    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     * @author Mick Killianey
    028     * @since 2.0 (imported from Google Collections Library)
    029     */
    030    @GwtCompatible
    031    public 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    }