    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;
    023
    024import org.apache.commons.collections4.FluentIterable;
    025
    026/**
    027 * Provides an interleaved iteration over the elements contained in a
    028 * collection of Iterators.
    029 * <p>
    030 * Given two {@link Iterator} instances {@code A} and {@code B}, the
    031 * {@link #next} method on this iterator will switch between {@code A.next()}
    032 * and {@code B.next()} until both iterators are exhausted.
    033 * </p>
    034 *
    035 * @since 4.1
    036 */
    037public class ZippingIterator<E> implements Iterator<E> {
    038
    039    /** The {@link Iterator}s to evaluate. */
    040    private final Iterator<Iterator<? extends E>> iterators;
    041
    042    /** The next iterator to use for next(). */
    043    private Iterator<? extends E> nextIterator = null;
    044
    045    /** The last iterator which was used for next(). */
    046    private Iterator<? extends E> lastReturned = null;
    047
    048    // Constructors
    049    // ----------------------------------------------------------------------
    050
    051    /**
    052     * Constructs a new <code>ZippingIterator</code> that will provide
    053     * interleaved iteration over the two given iterators.
    054     *
    055     * @param a  the first child iterator
    056     * @param b  the second child iterator
    057     * @throws NullPointerException if either iterator is null
    058     */
    059    @SuppressWarnings("unchecked")
    060    public ZippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {
    061        this(new Iterator[] {a, b});
    062    }
    063
    064    /**
    065     * Constructs a new <code>ZippingIterator</code> that will provide
    066     * interleaved iteration over the three given iterators.
    067     *
    068     * @param a  the first child iterator
    069     * @param b  the second child iterator
    070     * @param c  the third child iterator
    071     * @throws NullPointerException if either iterator is null
    072     */
    073    @SuppressWarnings("unchecked")
    074    public ZippingIterator(final Iterator<? extends E> a,
    075                           final Iterator<? extends E> b,
    076                           final Iterator<? extends E> c) {
    077        this(new Iterator[] {a, b, c});
    078    }
    079
    080    /**
    081     * Constructs a new <code>ZippingIterator</code> that will provide
    082     * interleaved iteration of the specified iterators.
    083     *
    084     * @param iterators  the array of iterators
    085     * @throws NullPointerException if any iterator is null
    086     */
    087    public ZippingIterator(final Iterator<? extends E>... iterators) {
    088        // create a mutable list to be able to remove exhausted iterators
    089        final List<Iterator<? extends E>> list = new ArrayList<>();
    090        for (final Iterator<? extends E> iterator : iterators) {
    091            if (iterator == null) {
    092                throw new NullPointerException("Iterator must not be null.");
    093            }
    094            list.add(iterator);
    095        }
    096        this.iterators = FluentIterable.of(list).loop().iterator();
    097    }
    098
    099    // Iterator Methods
    100    // -------------------------------------------------------------------
    101
    102    /**
    103     * Returns {@code true} if any child iterator has remaining elements.
    104     *
    105     * @return true if this iterator has remaining elements
    106     */
    107    @Override
    108    public boolean hasNext() {
    109        // the next iterator has already been determined
    110        // this might happen if hasNext() is called multiple
    111        if (nextIterator != null) {
    112            return true;
    113        }
    114
    115        while(iterators.hasNext()) {
    116            final Iterator<? extends E> childIterator = iterators.next();
    117            if (childIterator.hasNext()) {
    118                nextIterator = childIterator;
    119                return true;
    120            }
    121            // iterator is exhausted, remove it
    122            iterators.remove();
    123        }
    124        return false;
    125    }
    126
    127    /**
    128     * Returns the next element from a child iterator.
    129     *
    130     * @return the next interleaved element
    131     * @throws NoSuchElementException if no child iterator has any more elements
    132     */
    133    @Override
    134    public E next() throws NoSuchElementException {
    135        if (!hasNext()) {
    136            throw new NoSuchElementException();
    137        }
    138
    139        final E val = nextIterator.next();
    140        lastReturned = nextIterator;
    141        nextIterator = null;
    142        return val;
    143    }
    144
    145    /**
    146     * Removes the last returned element from the child iterator that produced it.
    147     *
    148     * @throws IllegalStateException if there is no last returned element, or if
    149     *   the last returned element has already been removed
    150     */
    151    @Override
    152    public void remove() {
    153        if (lastReturned == null) {
    154            throw new IllegalStateException("No value can be removed at present");
    155        }
    156        lastReturned.remove();
    157        lastReturned = null;
    158    }
    159
    160}