001/*
002 * Copyright (C) 2008 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import com.google.common.annotations.GwtCompatible;
020
021import java.util.Iterator;
022import java.util.NoSuchElementException;
023
024/**
025 * An iterator that supports a one-element lookahead while iterating.
026 *
027 * <p>See the Guava User Guide article on <a href=
028 * "https://github.com/google/guava/wiki/CollectionHelpersExplained#peekingiterator">
029 * {@code PeekingIterator}</a>.
030 *
031 * @author Mick Killianey
032 * @since 2.0
033 */
034@GwtCompatible
035public interface PeekingIterator<E> extends Iterator<E> {
036  /**
037   * Returns the next element in the iteration, without advancing the iteration.
038   *
039   * <p>Calls to {@code peek()} should not change the state of the iteration,
040   * except that it <i>may</i> prevent removal of the most recent element via
041   * {@link #remove()}.
042   *
043   * @throws NoSuchElementException if the iteration has no more elements
044   *     according to {@link #hasNext()}
045   */
046  E peek();
047
048  /**
049   * {@inheritDoc}
050   *
051   * <p>The objects returned by consecutive calls to {@link #peek()} then {@link
052   * #next()} are guaranteed to be equal to each other.
053   */
054  @Override
055  E next();
056
057  /**
058   * {@inheritDoc}
059   *
060   * <p>Implementations may or may not support removal when a call to {@link
061   * #peek()} has occurred since the most recent call to {@link #next()}.
062   *
063   * @throws IllegalStateException if there has been a call to {@link #peek()}
064   *     since the most recent call to {@link #next()} and this implementation
065   *     does not support this sequence of calls (optional)
066   */
067  @Override
068  void remove();
069}