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.
Ranking and selection structures.
I/O classes exploiting succinct data structures.
Implementations of ([compressed] static | minimal perfect hash) functions.
Solvers for linear systems.
Succinct data structures for collections.