    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
    017    package com.google.common.collect;
    018
    019    import com.google.common.annotations.GwtCompatible;
    020
    021    import java.util.Iterator;
    022    import 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     * "http://code.google.com/p/guava-libraries/wiki/CollectionHelpersExplained#PeekingIterator">
    029     * {@code PeekingIterator}</a>.
    030     *
    031     * @author Mick Killianey
    032     * @since 2.0 (imported from Google Collections Library)
    033     */
    034    @GwtCompatible
    035    public 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    }