/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Iterator;
import java.util.NoSuchElementException;

import com.google.common.annotations.GwtCompatible;

/**
 * An iterator that supports a one-element lookahead while iterating.
 * 
 * <p>
 * See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/CollectionHelpersExplained#PeekingIterator">
 * {@code PeekingIterator}</a>.
 *
 * @author Mick Killianey
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public interface PeekingIterator<E> extends Iterator<E> {
	/**
	 * Returns the next element in the iteration, without advancing the iteration.
	 *
	 * <p>
	 * Calls to {@code peek()} should not change the state of the iteration, except
	 * that it <i>may</i> prevent removal of the most recent element via
	 * {@link #remove()}.
	 *
	 * @throws NoSuchElementException if the iteration has no more elements
	 *                                according to {@link #hasNext()}
	 */
	E peek();

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * The objects returned by consecutive calls to {@link #peek()} then
	 * {@link #next()} are guaranteed to be equal to each other.
	 */
	@Override
	E next();

	/**
	 * {@inheritDoc}
	 *
	 * <p>
	 * Implementations may or may not support removal when a call to {@link #peek()}
	 * has occurred since the most recent call to {@link #next()}.
	 *
	 * @throws IllegalStateException if there has been a call to {@link #peek()}
	 *                               since the most recent call to {@link #next()}
	 *                               and this implementation does not support this
	 *                               sequence of calls (optional)
	 */
	@Override
	void remove();
}