Simulation speed is of primary concern here (though javascript isn't too shabby)! I actually have a C++ version too, but I thought Java might be more accessible to more people...
I completely understand the need for speed, which is why I'm happy you picked something like Java. It's just with the popularity of Javascript on HN, I was worried that it would be yet another JS project.
HearthSim is designed purely as a AI vs AI game simulation tool. It's not a "playable" Hearthstone clone. In the examples on my blog, I usually run 40,000 simulated games per data point, and each set of 40,000 games usually takes a few hours to run on a relatively modern PC.