    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one
    003 * or more contributor license agreements.  See the NOTICE file
    004 * distributed with this work for additional information
    005 * regarding copyright ownership.  The ASF licenses this file
    006 * to you under the Apache License, Version 2.0 (the
    007 * "License"); you may not use this file except in compliance
    008 * with the License.  You may obtain a copy of the License at
    009 *
    010 *     http://www.apache.org/licenses/LICENSE-2.0
    011 *
    012 * Unless required by applicable law or agreed to in writing, software
    013 * distributed under the License is distributed on an "AS IS" BASIS,
    014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    015 * See the License for the specific language governing permissions and
    016 * limitations under the License.
    017 */
    018package org.apache.commons.collections4.iterators;
    019
    020import java.util.ArrayDeque;
    021import java.util.Deque;
    022import java.util.Iterator;
    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 * @since 4.0
    034 */
    035public class PushbackIterator<E> implements Iterator<E> {
    036
    037    /** The iterator being decorated. */
    038    private final Iterator<? extends E> iterator;
    039
    040    /** The LIFO queue containing the pushed back items. */
    041    private final Deque<E> items = new ArrayDeque<>();
    042
    043    //-----------------------------------------------------------------------
    044    /**
    045     * Decorates the specified iterator to support one-element lookahead.
    046     * <p>
    047     * If the iterator is already a {@link PushbackIterator} it is returned directly.
    048     *
    049     * @param <E>  the element type
    050     * @param iterator  the iterator to decorate
    051     * @return a new peeking iterator
    052     * @throws NullPointerException if the iterator is null
    053     */
    054    public static <E> PushbackIterator<E> pushbackIterator(final Iterator<? extends E> iterator) {
    055        if (iterator == null) {
    056            throw new NullPointerException("Iterator must not be null");
    057        }
    058        if (iterator instanceof PushbackIterator<?>) {
    059            @SuppressWarnings("unchecked") // safe cast
    060            final PushbackIterator<E> it = (PushbackIterator<E>) iterator;
    061            return it;
    062        }
    063        return new PushbackIterator<>(iterator);
    064    }
    065
    066    //-----------------------------------------------------------------------
    067
    068    /**
    069     * Constructor.
    070     *
    071     * @param iterator  the iterator to decorate
    072     */
    073    public PushbackIterator(final Iterator<? extends E> iterator) {
    074        super();
    075        this.iterator = iterator;
    076    }
    077
    078    /**
    079     * Push back the given element to the iterator.
    080     * <p>
    081     * Calling {@link #next()} immediately afterwards will return exactly this element.
    082     *
    083     * @param item  the element to push back to the iterator
    084     */
    085    public void pushback(final E item) {
    086        items.push(item);
    087    }
    088
    089    @Override
    090    public boolean hasNext() {
    091        return !items.isEmpty() || iterator.hasNext();
    092    }
    093
    094    @Override
    095    public E next() {
    096        return !items.isEmpty() ? items.pop() : iterator.next();
    097    }
    098
    099    /**
    100     * This iterator will always throw an {@link UnsupportedOperationException}.
    101     *
    102     * @throws UnsupportedOperationException always
    103     */
    104    @Override
    105    public void remove() {
    106        throw new UnsupportedOperationException();
    107    }
    108
    109}