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}