If the stack is empty then stack.pop() returns undefined

Cant get why we can have true in else statement. It seems that we are only adding '(' to the stack.

Could you use Math.sqrt(n) in the loop? Or this actually doesn't affect anything cause of nextProbablePrime?

I have a question. How I can use counter.count() in run() method? counter is initialized in another class. And run() method takes only int i as parameter. Should I override it, so that it would be like run(int i, Counter count). I'm not sure if it's possible to override including new parameter. Cause in the examples i found, only the body of a method has been changed.