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;
024import org.checkerframework.checker.nullness.qual.Nullable;
025
026/**
027 * An iterator that supports a one-element lookahead while iterating.
028 *
029 * <p>See the Guava User Guide article on <a href=
030 * "https://github.com/google/guava/wiki/CollectionHelpersExplained#peekingiterator"> {@code
031 * PeekingIterator}</a>.
032 *
033 * @author Mick Killianey
034 * @since 2.0
035 */
036@DoNotMock("Use Iterators.peekingIterator")
037@GwtCompatible
038@ElementTypesAreNonnullByDefault
039public interface PeekingIterator<E extends @Nullable Object> extends Iterator<E> {
040  /**
041   * Returns the next element in the iteration, without advancing the iteration.
042   *
043   * <p>Calls to {@code peek()} should not change the state of the iteration, except that it
044   * <i>may</i> prevent removal of the most recent element via {@link #remove()}.
045   *
046   * @throws NoSuchElementException if the iteration has no more elements according to {@link
047   *     #hasNext()}
048   */
049  @ParametricNullness
050  E peek();
051
052  /**
053   * {@inheritDoc}
054   *
055   * <p>The objects returned by consecutive calls to {@link #peek()} then {@link #next()} are
056   * guaranteed to be equal to each other.
057   */
058  @CanIgnoreReturnValue
059  @Override
060  @ParametricNullness
061  E next();
062
063  /**
064   * {@inheritDoc}
065   *
066   * <p>Implementations may or may not support removal when a call to {@link #peek()} has occurred
067   * since the most recent call to {@link #next()}.
068   *
069   * @throws IllegalStateException if there has been a call to {@link #peek()} since the most recent
070   *     call to {@link #next()} and this implementation does not support this sequence of calls
071   *     (optional)
072   */
073  @Override
074  void remove();
075}