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 
/*
	* 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.
	*/
package it.unimi.dsi.fastutil.objects;
import java.util.Set;

A type-specific Set; provides some additional methods that use polymorphism to avoid (un)boxing. 
 Additionally, this interface strengthens (again) iterator(). 
See Also:
/**
 * A type-specific {@link Set}; provides some additional methods that use
 * polymorphism to avoid (un)boxing.
 *
 * <p>
 * Additionally, this interface strengthens (again) {@link #iterator()}.
 *
 * @see Set
 */
public interface ObjectSet<K> extends ObjectCollection<K>, Set<K> {
	
Returns a type-specific iterator on the elements of this set.

 Note that this specification strengthens the one given in Iterable.iterator(), which was already strengthened in the corresponding type-specific class, but was weakened by the fact that this interface extends Set. 
Returns:a type-specific iterator on the elements of this set.
/**
	 * Returns a type-specific iterator on the elements of this set.
	 *
	 * <p>
	 * Note that this specification strengthens the one given in
	 * {@link java.lang.Iterable#iterator()}, which was already strengthened in the
	 * corresponding type-specific class, but was weakened by the fact that this
	 * interface extends {@link Set}.
	 *
	 * @return a type-specific iterator on the elements of this set.
	 */
	@Override
	ObjectIterator<K> iterator();
}