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;
020import com.google.errorprone.annotations.CanIgnoreReturnValue;
021import com.google.errorprone.annotations.DoNotMock;
022import java.util.Iterator;
023import java.util.NoSuchElementException;
024
025/**
026 * An iterator that supports a one-element lookahead while iterating.
027 *
028 * <p>See the Guava User Guide article on <a href=
029 * "https://github.com/google/guava/wiki/CollectionHelpersExplained#peekingiterator"> {@code
030 * PeekingIterator}</a>.
031 *
032 * @author Mick Killianey
033 * @since 2.0
034 */
035@DoNotMock("Use Iterators.peekingIterator")
036@GwtCompatible
037public interface PeekingIterator<E> extends Iterator<E> {
038  /**
039   * Returns the next element in the iteration, without advancing the iteration.
040   *
041   * <p>Calls to {@code peek()} should not change the state of the iteration, except that it
042   * <i>may</i> prevent removal of the most recent element via {@link #remove()}.
043   *
044   * @throws NoSuchElementException if the iteration has no more elements according to {@link
045   *     #hasNext()}
046   */
047  E peek();
048
049  /**
050   * {@inheritDoc}
051   *
052   * <p>The objects returned by consecutive calls to {@link #peek()} then {@link #next()} are
053   * guaranteed to be equal to each other.
054   */
055  @CanIgnoreReturnValue
056  @Override
057  E next();
058
059  /**
060   * {@inheritDoc}
061   *
062   * <p>Implementations may or may not support removal when a call to {@link #peek()} has occurred
063   * since the most recent call to {@link #next()}.
064   *
065   * @throws IllegalStateException if there has been a call to {@link #peek()} since the most recent
066   *     call to {@link #next()} and this implementation does not support this sequence of calls
067   *     (optional)
068   */
069  @Override
070  void remove();
071}