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