1   /*
2    * Copyright (C) 2008 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  
21  import java.util.Iterator;
22  import java.util.NoSuchElementException;
23  
24  /**
25   * An iterator that supports a one-element lookahead while iterating.
26   * 
27   * <p>See the Guava User Guide article on <a href=
28   * "http://code.google.com/p/guava-libraries/wiki/CollectionHelpersExplained#PeekingIterator">
29   * {@code PeekingIterator}</a>.
30   *
31   * @author Mick Killianey
32   * @since 2.0 (imported from Google Collections Library)
33   */
34  @GwtCompatible
35  public interface PeekingIterator<E> extends Iterator<E> {
36    /**
37     * Returns the next element in the iteration, without advancing the iteration.
38     *
39     * <p>Calls to {@code peek()} should not change the state of the iteration,
40     * except that it <i>may</i> prevent removal of the most recent element via
41     * {@link #remove()}.
42     *
43     * @throws NoSuchElementException if the iteration has no more elements
44     *     according to {@link #hasNext()}
45     */
46    E peek();
47  
48    /**
49     * {@inheritDoc}
50     *
51     * <p>The objects returned by consecutive calls to {@link #peek()} then {@link
52     * #next()} are guaranteed to be equal to each other.
53     */
54    @Override
55    E next();
56  
57    /**
58     * {@inheritDoc}
59     *
60     * <p>Implementations may or may not support removal when a call to {@link
61     * #peek()} has occurred since the most recent call to {@link #next()}.
62     *
63     * @throws IllegalStateException if there has been a call to {@link #peek()}
64     *     since the most recent call to {@link #next()} and this implementation
65     *     does not support this sequence of calls (optional)
66     */
67    @Override
68    void remove();
69  }