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 boolean elements or keys.

 Not all classes are provided in a boolean-specific version: sorted sets and maps not generated, as they are useless. Unsorted sets and maps are kept around for orthogonality, whereas BooleanCollection is used by maps with boolean values. 
/**
 * Type-specific classes for boolean elements or keys.
 *
 * <p>
 * Not all classes are provided in a boolean-specific version: sorted sets and
 * maps not generated, as they are useless. Unsorted sets and maps are kept
 * around for orthogonality, whereas
 * {@link it.unimi.dsi.fastutil.booleans.BooleanCollection} is used by maps with
 * boolean values.
 */
package it.unimi.dsi.fastutil.booleans;