Sux4J is an effort to bring succinct data structures to Java. Presently it provides a number of related implementations covering ranking/selection over bit arrays, compressed lists and [[monotone] minimal perfect hash] functions.

Sux4J is free software distributed under either the GNU Lesser General Public License 2.1+ or the Apache Software License 2.0.

Packages
Package Description
it.unimi.dsi.sux4j.bits
Ranking and selection structures.
it.unimi.dsi.sux4j.io
I/O classes exploiting succinct data structures.
it.unimi.dsi.sux4j.mph
Implementations of ([compressed] static | minimal perfect hash) functions.
it.unimi.dsi.sux4j.mph.codec  
it.unimi.dsi.sux4j.mph.solve
Solvers for linear systems.
it.unimi.dsi.sux4j.scratch  
it.unimi.dsi.sux4j.test  
it.unimi.dsi.sux4j.util
Succinct data structures for collections.