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    }