/* |
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* Licensed to the Apache Software Foundation (ASF) under one or more |
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* contributor license agreements. See the NOTICE file distributed with |
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* this work for additional information regarding copyright ownership. |
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* The ASF licenses this file to You under the Apache License, Version 2.0 |
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* (the "License"); you may not use this file except in compliance with |
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* the License. You may obtain a copy of the License at |
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* |
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* http://www.apache.org/licenses/LICENSE-2.0 |
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* |
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* Unless required by applicable law or agreed to in writing, software |
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* distributed under the License is distributed on an "AS IS" BASIS, |
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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* See the License for the specific language governing permissions and |
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* limitations under the License. |
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*/ |
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package org.apache.commons.collections4.iterators; |
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import java.util.List; |
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import java.util.ListIterator; |
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import java.util.NoSuchElementException; |
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import org.apache.commons.collections4.ResettableListIterator; |
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/** |
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* A ListIterator that restarts when it reaches the end or when it |
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* reaches the beginning. |
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* <p> |
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* The iterator will loop continuously around the provided list, |
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* unless there are no elements in the collection to begin with, or |
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* all of the elements have been {@link #remove removed}. |
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* <p> |
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* Concurrent modifications are not directly supported, and for most |
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* collection implementations will throw a |
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* ConcurrentModificationException. |
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* |
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* @since 3.2 |
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*/ |
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public class LoopingListIterator<E> implements ResettableListIterator<E> { |
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/** The list to base the iterator on */ |
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private final List<E> list; |
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/** The current list iterator */ |
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private ListIterator<E> iterator; |
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/** |
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* Constructor that wraps a list. |
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* <p> |
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* There is no way to reset a ListIterator instance without |
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* recreating it from the original source, so the List must be |
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* passed in and a reference to it held. |
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* |
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* @param list the list to wrap |
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* @throws NullPointerException if the list it null |
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*/ |
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public LoopingListIterator(final List<E> list) { |
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if (list == null) { |
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throw new NullPointerException("The list must not be null"); |
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} |
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this.list = list; |
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_reset(); |
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} |
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/** |
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* Returns whether this iterator has any more elements. |
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* <p> |
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* Returns false only if the list originally had zero elements, or |
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* all elements have been {@link #remove removed}. |
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* |
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* @return <code>true</code> if there are more elements |
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*/ |
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@Override |
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public boolean hasNext() { |
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return !list.isEmpty(); |
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} |
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/** |
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* Returns the next object in the list. |
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* <p> |
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* If at the end of the list, returns the first element. |
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* |
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* @return the object after the last element returned |
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* @throws NoSuchElementException if there are no elements in the list |
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*/ |
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@Override |
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public E next() { |
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if (list.isEmpty()) { |
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throw new NoSuchElementException( |
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"There are no elements for this iterator to loop on"); |
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} |
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if (iterator.hasNext() == false) { |
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reset(); |
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} |
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return iterator.next(); |
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} |
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/** |
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* Returns the index of the element that would be returned by a |
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* subsequent call to {@link #next}. |
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* <p> |
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* As would be expected, if the iterator is at the physical end of |
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* the underlying list, 0 is returned, signifying the beginning of |
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* the list. |
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* |
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* @return the index of the element that would be returned if next() were called |
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* @throws NoSuchElementException if there are no elements in the list |
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*/ |
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@Override |
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public int nextIndex() { |
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if (list.isEmpty()) { |
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throw new NoSuchElementException( |
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"There are no elements for this iterator to loop on"); |
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} |
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if (iterator.hasNext() == false) { |
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return 0; |
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} |
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return iterator.nextIndex(); |
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} |
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/** |
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* Returns whether this iterator has any more previous elements. |
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* <p> |
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* Returns false only if the list originally had zero elements, or |
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* all elements have been {@link #remove removed}. |
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* |
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* @return <code>true</code> if there are more elements |
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*/ |
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@Override |
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public boolean hasPrevious() { |
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return !list.isEmpty(); |
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} |
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/** |
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* Returns the previous object in the list. |
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* <p> |
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* If at the beginning of the list, return the last element. Note |
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* that in this case, traversal to find that element takes linear time. |
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* |
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* @return the object before the last element returned |
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* @throws NoSuchElementException if there are no elements in the list |
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*/ |
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@Override |
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public E previous() { |
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if (list.isEmpty()) { |
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throw new NoSuchElementException( |
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"There are no elements for this iterator to loop on"); |
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} |
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if (iterator.hasPrevious() == false) { |
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E result = null; |
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while (iterator.hasNext()) { |
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result = iterator.next(); |
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} |
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iterator.previous(); |
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return result; |
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} |
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return iterator.previous(); |
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} |
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/** |
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* Returns the index of the element that would be returned by a |
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* subsequent call to {@link #previous}. |
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* <p> |
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* As would be expected, if at the iterator is at the physical |
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* beginning of the underlying list, the list's size minus one is |
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* returned, signifying the end of the list. |
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* |
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* @return the index of the element that would be returned if previous() were called |
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* @throws NoSuchElementException if there are no elements in the list |
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*/ |
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@Override |
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public int previousIndex() { |
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if (list.isEmpty()) { |
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throw new NoSuchElementException( |
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"There are no elements for this iterator to loop on"); |
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} |
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if (iterator.hasPrevious() == false) { |
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return list.size() - 1; |
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} |
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return iterator.previousIndex(); |
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} |
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/** |
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* Removes the previously retrieved item from the underlying list. |
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* <p> |
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* This feature is only supported if the underlying list's |
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* {@link List#iterator iterator} method returns an implementation |
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* that supports it. |
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* <p> |
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* This method can only be called after at least one {@link #next} |
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* or {@link #previous} method call. After a removal, the remove |
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* method may not be called again until another {@link #next} or |
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* {@link #previous} has been performed. If the {@link #reset} is |
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* called, then remove may not be called until {@link #next} or |
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* {@link #previous} is called again. |
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* |
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* @throws UnsupportedOperationException if the remove method is |
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* not supported by the iterator implementation of the underlying |
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* list |
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*/ |
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@Override |
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public void remove() { |
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iterator.remove(); |
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} |
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/** |
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* Inserts the specified element into the underlying list. |
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* <p> |
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* The element is inserted before the next element that would be |
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* returned by {@link #next}, if any, and after the next element |
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* that would be returned by {@link #previous}, if any. |
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* <p> |
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* This feature is only supported if the underlying list's |
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* {@link List#listIterator} method returns an implementation |
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* that supports it. |
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* |
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* @param obj the element to insert |
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* @throws UnsupportedOperationException if the add method is not |
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* supported by the iterator implementation of the underlying list |
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*/ |
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@Override |
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public void add(final E obj) { |
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iterator.add(obj); |
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} |
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/** |
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* Replaces the last element that was returned by {@link #next} or |
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* {@link #previous}. |
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* <p> |
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* This feature is only supported if the underlying list's |
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* {@link List#listIterator} method returns an implementation |
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* that supports it. |
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* |
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* @param obj the element with which to replace the last element returned |
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* @throws UnsupportedOperationException if the set method is not |
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* supported by the iterator implementation of the underlying list |
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*/ |
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@Override |
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public void set(final E obj) { |
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iterator.set(obj); |
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} |
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/** |
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* Resets the iterator back to the start of the list. |
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*/ |
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@Override |
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public void reset() { |
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_reset(); |
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} |
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private void _reset() { |
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iterator = list.listIterator(); |
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} |
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/** |
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* Gets the size of the list underlying the iterator. |
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* |
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* @return the current list size |
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*/ |
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public int size() { |
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return list.size(); |
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} |
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} |