Fastutil
From charlesreid1
Fastutil extends Java Collections framework by providing maps, sets, lists, queues that have small memory footprints and fast access/insertion. It also provides support for many types and for I/O. Distributed under Apache license, requires Java 7+.
Performance comparisons: http://java-performance.info/hashmap-overview-jdk-fastutil-goldman-sachs-hppc-koloboke-trove-january-2015/
HPPC high perf primitive collections Java: http://labs.carrotsearch.com/hppc.html