http://fastutil.di.unimi.it/: fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and priority queues with a small memory footprint and fast access and insertion; provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.
  • Sebastiano Vigna 
package it.unimi.dsi.fastutil;

/*
 * Copyright (C) 2002-2019 Sebastiano Vigna
 *
 * 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.
 */


import java.util.Iterator;
import java.util.ListIterator;


A bidirectional Iterator. 
This kind of iterator is essentially a ListIterator that does not support ListIterator.previousIndex() and ListIterator.nextIndex(). It is useful for those maps that can easily provide bidirectional iteration, but provide no index. 
Note that iterators returned by fastutil classes are more specific, and support skipping. This class serves the purpose of organising in a cleaner way the relationships between various iterators. 
See Also:
/** A bidirectional {@link Iterator}.
 *
 * <p>This kind of iterator is essentially a {@link ListIterator} that
 * does not support {@link ListIterator#previousIndex()} and {@link
 * ListIterator#nextIndex()}. It is useful for those maps that can easily
 * provide bidirectional iteration, but provide no index.
 *
 * <p>Note that iterators returned by {@code fastutil} classes are more
 * specific, and support skipping. This class serves the purpose of organising
 * in a cleaner way the relationships between various iterators.
 *
 * @see Iterator
 * @see ListIterator
 */

public interface BidirectionalIterator<K> extends Iterator<K> {

	
Returns the previous element from the collection.

See Also:
Returns:the previous element from the collection.
/** Returns the previous element from the collection.
	 *
	 * @return the previous element from the collection.
	 * @see java.util.ListIterator#previous()
	 */

	K previous();

	
Returns whether there is a previous element.

See Also:
Returns:whether there is a previous element.
/** Returns whether there is a previous element.
	 *
	 * @return whether there is a previous element.
	 * @see java.util.ListIterator#hasPrevious()
	 */

	boolean hasPrevious();
}