportedOperationException if the remove method is 220 * not supported by the iterator implementation of the underlying 221 * list 222 */ 223 @Override 224 public void remove() { 225 iterator.remove(); 226 } 227 228 /** 229 * Resets the iterator back to the start of the list. 230 */ 231 @Override 232 public void reset() { 233 _reset(); 234 } 235 236 /** 237 * Replaces the last element that was returned by {@link #next} or 238 * {@link #previous}. 239 * <p> 240 * This feature is only supported if the underlying list's 241 * {@link List#listIterator} method returns an implementation 242 * that supports it. 243 * 244 * @param obj the element with which to replace the last element returned 245 * @throws UnsupportedOperationException if the set method is not 246 * supported by the iterator implementation of the underlying list 247 */ 248 @Override 249 public void set(final E obj) { 250 iterator.set(obj); 251 } 252 253 /** 254 * Gets the size of the list underlying the iterator. 255 * 256 * @return the current list size 257 */ 258 public int size() { 259 return list.size(); 260 } 261 262}