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
17package com.google.common.collect;
18
19import com.google.common.annotations.GwtCompatible;
20
21import java.util.NoSuchElementException;
22
23import javax.annotation.Nullable;
24
25/**
26 * This class provides a skeletal implementation of the {@code Iterator}
27 * interface for sequences whose next element can always be derived from the
28 * previous element. Null elements are not supported, nor is the
29 * {@link #remove()} method.
30 *
31 * <p>Example: <pre>   {@code
32 *
33 *   Iterator<Integer> powersOfTwo =
34 *       new AbstractSequentialIterator<Integer>(1) {
35 *         protected Integer computeNext(Integer previous) {
36 *           return (previous == 1 << 30) ? null : previous * 2;
37 *         }
38 *       };}</pre>
39 *
40 * @author Chris Povirk
41 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
42 */
43@GwtCompatible
44public abstract class AbstractSequentialIterator<T>
45    extends UnmodifiableIterator<T> {
46  private T nextOrNull;
47
48  /**
49   * Creates a new iterator with the given first element, or, if {@code
50   * firstOrNull} is null, creates a new empty iterator.
51   */
52  protected AbstractSequentialIterator(@Nullable T firstOrNull) {
53    this.nextOrNull = firstOrNull;
54  }
55
56  /**
57   * Returns the element that follows {@code previous}, or returns {@code null}
58   * if no elements remain. This method is invoked during each call to
59   * {@link #next()} in order to compute the result of a <i>future</i> call to
60   * {@code next()}.
61   */
62  protected abstract T computeNext(T previous);
63
64  @Override
65  public final boolean hasNext() {
66    return nextOrNull != null;
67  }
68
69  @Override
70  public final T next() {
71    if (!hasNext()) {
72      throw new NoSuchElementException();
73    }
74    try {
75      return nextOrNull;
76    } finally {
77      nextOrNull = computeNext(nextOrNull);
78    }
79  }
80}
81