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Commons Collections example source code file (LoopingListIterator.java)

This example Commons Collections source code file (LoopingListIterator.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

list, listiterator, loopinglistiterator, loopinglistiterator, nosuchelementexception, nosuchelementexception, nullpointerexception, object, object, resettablelistiterator, the, there, there, util

The Commons Collections LoopingListIterator.java source code

/*
 *  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.
 * <p>
 * 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}.
 * <p>
 * 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 13:33:15 +0100 (Thu, 10 Apr 2008) $
 *
 * @author Eric Crampton <ccesc@eonomine.com>
 */
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.
     * <p>
     * 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.
     * <p>
     * Returns false only if the list originally had zero elements, or
     * all elements have been {@link #remove removed}.
     *
     * @return <code>true if there are more elements
     */
    public boolean hasNext() {
        return !list.isEmpty();
    }

    /**
     * Returns the next object in the list.
     * <p>
     * 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}.
     * <p>
     * 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.
     * <p>
     * Returns false only if the list originally had zero elements, or
     * all elements have been {@link #remove removed}.
     *
     * @return <code>true if there are more elements
     */
    public boolean hasPrevious() {
        return !list.isEmpty();
    }

    /**
     * Returns the previous object in the list.
     * <p>
     * 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}.
     * <p>
     * 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.
     * <p>
     * This feature is only supported if the underlying list's
     * {@link List#iterator iterator} method returns an implementation
     * that supports it.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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}.
     * <p>
     * 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();
    }

}

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