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 * @author Mick Killianey
028 * @since 2 (imported from Google Collections Library)
029 */
030@GwtCompatible
031public interface PeekingIterator<E> extends Iterator<E> {
032  /**
033   * Returns the next element in the iteration, without advancing the iteration.
034   *
035   * <p>Calls to {@code peek()} should not change the state of the iteration,
036   * except that it <i>may</i> prevent removal of the most recent element via
037   * {@link #remove()}.
038   *
039   * @throws NoSuchElementException if the iteration has no more elements
040   *     according to {@link #hasNext()}
041   */
042  E peek();
043
044  /**
045   * {@inheritDoc}
046   *
047   * <p>The objects returned by consecutive calls to {@link #peek()} then {@link
048   * #next()} are guaranteed to be equal to each other.
049   */
050  @Override
051  E next();
052
053  /**
054   * {@inheritDoc}
055   *
056   * <p>Implementations may or may not support removal when a call to {@link
057   * #peek()} has occurred since the most recent call to {@link #next()}.
058   *
059   * @throws IllegalStateException if there has been a call to {@link #peek()}
060   *     since the most recent call to {@link #next()} and this implementation
061   *     does not support this sequence of calls (optional)
062   */
063  @Override
064  void remove();
065}