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 */
017
018package org.apache.commons.io;
019
020import java.util.Iterator;
021import java.util.Objects;
022import java.util.stream.Stream;
023
024/**
025 * Wraps and presents a {@link Stream} as a {@link AutoCloseable} {@link Iterator} resource that automatically closes itself when reaching the end of stream.
026 *
027 * <h2>Warning</h2>
028 * <p>
029 * In order to close the stream, the call site MUST either close the stream it allocated OR call this iterator until the end.
030 * </p>
031 *
032 * @param <E> The {@link Stream} and {@link Iterator} type.
033 * @since 2.15.0
034 */
035public final class StreamIterator<E> implements Iterator<E>, AutoCloseable {
036
037    /**
038     * Wraps and presents a stream as a closable resource that automatically closes itself when reaching the end of stream.
039     * <p>
040     * <b>Warning</b>
041     * </p>
042     * <p>
043     * In order to close the stream, the call site MUST either close the stream it allocated OR call this iterator until the end.
044     * </p>
045     *
046     * @param <T>    The stream and iterator type.
047     * @param stream The stream iterate.
048     * @return A new iterator.
049     */
050    public static <T> StreamIterator<T> iterator(final Stream<T> stream) {
051        return new StreamIterator<>(stream);
052    }
053
054    /**
055     * The given stream's Iterator.
056     */
057    private final Iterator<E> iterator;
058
059    /**
060     * The given stream.
061     */
062    private final Stream<E> stream;
063
064    /**
065     * Whether {@link #close()} has been called.
066     */
067    private boolean closed;
068
069    private StreamIterator(final Stream<E> stream) {
070        this.stream = Objects.requireNonNull(stream, "stream");
071        this.iterator = stream.iterator();
072    }
073
074    /**
075     * Closes the underlying stream.
076     */
077    @Override
078    public void close() {
079        closed = true;
080        stream.close();
081    }
082
083    @Override
084    public boolean hasNext() {
085        if (closed) {
086            // Calling Iterator#hasNext() on a closed java.nio.file.FileTreeIterator causes an IllegalStateException.
087            return false;
088        }
089        final boolean hasNext = iterator.hasNext();
090        if (!hasNext) {
091            close();
092        }
093        return hasNext;
094    }
095
096    @Override
097    public E next() {
098        final E next = iterator.next();
099        if (next == null) {
100            close();
101        }
102        return next;
103    }
104
105}