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}