Version 24 of ojAlgo has been released (not too long ago) and work on Version 25 has begun.
ojAlgo is Open Source Java code that has to do with mathematics, linear algebra and optimisation particularly (but certainly not exclusively) suitable for the financial domain.
The linear algebra framework is built using highly efficient multidimensional arrays.
Built on top of the linear algebra framework ojAlgo contains models and solvers for quadratic optimisation problems, and on top of that finance specific models/solvers for portfolio selection.
There's lots more; time series, random numbers, stochastic processes, polynomials as well as code to interact with Excel, JFreeChart and AMPL.