org.apache.commons.collections.iterators.LoopingListIterator Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections.iterators;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import org.apache.commons.collections.ResettableListIterator;
/**
* A ListIterator that restarts when it reaches the end or when it
* reaches the beginning.
*
* The iterator will loop continuously around the provided list,
* unless there are no elements in the collection to begin with, or
* all of the elements have been {@link #remove removed}.
*
* Concurrent modifications are not directly supported, and for most
* collection implementations will throw a
* ConcurrentModificationException.
*
* @since Commons Collections 3.2
* @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
*
* @author Eric Crampton
*/
public class LoopingListIterator implements ResettableListIterator {
/** The list to base the iterator on */
private List list;
/** The current list iterator */
private ListIterator iterator;
/**
* Constructor that wraps a list.
*
* There is no way to reset a ListIterator instance without
* recreating it from the original source, so the List must be
* passed in and a reference to it held.
*
* @param list the list to wrap
* @throws NullPointerException if the list it null
*/
public LoopingListIterator(List list) {
if (list == null) {
throw new NullPointerException("The list must not be null");
}
this.list = list;
reset();
}
/**
* Returns whether this iterator has any more elements.
*
* Returns false only if the list originally had zero elements, or
* all elements have been {@link #remove removed}.
*
* @return true if there are more elements
*/
public boolean hasNext() {
return !list.isEmpty();
}
/**
* Returns the next object in the list.
*
* If at the end of the list, returns the first element.
*
* @return the object after the last element returned
* @throws NoSuchElementException if there are no elements in the list
*/
public Object next() {
if (list.isEmpty()) {
throw new NoSuchElementException(
"There are no elements for this iterator to loop on");
}
if (iterator.hasNext() == false) {
reset();
}
return iterator.next();
}
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #next}.
*
* As would be expected, if the iterator is at the physical end of
* the underlying list, 0 is returned, signifying the beginning of
* the list.
*
* @return the index of the element that would be returned if next() were called
* @throws NoSuchElementException if there are no elements in the list
*/
public int nextIndex() {
if (list.isEmpty()) {
throw new NoSuchElementException(
"There are no elements for this iterator to loop on");
}
if (iterator.hasNext() == false) {
return 0;
} else {
return iterator.nextIndex();
}
}
/**
* Returns whether this iterator has any more previous elements.
*
* Returns false only if the list originally had zero elements, or
* all elements have been {@link #remove removed}.
*
* @return true if there are more elements
*/
public boolean hasPrevious() {
return !list.isEmpty();
}
/**
* Returns the previous object in the list.
*
* If at the beginning of the list, return the last element. Note
* that in this case, traversal to find that element takes linear time.
*
* @return the object before the last element returned
* @throws NoSuchElementException if there are no elements in the list
*/
public Object previous() {
if (list.isEmpty()) {
throw new NoSuchElementException(
"There are no elements for this iterator to loop on");
}
if (iterator.hasPrevious() == false) {
Object result = null;
while (iterator.hasNext()) {
result = iterator.next();
}
iterator.previous();
return result;
} else {
return iterator.previous();
}
}
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #previous}.
*
* As would be expected, if at the iterator is at the physical
* beginning of the underlying list, the list's size minus one is
* returned, signifying the end of the list.
*
* @return the index of the element that would be returned if previous() were called
* @throws NoSuchElementException if there are no elements in the list
*/
public int previousIndex() {
if (list.isEmpty()) {
throw new NoSuchElementException(
"There are no elements for this iterator to loop on");
}
if (iterator.hasPrevious() == false) {
return list.size() - 1;
} else {
return iterator.previousIndex();
}
}
/**
* Removes the previously retrieved item from the underlying list.
*
* This feature is only supported if the underlying list's
* {@link List#iterator iterator} method returns an implementation
* that supports it.
*
* This method can only be called after at least one {@link #next}
* or {@link #previous} method call. After a removal, the remove
* method may not be called again until another {@link #next} or
* {@link #previous} has been performed. If the {@link #reset} is
* called, then remove may not be called until {@link #next} or
* {@link #previous} is called again.
*
* @throws UnsupportedOperationException if the remove method is
* not supported by the iterator implementation of the underlying
* list
*/
public void remove() {
iterator.remove();
}
/**
* Inserts the specified element into the underlying list.
*
* The element is inserted before the next element that would be
* returned by {@link #next}, if any, and after the next element
* that would be returned by {@link #previous}, if any.
*
* This feature is only supported if the underlying list's
* {@link List#listIterator} method returns an implementation
* that supports it.
*
* @param obj the element to insert
* @throws UnsupportedOperationException if the add method is not
* supported by the iterator implementation of the underlying list
*/
public void add(Object obj) {
iterator.add(obj);
}
/**
* Replaces the last element that was returned by {@link #next} or
* {@link #previous}.
*
* This feature is only supported if the underlying list's
* {@link List#listIterator} method returns an implementation
* that supports it.
*
* @param obj the element with which to replace the last element returned
* @throws UnsupportedOperationException if the set method is not
* supported by the iterator implementation of the underlying list
*/
public void set(Object obj) {
iterator.set(obj);
}
/**
* Resets the iterator back to the start of the list.
*/
public void reset() {
iterator = list.listIterator();
}
/**
* Gets the size of the list underlying the iterator.
*
* @return the current list size
*/
public int size() {
return list.size();
}
}