Java Statistical Analysis Tool (JSAT) is a Machine Learning library written in pure Java. 
It works to fill a void in the Java ecosystem for a general purpose library that is relatively high performance and flexible, which is not adequately fulfilled by Weka (Hall et al., 2009) and Java-ML (Abeel et al., 2009). 
Almost all of the algorithms are independently implemented using an Object-Oriented framework. 
JSAT is made available under the GNU GPL license here: 

github.com/EdwardRaff/JSAT

https://jmlr.org/papers/v18/16-131.html

https://jmlr.org/papers/volume18/16-131/16-131.pdf