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, sorting algorithms, and fast, practical I/O classes for binary and text files.
  • Sebastiano Vigna 
Type-specific classes for object elements or keys.

 Whenever possible and useful, fastutil provides both typical collections, which compare objects using equals(), and reference-based collections, which use identity (==). See the related comments in the overview. 
/**
 * Type-specific classes for object elements or keys.
 *
 * <p>
 * Whenever possible and useful, {@code fastutil} provides both typical
 * collections, which compare objects using {@code equals()}, and
 * {@linkplain it.unimi.dsi.fastutil.objects.ReferenceSet
 * <em>reference-based</em> collections}, which use identity ({@code ==}). See
 * the related comments in the overview.
 */
package it.unimi.dsi.fastutil.objects;