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.Iterator;
020
021import org.apache.commons.collections4.ResettableIterator;
022
023/**
024 * Adapter to make an {@link Iterator Iterator} instance appear to be an
025 * {@link Iterable Iterable} instance.  The iterable can be constructed in one
026 * of two variants:  single use, multiple use.
027 *
028 * <p>
029 * In the single use iterable case, the iterable is only usable for one
030 * iterative operation over the source iterator.  Subsequent iterative
031 * operations use the same, exhausted source iterator.  To create a single use
032 * iterable, construct a new {@link IteratorIterable} using a {@link Iterator}
033 * that is NOT a {@link ResettableIterator} iterator:
034 * </p>
035 *
036 * <pre>
037 *   Iterator&lt;Integer&gt; iterator = // some non-resettable iterator
038 *   Iterable&lt;Integer&gt; iterable = new IteratorIterable&lt;Integer&gt;(iterator);
039 * </pre>
040 *
041 * <p>
042 * In the multiple use iterable case, the iterable is usable for any number of
043 * iterative operations over the source iterator.  Of special note, even though
044 * the iterable supports multiple iterations, it does not support concurrent
045 * iterations. To implicitly create a multiple use iterable, construct a new
046 * {@link IteratorIterable} using a {@link ResettableIterator} iterator:
047 * </p>
048 *
049 * <pre>
050 *   Integer[] array = {Integer.valueOf(1),Integer.valueOf(2),Integer.valueOf(3)};
051 *   Iterator&lt;Integer&gt; iterator = IteratorUtils.arrayIterator(array); // a resettable iterator
052 *   Iterable&lt;Integer&gt; iterable = new IteratorIterable&lt;Integer&gt;(iterator);
053 * </pre>
054 *
055 * <p>
056 * A multiple use iterable can also be explicitly constructed using any
057 * {@link Iterator} and specifying {@code true} for the
058 * {@code multipleUse} flag:
059 * </p>
060 *
061 * <pre>
062 *   Iterator&lt;Integer&gt; iterator = // some non-resettable iterator
063 *   Iterable&lt;Integer&gt; iterable = new IteratorIterable&lt;Integer&gt;(iterator, true);
064 * </pre>
065 *
066 * @param <E> the type of elements returned by this iterator.
067 * @since 4.0
068 */
069public class IteratorIterable<E> implements Iterable<E> {
070
071    /**
072     * Factory method to create an {@link Iterator Iterator} from another
073     * iterator over objects of a different subtype.
074     */
075    private static <E> Iterator<E> createTypesafeIterator(final Iterator<? extends E> iterator) {
076        return new Iterator<E>() {
077            @Override
078            public boolean hasNext() {
079                return iterator.hasNext();
080            }
081
082            @Override
083            public E next() {
084                return iterator.next();
085            }
086
087            @Override
088            public void remove() {
089                iterator.remove();
090            }
091        };
092    }
093
094    /** The iterator being adapted into an iterable. */
095    private final Iterator<? extends E> iterator;
096
097    /** The iterator parameterized as the {@link #iterator()} return type. */
098    private final Iterator<E> typeSafeIterator;
099
100    /**
101     * Constructs a new {@code IteratorIterable} that will use the given
102     * iterator.
103     *
104     * @param iterator the iterator to use.
105     */
106    public IteratorIterable(final Iterator<? extends E> iterator) {
107        this(iterator, false);
108    }
109
110    /**
111     * Constructs a new {@code IteratorIterable} that will use the given
112     * iterator.
113     *
114     * @param iterator the iterator to use.
115     * @param multipleUse {@code true} if the new iterable can be used in multiple iterations
116     */
117    public IteratorIterable(final Iterator<? extends E> iterator, final boolean multipleUse) {
118        if (multipleUse && !(iterator instanceof ResettableIterator)) {
119            this.iterator = new ListIteratorWrapper<>(iterator);
120        } else {
121            this.iterator = iterator;
122        }
123        this.typeSafeIterator = createTypesafeIterator(this.iterator);
124    }
125
126    /**
127     * Gets the iterator wrapped by this iterable.
128     *
129     * @return the iterator
130     */
131    @Override
132    public Iterator<E> iterator() {
133        if (iterator instanceof ResettableIterator) {
134            ((ResettableIterator<? extends E>) iterator).reset();
135        }
136        return typeSafeIterator;
137    }
138}