A collection of index based algorithms for comparison purposes. Also contains RStarTrees for indexing and the DBSCAN algorithm. Yet, this library is not intended as a library and really is complicated to use (see http://elki.dbs.ifi.lmu.de/wiki/HowTo/InvokingELKIFromJava).
S. Günnemann, B. Boden, and T. Seidl. Joint European Conference on Machine Learning and Knowledge Discovery in Databases, page 565--580. Springer, (2011)