1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 package org.apache.commons.collections4.iterators; 18 19 import java.util.List; 20 import java.util.ListIterator; 21 import java.util.Objects; 22 23 import org.apache.commons.collections4.ResettableListIterator; 24 25 /** 26 * Iterates backwards through a List, starting with the last element 27 * and continuing to the first. This is useful for looping around 28 * a list in reverse order without needing to actually reverse the list. 29 * <p> 30 * The first call to {@code next()} will return the last element 31 * from the list, and so on. The {@code hasNext()} method works 32 * in concert with the {@code next()} method as expected. 33 * However, the {@code nextIndex()} method returns the correct 34 * index in the list, thus it starts high and reduces as the iteration 35 * continues. The previous methods work similarly. 36 * 37 * @param <E> the type of elements returned by this iterator. 38 * @since 3.2 39 */ 40 public class ReverseListIterator<E> implements ResettableListIterator<E> { 41 42 /** The list being wrapped. */ 43 private final List<E> list; 44 /** The list iterator being wrapped. */ 45 private ListIterator<E> iterator; 46 /** Flag to indicate if updating is possible at the moment. */ 47 private boolean validForUpdate = true; 48 49 /** 50 * Constructor that wraps a list. 51 * 52 * @param list the list to create a reversed iterator for 53 * @throws NullPointerException if the list is null 54 */ 55 public ReverseListIterator(final List<E> list) { 56 this.list = Objects.requireNonNull(list, "list"); 57 iterator = list.listIterator(list.size()); 58 } 59 60 /** 61 * Adds a new element to the list between the next and previous elements. 62 * 63 * @param obj the object to add 64 * @throws UnsupportedOperationException if the list is unmodifiable 65 * @throws IllegalStateException if the iterator is not in a valid state for set 66 */ 67 @Override 68 public void add(final E obj) { 69 // the validForUpdate flag is needed as the necessary previous() 70 // method call re-enables remove and add 71 if (!validForUpdate) { 72 throw new IllegalStateException("Cannot add to list until next() or previous() called"); 73 } 74 validForUpdate = false; 75 iterator.add(obj); 76 iterator.previous(); 77 } 78 79 /** 80 * Checks whether there is another element. 81 * 82 * @return true if there is another element 83 */ 84 @Override 85 public boolean hasNext() { 86 return iterator.hasPrevious(); 87 } 88 89 /** 90 * Checks whether there is a previous element. 91 * 92 * @return true if there is a previous element 93 */ 94 @Override 95 public boolean hasPrevious() { 96 return iterator.hasNext(); 97 } 98 99 /** 100 * Gets the next element. 101 * The next element is the previous in the list. 102 * 103 * @return the next element in the iterator 104 */ 105 @Override 106 public E next() { 107 final E obj = iterator.previous(); 108 validForUpdate = true; 109 return obj; 110 } 111 112 /** 113 * Gets the index of the next element. 114 * 115 * @return the index of the next element in the iterator 116 */ 117 @Override 118 public int nextIndex() { 119 return iterator.previousIndex(); 120 } 121 122 /** 123 * Gets the previous element. 124 * The next element is the previous in the list. 125 * 126 * @return the previous element in the iterator 127 */ 128 @Override 129 public E previous() { 130 final E obj = iterator.next(); 131 validForUpdate = true; 132 return obj; 133 } 134 135 /** 136 * Gets the index of the previous element. 137 * 138 * @return the index of the previous element in the iterator 139 */ 140 @Override 141 public int previousIndex() { 142 return iterator.nextIndex(); 143 } 144 145 /** 146 * Removes the last returned element. 147 * 148 * @throws UnsupportedOperationException if the list is unmodifiable 149 * @throws IllegalStateException if there is no element to remove 150 */ 151 @Override 152 public void remove() { 153 if (!validForUpdate) { 154 throw new IllegalStateException("Cannot remove from list until next() or previous() called"); 155 } 156 iterator.remove(); 157 } 158 159 /** 160 * Resets the iterator back to the start (which is the 161 * end of the list as this is a reversed iterator) 162 */ 163 @Override 164 public void reset() { 165 iterator = list.listIterator(list.size()); 166 } 167 168 /** 169 * Replaces the last returned element. 170 * 171 * @param obj the object to set 172 * @throws UnsupportedOperationException if the list is unmodifiable 173 * @throws IllegalStateException if the iterator is not in a valid state for set 174 */ 175 @Override 176 public void set(final E obj) { 177 if (!validForUpdate) { 178 throw new IllegalStateException("Cannot set to list until next() or previous() called"); 179 } 180 iterator.set(obj); 181 } 182 183 }