    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.Collection;
    020import java.util.Iterator;
    021import java.util.NoSuchElementException;
    022import java.util.Objects;
    023
    024import org.apache.commons.collections4.ResettableIterator;
    025
    026/**
    027 * An Iterator that restarts when it reaches the end.
    028 * <p>
    029 * The iterator will loop continuously around the provided elements, unless
    030 * there are no elements in the collection to begin with, or all the elements
    031 * have been {@link #remove removed}.
    032 * </p>
    033 * <p>
    034 * Concurrent modifications are not directly supported, and for most collection
    035 * implementations will throw a ConcurrentModificationException.
    036 * </p>
    037 *
    038 * @param <E> the type of elements returned by this iterator.
    039 * @since 3.0
    040 */
    041public class LoopingIterator<E> implements ResettableIterator<E> {
    042
    043    /** The collection to base the iterator on */
    044    private final Collection<? extends E> collection;
    045    /** The current iterator */
    046    private Iterator<? extends E> iterator;
    047
    048    /**
    049     * Constructor that wraps a collection.
    050     * <p>
    051     * There is no way to reset an Iterator instance without recreating it from
    052     * the original source, so the Collection must be passed in.
    053     * </p>
    054     *
    055     * @param collection  the collection to wrap
    056     * @throws NullPointerException if the collection is null
    057     */
    058    public LoopingIterator(final Collection<? extends E> collection) {
    059        this.collection = Objects.requireNonNull(collection, "collection");
    060        reset();
    061    }
    062
    063    /**
    064     * Has the iterator any more elements.
    065     * <p>
    066     * Returns false only if the collection originally had zero elements, or
    067     * all the elements have been {@link #remove removed}.
    068     * </p>
    069     *
    070     * @return {@code true} if there are more elements
    071     */
    072    @Override
    073    public boolean hasNext() {
    074        return !collection.isEmpty();
    075    }
    076
    077    /**
    078     * Returns the next object in the collection.
    079     * <p>
    080     * If at the end of the collection, return the first element.
    081     * </p>
    082     *
    083     * @return the next object
    084     * @throws NoSuchElementException if there are no elements
    085     *         at all.  Use {@link #hasNext} to avoid this error.
    086     */
    087    @Override
    088    public E next() {
    089        if (collection.isEmpty()) {
    090            throw new NoSuchElementException("There are no elements for this iterator to loop on");
    091        }
    092        if (!iterator.hasNext()) {
    093            reset();
    094        }
    095        return iterator.next();
    096    }
    097
    098    /**
    099     * Removes the previously retrieved item from the underlying collection.
    100     * <p>
    101     * This feature is only supported if the underlying collection's
    102     * {@link Collection#iterator()} method returns an implementation
    103     * that supports it.
    104     * </p>
    105     * <p>
    106     * This method can only be called after at least one {@link #next} method call.
    107     * After a removal, the remove method may not be called again until another
    108     * next has been performed. If the {@link #reset} is called, then remove may
    109     * not be called until {@link #next} is called again.
    110     * </p>
    111     */
    112    @Override
    113    public void remove() {
    114        iterator.remove();
    115    }
    116
    117    /**
    118     * Resets the iterator back to the start of the collection.
    119     */
    120    @Override
    121    public void reset() {
    122        iterator = collection.iterator();
    123    }
    124
    125    /**
    126     * Gets the size of the collection underlying the iterator.
    127     *
    128     * @return the current collection size
    129     */
    130    public int size() {
    131        return collection.size();
    132    }
    133
    134}