    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.Iterator;
    020import java.util.NoSuchElementException;
    021import java.util.Objects;
    022
    023/**
    024 * Decorates an iterator to support one-element lookahead while iterating.
    025 * <p>
    026 * The decorator supports the removal operation, but an {@link IllegalStateException}
    027 * will be thrown if {@link #remove()} is called directly after a call to
    028 * {@link #peek()} or {@link #element()}.
    029 *
    030 * @param <E> the type of elements returned by this iterator.
    031 * @since 4.0
    032 */
    033public class PeekingIterator<E> implements Iterator<E> {
    034
    035    /**
    036     * Decorates the specified iterator to support one-element lookahead.
    037     * <p>
    038     * If the iterator is already a {@link PeekingIterator} it is returned directly.
    039     *
    040     * @param <E>  the element type
    041     * @param iterator  the iterator to decorate
    042     * @return a new peeking iterator
    043     * @throws NullPointerException if the iterator is null
    044     */
    045    public static <E> PeekingIterator<E> peekingIterator(final Iterator<? extends E> iterator) {
    046        Objects.requireNonNull(iterator, "iterator");
    047        if (iterator instanceof PeekingIterator<?>) {
    048            @SuppressWarnings("unchecked") // safe cast
    049            final PeekingIterator<E> it = (PeekingIterator<E>) iterator;
    050            return it;
    051        }
    052        return new PeekingIterator<>(iterator);
    053    }
    054
    055    /** The iterator being decorated. */
    056    private final Iterator<? extends E> iterator;
    057
    058    /** Indicates that the decorated iterator is exhausted. */
    059    private boolean exhausted;
    060
    061    /** Indicates if the lookahead slot is filled. */
    062    private boolean slotFilled;
    063
    064    /** The current slot for lookahead. */
    065    private E slot;
    066
    067    /**
    068     * Constructs a new instance.
    069     *
    070     * @param iterator  the iterator to decorate
    071     */
    072    public PeekingIterator(final Iterator<? extends E> iterator) {
    073        this.iterator = iterator;
    074    }
    075
    076    /**
    077     * Returns the next element in iteration without advancing the underlying iterator.
    078     * If the iterator is already exhausted, null will be returned.
    079     *
    080     * @return the next element from the iterator
    081     * @throws NoSuchElementException if the iterator is already exhausted according to {@link #hasNext()}
    082     */
    083    public E element() {
    084        fill();
    085        if (exhausted) {
    086            throw new NoSuchElementException();
    087        }
    088        return slot;
    089    }
    090
    091    private void fill() {
    092        if (exhausted || slotFilled) {
    093            return;
    094        }
    095        if (iterator.hasNext()) {
    096            slot = iterator.next();
    097            slotFilled = true;
    098        } else {
    099            exhausted = true;
    100            slot = null;
    101            slotFilled = false;
    102        }
    103    }
    104
    105    @Override
    106    public boolean hasNext() {
    107        if (exhausted) {
    108            return false;
    109        }
    110        return slotFilled || iterator.hasNext();
    111    }
    112
    113    @Override
    114    public E next() {
    115        if (!hasNext()) {
    116            throw new NoSuchElementException();
    117        }
    118        final E x = slotFilled ? slot : iterator.next();
    119        // reset the lookahead slot
    120        slot = null;
    121        slotFilled = false;
    122        return x;
    123    }
    124
    125    /**
    126     * Returns the next element in iteration without advancing the underlying iterator.
    127     * If the iterator is already exhausted, null will be returned.
    128     * <p>
    129     * Note: this method does not throw a {@link NoSuchElementException} if the iterator
    130     * is already exhausted. If you want such a behavior, use {@link #element()} instead.
    131     * <p>
    132     * The rationale behind this is to follow the {@link java.util.Queue} interface
    133     * which uses the same terminology.
    134     *
    135     * @return the next element from the iterator
    136     */
    137    public E peek() {
    138        fill();
    139        return exhausted ? null : slot;
    140    }
    141
    142    /**
    143     * {@inheritDoc}
    144     *
    145     * @throws IllegalStateException if {@link #peek()} or {@link #element()} has been called
    146     *   prior to the call to {@link #remove()}
    147     */
    148    @Override
    149    public void remove() {
    150        if (slotFilled) {
    151            throw new IllegalStateException("peek() or element() called before remove()");
    152        }
    153        iterator.remove();
    154    }
    155
    156}