There are quite a few machine learning and data mining libraries available for Java and other JVM languages. Some of them are as follows:
Weka (http://www.cs.waikato.ac.nz/ml/weka/) is probably the most famous data mining library in Java, contains a lot of algorithms and has many extensions.
Smile (http://haifengl.github.io/smile/) is a promising ML library that is under active development at the moment and a lot of new methods are being added there.
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