Minpack, for Java! Easy to use algorithm for solving systems of equations. Works with linear, non-linear, and multivariate systems.
Still currently a work in progress to get other functions working, but Hybrd seesms to be fully functional. As I havent had a chance to finish converting the tests, its unclear if it performs to the same standard as the original yet, but so far has worked well on what I have tested.
Hybrd is what Python's SciPy Fsolve uses in conjunction with Hybrj.
This conversion was created to be an easy to use optimization tool. The hybrd1 method has been modified to accept a system of equations, an initial guess, and desired tolerance, returning the solution as an array. Some more work is needed to fully flesh out all the features of Hybrd, but the important part is there.
Any issues, comments or concerns are welcomed.