    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.ArrayList;
    020import java.util.Iterator;
    021import java.util.List;
    022import java.util.NoSuchElementException;
    023import java.util.Objects;
    024
    025import org.apache.commons.collections4.FluentIterable;
    026
    027/**
    028 * Provides an interleaved iteration over the elements contained in a
    029 * collection of Iterators.
    030 * <p>
    031 * Given two {@link Iterator} instances {@code A} and {@code B}, the
    032 * {@link #next} method on this iterator will switch between {@code A.next()}
    033 * and {@code B.next()} until both iterators are exhausted.
    034 * </p>
    035 *
    036 * @param <E> the type of elements returned by this iterator.
    037 * @since 4.1
    038 */
    039public class ZippingIterator<E> implements Iterator<E> {
    040
    041    /** The {@link Iterator}s to evaluate. */
    042    private final Iterator<Iterator<? extends E>> iterators;
    043
    044    /** The next iterator to use for next(). */
    045    private Iterator<? extends E> nextIterator;
    046
    047    /** The last iterator which was used for next(). */
    048    private Iterator<? extends E> lastReturned;
    049
    050    /**
    051     * Constructs a new {@code ZippingIterator} that will provide
    052     * interleaved iteration of the specified iterators.
    053     *
    054     * @param iterators  the array of iterators
    055     * @throws NullPointerException if any iterator is null
    056     */
    057    public ZippingIterator(final Iterator<? extends E>... iterators) {
    058        // create a mutable list to be able to remove exhausted iterators
    059        final List<Iterator<? extends E>> list = new ArrayList<>();
    060        for (final Iterator<? extends E> iterator : iterators) {
    061            Objects.requireNonNull(iterator, "iterator");
    062            list.add(iterator);
    063        }
    064        this.iterators = FluentIterable.of(list).loop().iterator();
    065    }
    066
    067    /**
    068     * Constructs a new {@code ZippingIterator} that will provide
    069     * interleaved iteration over the two given iterators.
    070     *
    071     * @param a  the first child iterator
    072     * @param b  the second child iterator
    073     * @throws NullPointerException if either iterator is null
    074     */
    075    @SuppressWarnings("unchecked")
    076    public ZippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {
    077        this(new Iterator[] {a, b});
    078    }
    079
    080    /**
    081     * Constructs a new {@code ZippingIterator} that will provide
    082     * interleaved iteration over the three given iterators.
    083     *
    084     * @param a  the first child iterator
    085     * @param b  the second child iterator
    086     * @param c  the third child iterator
    087     * @throws NullPointerException if either iterator is null
    088     */
    089    @SuppressWarnings("unchecked")
    090    public ZippingIterator(final Iterator<? extends E> a,
    091                           final Iterator<? extends E> b,
    092                           final Iterator<? extends E> c) {
    093        this(new Iterator[] {a, b, c});
    094    }
    095
    096    /**
    097     * Returns {@code true} if any child iterator has remaining elements.
    098     *
    099     * @return true if this iterator has remaining elements
    100     */
    101    @Override
    102    public boolean hasNext() {
    103        // the next iterator has already been determined
    104        // this might happen if hasNext() is called multiple
    105        if (nextIterator != null) {
    106            return true;
    107        }
    108
    109        while (iterators.hasNext()) {
    110            final Iterator<? extends E> childIterator = iterators.next();
    111            if (childIterator.hasNext()) {
    112                nextIterator = childIterator;
    113                return true;
    114            }
    115            // iterator is exhausted, remove it
    116            iterators.remove();
    117        }
    118        return false;
    119    }
    120
    121    /**
    122     * Returns the next element from a child iterator.
    123     *
    124     * @return the next interleaved element
    125     * @throws NoSuchElementException if no child iterator has any more elements
    126     */
    127    @Override
    128    public E next() throws NoSuchElementException {
    129        if (!hasNext()) {
    130            throw new NoSuchElementException();
    131        }
    132
    133        final E val = nextIterator.next();
    134        lastReturned = nextIterator;
    135        nextIterator = null;
    136        return val;
    137    }
    138
    139    /**
    140     * Removes the last returned element from the child iterator that produced it.
    141     *
    142     * @throws IllegalStateException if there is no last returned element, or if
    143     *   the last returned element has already been removed
    144     */
    145    @Override
    146    public void remove() {
    147        if (lastReturned == null) {
    148            throw new IllegalStateException("No value can be removed at present");
    149        }
    150        lastReturned.remove();
    151        lastReturned = null;
    152    }
    153
    154}