1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections4.iterators;
18
19 import java.util.ArrayDeque;
20 import java.util.Deque;
21 import java.util.Iterator;
22 import java.util.Objects;
23
24 /**
25 * Decorates an iterator to support pushback of elements.
26 * <p>
27 * The decorator stores the pushed back elements in a LIFO manner: the last element
28 * that has been pushed back, will be returned as the next element in a call to {@link #next()}.
29 * <p>
30 * The decorator does not support the removal operation. Any call to {@link #remove()} will
31 * result in an {@link UnsupportedOperationException}.
32 *
33 * @param <E> the type of elements returned by this iterator.
34 * @since 4.0
35 */
36 public class PushbackIterator<E> implements Iterator<E> {
37
38 /**
39 * Decorates the specified iterator to support one-element lookahead.
40 * <p>
41 * If the iterator is already a {@link PushbackIterator} it is returned directly.
42 *
43 * @param <E> the element type
44 * @param iterator the iterator to decorate
45 * @return a new peeking iterator
46 * @throws NullPointerException if the iterator is null
47 */
48 public static <E> PushbackIterator<E> pushbackIterator(final Iterator<? extends E> iterator) {
49 Objects.requireNonNull(iterator, "iterator");
50 if (iterator instanceof PushbackIterator<?>) {
51 @SuppressWarnings("unchecked") // safe cast
52 final PushbackIterator<E> it = (PushbackIterator<E>) iterator;
53 return it;
54 }
55 return new PushbackIterator<>(iterator);
56 }
57
58 /** The iterator being decorated. */
59 private final Iterator<? extends E> iterator;
60
61 /** The LIFO queue containing the pushed back items. */
62 private final Deque<E> items = new ArrayDeque<>();
63
64 /**
65 * Constructs a new instance.
66 *
67 * @param iterator the iterator to decorate
68 */
69 public PushbackIterator(final Iterator<? extends E> iterator) {
70 this.iterator = iterator;
71 }
72
73 @Override
74 public boolean hasNext() {
75 return !items.isEmpty() || iterator.hasNext();
76 }
77
78 @Override
79 public E next() {
80 return !items.isEmpty() ? items.pop() : iterator.next();
81 }
82
83 /**
84 * Push back the given element to the iterator.
85 * <p>
86 * Calling {@link #next()} immediately afterwards will return exactly this element.
87 *
88 * @param item the element to push back to the iterator
89 */
90 public void pushback(final E item) {
91 items.push(item);
92 }
93
94 /**
95 * This iterator will always throw an {@link UnsupportedOperationException}.
96 *
97 * @throws UnsupportedOperationException always
98 */
99 @Override
100 public void remove() {
101 throw new UnsupportedOperationException();
102 }
103
104 }