1 /*
2 * Copyright (C) 2010 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 package com.google.common.collect;
18
19 import com.google.common.annotations.GwtCompatible;
20 import java.util.NoSuchElementException;
21 import org.checkerframework.checker.nullness.qual.Nullable;
22
23 /**
24 * This class provides a skeletal implementation of the {@code Iterator} interface for sequences
25 * whose next element can always be derived from the previous element. Null elements are not
26 * supported, nor is the {@link #remove()} method.
27 *
28 * <p>Example:
29 *
30 * <pre>{@code
31 * Iterator<Integer> powersOfTwo =
32 * new AbstractSequentialIterator<Integer>(1) {
33 * protected Integer computeNext(Integer previous) {
34 * return (previous == 1 << 30) ? null : previous * 2;
35 * }
36 * };
37 * }</pre>
38 *
39 * @author Chris Povirk
40 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
41 */
42 @GwtCompatible
43 public abstract class AbstractSequentialIterator<T> extends UnmodifiableIterator<T> {
44 private @Nullable T nextOrNull;
45
46 /**
47 * Creates a new iterator with the given first element, or, if {@code firstOrNull} is null,
48 * creates a new empty iterator.
49 */
50 protected AbstractSequentialIterator(@Nullable T firstOrNull) {
51 this.nextOrNull = firstOrNull;
52 }
53
54 /**
55 * Returns the element that follows {@code previous}, or returns {@code null} if no elements
56 * remain. This method is invoked during each call to {@link #next()} in order to compute the
57 * result of a <i>future</i> call to {@code next()}.
58 */
59 protected abstract @Nullable T computeNext(T previous);
60
61 @Override
62 public final boolean hasNext() {
63 return nextOrNull != null;
64 }
65
66 @Override
67 public final T next() {
68 if (!hasNext()) {
69 throw new NoSuchElementException();
70 }
71 try {
72 return nextOrNull;
73 } finally {
74 nextOrNull = computeNext(nextOrNull);
75 }
76 }
77 }