    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 *      https://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.ArrayDeque;
    020import java.util.Deque;
    021import java.util.Iterator;
    022import java.util.Objects;
    023
    024/**
    025 * Decorates an iterator to support pushback of elements.
    026 * <p>
    027 * The decorator stores the pushed back elements in a LIFO manner: the last element
    028 * that has been pushed back, will be returned as the next element in a call to {@link #next()}.
    029 * </p>
    030 * <p>
    031 * The decorator does not support the removal operation. Any call to {@link #remove()} will
    032 * result in an {@link UnsupportedOperationException}.
    033 * </p>
    034 *
    035 * @param <E> The type of elements returned by this iterator.
    036 * @since 4.0
    037 */
    038public class PushbackIterator<E> implements Iterator<E> {
    039
    040    /**
    041     * Decorates the specified iterator to support one-element lookahead.
    042     * <p>
    043     * If the iterator is already a {@link PushbackIterator} it is returned directly.
    044     * </p>
    045     *
    046     * @param <E>  the element type
    047     * @param iterator  The iterator to decorate
    048     * @return A new peeking iterator
    049     * @throws NullPointerException if the iterator is null
    050     */
    051    public static <E> PushbackIterator<E> pushbackIterator(final Iterator<? extends E> iterator) {
    052        Objects.requireNonNull(iterator, "iterator");
    053        if (iterator instanceof PushbackIterator<?>) {
    054            @SuppressWarnings("unchecked") // safe cast
    055            final PushbackIterator<E> it = (PushbackIterator<E>) iterator;
    056            return it;
    057        }
    058        return new PushbackIterator<>(iterator);
    059    }
    060
    061    /** The iterator being decorated. */
    062    private final Iterator<? extends E> iterator;
    063
    064    /** The LIFO queue containing the pushed back items. */
    065    private final Deque<E> items = new ArrayDeque<>();
    066
    067    /**
    068     * Constructs a new instance.
    069     *
    070     * @param iterator  The iterator to decorate
    071     */
    072    public PushbackIterator(final Iterator<? extends E> iterator) {
    073        this.iterator = iterator;
    074    }
    075
    076    @Override
    077    public boolean hasNext() {
    078        return !items.isEmpty() || iterator.hasNext();
    079    }
    080
    081    @Override
    082    public E next() {
    083        return !items.isEmpty() ? items.pop() : iterator.next();
    084    }
    085
    086    /**
    087     * Push back the given element to the iterator.
    088     * <p>
    089     * Calling {@link #next()} immediately afterwards will return exactly this element.
    090     * </p>
    091     *
    092     * @param item  The element to push back to the iterator
    093     */
    094    public void pushback(final E item) {
    095        items.push(item);
    096    }
    097
    098    /**
    099     * Always throws {@link UnsupportedOperationException}.
    100     *
    101     * @throws UnsupportedOperationException Always thrown.
    102     */
    103    @Override
    104    public void remove() {
    105        throw new UnsupportedOperationException();
    106    }
    107
    108}