oj! Algorithms - ojAlgo - is Open Source Java code that has to do with mathematics, linear algebra and optimisation.
General information about ojAlgo can be found at: http://ojalgo.org/
User documentation: https://github.com/optimatika/ojAlgo/wiki
ojAlgo is available at The Central (Maven) Repository to be used with your favourite dependency management tool.
For bug reports, questions or discussions related to ojAlgo; use the ojAlgo-user mailing list: http://sourceforge.net/p/ojalgo/mailman/ojalgo-user/ (Note that you have to be a member to post to that list!)
If you prefer you may also use "Issues" here at GitHub, but then please make sure your post actually relates to an issue with the source code (bug reports or suggested improvements). The issue tracker is not for user support or general discussions. Use the mailing list for that.
You can ofcourse also use something like stack overflow.
Feel free to contribute any way you want. One very good way to contribute is to supply test cases that verify that the functionality you use works as expected. The ojAlgo code base is often refactored. Test cases specific to your use case can be very beneficial (for you) in assuring future releases.